Foundation Bhawan Bharti Biology

48 Foundation Science: Biology for Class 10 Gland Hormone Effect Pituitary Growth hormone Others Controls growth Thyroid Thyroxine Parathyroid Parathormone Controls other endocrine glands Pancreas (islets of Langerhans) Insulin Adrenal Adrenaline Controls general metabolism Testis Testosterone Controls calcium level in blood Ovary Oestrogen Controls blood glucose level · Animal hormones are produced by ductless glands called endocrine glands, and are Causes excitement, increases blood pressure, discharged in the blood. Some important heartbeat and respiration rate endocrine glands, their hormones and functions are given in the above table. Promotes development of secondary sexual characters in males · The human nervous system consists of central, peripheral and autonomic parts. Promotes development of secondary sexual characters in females · The central nervous system consists of the brain and spinal cord. The brain has three main parts—forebrain, midbrain and hindbrain. · The spinal cord is the centre of reflex action. · Reflex actions are quick involuntary responses to stimuli. • EXERCISES • A. Very-Short-Answer Questions 7. Which endocrine gland is called the master gland of the human body? Why? 1. Name any two types of tropism. 8. Distinguish between gigantism and dwarfism. 2. What happens if a young green plant receives sunlight from one direction only? 9. Differentiate between the functions of sensory and motor nerves. 3. Mention one important function of gibberellins. 10. Why are diabetes patients treated by insulin 4. Name the plant hormone that promotes cell injections? division. 11. Why is the use of iodised salt advisable? 5. What is reflex action? 12. What is a nerve impulse? Which structure in a 6. What is synapse? neuron helps to conduct a nerve impulse (i) towards the cell body and (ii) away from the body? [CBSE] 7. Which part of the brain maintains the posture and balance of the body? 13. Name one hormone secreted by (a) the testis 8. Name the two sets of nerves that constitute the (b) the islets of Langerhans peripheral nervous system. (c) the thyroid gland (d) the ovary 9. Name one hormone which controls growth in animals. C. Long-Answer Questions B. Short-Answer Questions 1. What is the difference between the movement in a sensitive plant and the movement in our legs? 1. What is coordination? Give an example. 2. Compare and contrast the systems of nervous and 2. What are hormones? hormonal control in animals. 3. What are tropic movements? Name the types of 3. Which gland secretes adrenaline? Why is tropic movement observed in plants. adrenaline called the emergency hormone? 4. Differentiate between tropic and nastic movements 4. Briefly describe the structure of the neuron. in plants. Give one example of each. [CBSE] 5. Explain with an example how a reflex action takes place. 5. Design an experiment to demonstrate hydrotropism. 6. Where are auxins produced? Mention any two important functions of auxins.

Control and Coordination 49 6. (a) Draw the structure of a neuron and label its (a) insulin (b) adrenaline (c) thyroxine (d) testosterone nucleus, dendrite, cyton and axon. (b) Name that part of a neuron 7. The seat of intelligence and voluntary action in the (i) which receives stimulus brain is the (ii) which transmits impulse [CBSE] (a) pons (b) cerebellum 7. (a) What is (i) phototropism and (ii) geotropism? (c) cerebrum (d) medulla oblongata Give an activity supported by labelled diagrams to 8. The activities of the internal organs are controlled by (a) the reflex arc show that light and gravity affect the direction of (b) the peripheral nervous system (c) the autonomic nervous system plant growth. (d) none of these (b) Mention the role of the following plant hormones. (i) Auxin (ii) Abscisic acid [CBSE] D. Objective Questions 9. The junction between two neurons is called I. Pick the correct option. (a) synapse (b) dendrite 1. The plant hormone essential for cell division is (c) joint (d) axon (a) abscisic acid (b) auxin 10. Reflex action is controlled mainly by the (c) gibberellin (d) cytokinin (a) brain 2. The root of a plant is (b) autonomous nervous system (a) positively geotropic (c) peripheral nervous system (b) positively phototropic (d) spinal cord (c) negatively geotropic II. Fill in the blanks. (d) positively thigmotropic 1. The sensitive plant _____ folds up its leaflets on being touched. 3. Which of these plant hormones is a growth 2. In animals, hormones are secreted by _____. inhibitor? 3. Simple goitre is caused by the deficiency of _____ in (a) Gibberellin (b) Auxin the diet. 4. _____ promotes the development of secondary (c) Abscisic acid (d) Cytokinin sexual characters in a female. 4. The response of plant parts towards light is called 5. The cell body of a neuron is called _____. 6. The brain is enclosed in a box called _____. (a) geotropism (b) phototropism 7. The membranes covering the brain are called _____. 8. The _____ gland is located on the kidney. (c) hydrotropism (d) chemotropism 5. The master gland of the body is the (a) testis (b) pituitary (c) thyroid (d) adrenal 6. People suffering from diabetes mellitus are unable to secrete sufficient • ANSWERS • Objective Questions 1. (d) 2. (a) 3. (c) 4. (b) 5. (b) 8. (c) 9. (a) 10. (d) 6. (a) 7. (c) v

6 Reproduction ReprRoedpurcotdiounctiisonone of the fundamental attributes of a living organism, through which it is able to produce more of its own kind. Through this process, new individuals are produced, that grow and reproduce again, thus increasing the population of a species. How do we ascertain whether two organisms belong to the same species or not? Consider the example of a common urban crow and a black jungle crow. You may think they belong to the same species because they resemble each other. Actually, however, they belong to two different species because they cannot interbreed. A species is defined as a group of organisms (plants or animals) that can interbreed to produce fertile offspring. Compared to life processes like nutrition, respiration, etc., reproduction may appear to be a waste of energy as it is not essential for the survival of an individual. But it is an important function of a living being as it helps an organism to perpetuate its own kind. Continuity of life, from the time of its origin (millions of years ago) to the present day, has been possible through reproduction. ORGANISMS PRODUCE SIMILAR OFFSPRING Organisms produce similar offspring, but not exact copies of themselves. The offspring have similar body structures and similar genetic blueprints (DNA) in their cells. At the cellular level, reproduction involves first making a copy of its DNA and creating the cellular apparatus for the new cell. Then the nucleus divides and separates the two copies of DNA. Finally, the cytoplasm divides and separates the cellular apparatus. Thus, a cell divides to produce two new similar cells. DNA carries information for making proteins. Any change, or error, in the copying of the DNA sequence will change the structure of the protein formed. This altered protein may change the basic body design. If this protein happens to be an enzyme then the biochemical reaction it catalyzes will also be affected. Errors in the copying of DNA cause variation (change) in the offspring. Due to this tendency for variation, organisms do not create exact copies during reproduction. IMPORTANCE OF VARIATION Reproduction involves the replication of DNA. If the replication is exact the body design and features of the organism are maintained across generations. Thus reproduction provides stability to the populations of a species. But reproduction also provides variation. Variation is important for the survival of a species over a period of time, but it may bring advantages and/or disadvantages to the organisms in which it occurs. You will learn more about this in the next chapter. All organisms are exposed to certain environmental conditions. They adapt to cope with even slight changes in the environment. Any drastic change in the environment, like extreme temperature change, varying water level, an earthquake or a meteorite hit, may take a heavy toll 50

Reproduction 51 on organisms that are unable to cope with its effects. Variation gives some individuals in a population the ability to cope with changes that others cannot withstand. Suppose, there is a sudden drop in environmental temperature. As a result, some organisms will die due to severe cold. But some of them that have developed the ability to cope with extreme cold will survive and reproduce. This ability comes from variation caused by changes in the DNA. Reproduction in living organisms takes place by two general methods: asexual and sexual. ASEXUAL REPRODUCTION The type of reproduction that takes place without the process of gamete (sex cell) formation is called asexual reproduction. This type of reproduction takes place commonly in lower plants and animals, where the body is not very complex. There are different forms of asexual reproduction. Binary Fission Binary fission occurs under favourable environmental conditions. Binary fission is the division of one cell into two similar cells. This is the simplest method of asexual reproduction. It occurs in unicellular organisms like bacteria, yeast, Euglena, Amoeba and Paramoecium. In some organisms (e.g., Leishmania, which causes kala-azar) binary fission takes place in definite orientation due to their specific body structure. Take a permanent slide of Amoeba showing binary fission. Observe it under a microscope. You will see that the nucleus first divides amitotically into two, followed by the division of the cytoplasm. The Amoeba finally splits into two daughter cells. Fig. 6.1 (a) Binary fission in Amoeba (b) Multiple fission in Chlorella Multiple Fission Under unfavourable circumstances some unicellular organisms develop a hard protective covering over the cell, called cyst. The nucleus of the cell divides repeatedly, producing many nuclei. Each nucleus is surrounded by a small amount of cytoplasm and many daughter cells are produced within the cyst. When favourable conditions return, the offspring are released. Multiple fission is seen in many algae and the malarial parasite (Plasmodium). Budding Sometimes new individuals develop from the body wall of the parent as bulblike projections called buds. The buds may be unicellular or multicellular depending upon the type of parent

52 Foundation Science: Biology for Class 10 organism. The buds finally separate to form new individuals. Budding occurs in yeast, Hydra and sponges. Put some yeast in 10% sugar solution Fig. 6.2 (a) Hydra and (b) yeast reproduce by budding. kept in a glass. Cover the glass and keep it in a warm place for a day. The yeast cells grow and reproduce in the sugar solution. These cells are known as a culture of yeast cells. Take a drop of the yeast culture solution on a slide and cover it with a coverslip. Examine it under the microscope. You will see buds on the yeast cells. Fragmentation Obtain some pond water. You may see green filamentous structures floating in it. Take some of these structures on a slide. Put a drop of glycerine on them and cover them with a coverslip. Observe under a microscope. The green filamentous structures you see are an alga named Spirogyra, which grows in ponds, ditches and springs. Each filament has a single row of cylindrical cells. Each cell has spiral bands of chloroplasts. When a Spirogyra filament breaks into pieces, each piece grows into a new filament by cell division. This process is fragmentation. During this process the body of an individual breaks up into two or more parts and each part develops into a complete organism Fig. 6.3 Fragmentation in Spirogyra (Figure 6.3). Some animals like sponges, Hydra and flatworms (Planaria) also reproduce by a similar method known as regeneration. If they are cut into pieces, each piece can regenerate into an entire individual. In complex organisms all cells are not similar. The cells are organized into tissues and tissues into organs. The different organs are placed at definite positions. If such an organism breaks off at any point, the broken part cannot grow into a complete organism with all organs. Spore Formation Spores are asexual reproductive bodies enclosed in a thick-walled structure called sporangium, which can tide over unfavourable conditions such as extreme heat, dryness, acidity, and so on. Spore formation is a common method of asexual reproduction in many lower forms of life such as algae, bacteria and fungi. Under favourable conditions, the spores are released by the breaking of the thick wall of the sporangium. The spores then germinate into new individuals. In fungi, sporangia burst and release spores (Figure. 6.4). By this method of asexual reproduction, organisms can overcome unfavourable conditions. Some fungi, e.g., Rhizopus and Mucor reproduce by producing spores.



54 Foundation Science: Biology for Class 10 The fleshy leaves of Bryophyllum bear adventitious buds in the notches along the leaf margin. These buds develop into small plants (plantlets) under favourable conditions. These plantlets can be easily separated to grow as independent plants. Artificial modes of vegetative propagation Farmers, gardeners and horticulturists have developed various artificial methods of vegetative propagation, like grafting, layering, cutting and tissue culture for growing plants in gardens and nurseries. Cutting is a very simple Fig. 6.6 Buds arise from the leaf notches in Bryophyllum method of propagation in which a piece of the parent plant’s stem with nodes and internodes is placed in moist soil. This grows into a new plant. In grafting the cutting of a plant is attached to the stem of a rooted plant. The attached cutting becomes a part of the rooted plant, draws nutrition from it and grows roots at the joint. Now if it is separated, it grows into a new plant. In layering, one or more branches of the parent plant are bent close to the ground and covered with moist soil. The covered portions grow roots and develop into new plants. Cut two pieces from a money plant—one with leaves (i.e., a portion with nodes) and the other without leaves (i.e., a portion of an internode). Place these with one end immersed in water kept in a transparent bottle. Leave them like this for a week. You will see that roots and new leaves grow on the piece with leaves, while the other piece gradually withers. This is because a plant can grow new leaves and branches only if it has nodes. (New leaves and branches arise at the nodes.) The piece of money plant which does not have nodes cannot grow because it cannot produce new leaves. Tissue culture In this technique some tissue from a desired plant is placed in a suitable nutrient medium under proper conditions. The tissue grows into an unorganized mass, known as callus. A small part of this is put in another medium, which contains growth hormones that induce the formation of plantlets from the callus. When plantlets grow, they can be transplanted in the soil or in pots for developing to maturity. Tissue culture allows us to grow a whole plant from cells taken from any part of the plant body. Many plants can be grown from one parent plant in the laboratory under controlled, disease-free conditions. SEXUAL REPRODUCTION Sexual reproduction involves the two sexes, namely, male and female. The male sexual unit is known as male gamete, while the female sexual unit is known as female gamete. The formation of gametes and their fusion constitute sexual reproduction. The male gamete is smaller and more active than the female gamete. The female gamete is larger, filled with reserve food and remains passive. The cell formed after the fusion of the male and female gametes is called zygote. The zygote divides repeatedly to form a new individual. Although sexual reproduction also occurs in unicellular organisms like algae and Paramoecium, it is most common in multicellular organisms. Genetic basis and advantage of variations You know that variations help in the survival of a species over time. During asexual reproduction cells divide and DNA replication takes place. At the time of replication, some variation may occur but this variation does not usually cause any drastic change. So, in asexual reproduction offspring are more or less similar to the parent and variation is slow. During sexual reproduction two types of gametes (male and female) are formed. During the fusion of gametes there is recombination of genetic material from two parents. This leads to greater variation in the offspring. As the offspring gets more variations, it is more likely to adjust better to environmental fluctuations.

Reproduction 55 Gametes contain half the usual number of chromosomes During sexual reproduction, the combination of DNA from two parents would result in the offspring having twice the amount of DNA. To solve this problem, sexually reproducing individuals have special germ cells (gametes) with only half the normal number of chromosomes and, therefore, half the amount of DNA compared to the other cells of the body. When such germ cells from two individuals unite during sexual reproduction, the normal chromosome number and DNA content are restored. Significance of sexual reproduction Sexual reproduction results in new combinations of genes that are brought together during gamete formation. This reshuffling of genes in the gametes increases the chances of variation in the offspring. Moreover, the combination of two sets of chromosomes, one set from each parent, during zygote formation, leads to variation within a species. SEXUAL REPRODUCTION IN FLOWERING PLANTS The reproductive part of a flowering plant is the flower. Flowers are considered to be modified shoots. Parts of a Flower Most flowers have both the male and female reproductive organs, but some bear either the male or the female sex organs. Such flowers are known as unisexual flowers (e.g., watermelon, cucumber, etc.) Those flowers which have both sex organs are known as bisexual flowers (e.g., Hibiscus, pea, etc.) A flower generally bears a long or short axis. This axis has two parts—the stalk of the flower, called pedicel, and its swollen top called thalamus. The parts of a flower are arranged on the thalamus (Figure 6.7). Fig. 6.7 Diagram of the longitudinal section of a flower A typical flower consists of four sets of floral parts, or whorls: calyx (sepals), corolla (petals), androecium (stamens) and gynoecium (carpels). Calyx and corolla are not directly involved in reproduction. Androecium and gynoecium are directly concerned with sexual reproduction. The androecium is the male part of the flower and consists of stamens. The gynoecium (or pistil) consists of carpels and is the female reproductive part. The whorls are arranged on the thalamus of a flower in a definite sequence.

56 Foundation Science: Biology for Class 10 Calyx Calyx is the outermost whorl. It consists of sepals. The sepals are usually green, but sometimes they may be coloured. Calyx protects the floral whorls in the bud stage. Corolla Corolla, the next inner whorl, consists of petals. Petals may be white or brightly coloured. They attract insects towards the flower and thus help in pollination. Corolla protects the reproductive whorls in the bud stage. Androecium The stamens are collectively called androecium, which Fig. 6.8 Germination of pollen grains on is the third whorl. Each stamen consists of a filament stigma and fertilization and an anther. Each anther has two chambers called pollen sacs. If you touch the stamens of a flower, a yellowish powder may come off on your hands. Anthers produce these numerous yellowish pollen grains, which contain male gametes. Gynoecium The gynoecium (or pistil) is in the centre of the flower. It is the fourth whorl. It bears the female reproductive organ, called carpel. Each carpel consists of three parts—a basal swollen portion called ovary, a narrow stalklike middle portion called style and a one- to many-lobed flattened disclike sticky structure called stigma at the top of the style. The ovary is surrounded by an outer wall. The ovary may be divided into chambers. The chambers contain ovules. Each ovule has an egg cell (female gamete). Pollination The transfer of pollen grains from the anthers of a flower to the stigma of the same or another flower is known as pollination. The transfer of pollen to the stigma of the same flower or of flowers borne by the same plant is known as self-pollination (as in pea and Hibiscus). Cross-pollination is the transfer of pollen from the anthers of a flower to the stigma of a flower on another plant of the same species. It is very common in most flowering plants. Pollen can be carried with the help of many agents such as insects, birds, wind and water. Flowers and pollen grains are modified to facilitate the process of cross-pollination. For example, insect-pollinated flowers are colourful so that they attract insects. Wind-pollinated flowers produce light pollen grains which can be carried by the wind. Fertilization The pollen grains germinate on the stigma after pollination. The inner wall of the pollen grain grows into a pollen tube, which grows down through the style and finally reaches the ovule. Inside the ovule, a male gamete fuses with the female gamete and a zygote is formed. This is known as fertilization. The zygote divides repeatedly to form the embryo (future plant) in the ovule. The embryo possesses a tiny future root (radicle), a tiny future shoot (plumule) and cotyledons to store food. The ovary grows rapidly to form the fruit. The ovary wall ripens and forms the fruit wall. The sepals, petals, stamens, style and stigma of the flower degenerate and usually fall off. Sometimes the sepals may persist in the fruit. The ovule develops into a seed. The wall of the ovule thickens to form the protective seed coat. The seed loses water and becomes hard and dry. Seeds can withstand drought and other adverse conditions in this state. This is an advantage for seed-producing plants. The embryo lies dormant in the seed, but under favourable environmental conditions it becomes active and germinates to form a small seedling. The radicle forms the root while the plumule forms the shoot. The growing root and shoot utilize the food stored in the cotyledons.

Reproduction 57 Keep some gram seeds in water for one day. Now take out the soaked seeds from water and keep them at normal room temperature. After 2–3 days remove the seed coat of a healthy seed, separate its cotyledons and place them on a clean glass slide. Observe the parts of the radicle, plumule and cotyledons, with the help of a hand lens. Fig. 6.9 The embryo has a plumule, a radicle and cotyledons. HUMAN REPRODUCTIVE SYSTEM Changes in the Human Body When a baby begins to grow, the different parts of its body such as the head, arms, legs and chest grow at different rates. For about the first 12 years of its life it goes through a phase of body enlargement and mental development. During this phase its reproductive organs develop at a slower rate. At about 12 years of age, body enlargement slows down and certain other changes begin to appear. These changes prepare the body for sexual reproduction. This phase is known as adolescence. During this phase, certain parts of the body change in appearance and the person also experiences new sensations such as extreme happiness, sadness, anger, insecurity, and so on. All this is due to the beginning of the secretion of hormones from the ovary and testis. The age when this begins is called the age of sexual maturity (puberty). It varies from person to person. It is marked by the growth of thick hair in the armpits and pubic area. In males, facial hair begins to grow. The vocal cords become wide. Therefore, the voice begins to deepen. The testes become active and begin to produce sperms. The penis and scrotum become larger. In females the menstrual cycle begins and the breasts become enlarged. These changes are slow and take place over six years or so. They serve as signals identifiable by other individuals that sexual maturation is taking place. From this period onwards sexual reproduction becomes a possibility as the body becomes capable of producing the specialized germ cells that are needed for sexual reproduction. But childbearing and lactation (milk secretion) need the female reproductive organs and breasts to be fully developed. Male Reproductive System The male reproductive organs include the testes, seminal vesicles, penis and some associated glands such as the prostate gland. Testis The most important male reproductive organ is the testis, which produces sperms. There are two oval testes, each contained in a protective bag called scrotum (or scrotal sac), lying outside the abdominal cavity. The scrotal sac can elongate and contract depending upon the body temperature and external temperature. This is necessary because sperm formation occurs at a temperature lower than normal body temperature. The testes produce sperms continuously from the stage of puberty onwards. Sperms from the testis pass through the sperm duct, known as vas deferens. The vas deferens runs anteriorly up to the urinary bladder, from where it leads downward and is joined by a duct from the seminal vesicle.

58 Foundation Science: Biology for Class 10 Fig. 6.10 Human male sex organs Seminal vesicle The seminal vesicle is an elongated sac at the base of the urinary bladder. For each testis, there is one vas deferens and one seminal vesicle. The functions of a seminal vesicle are to store the sperms that have come from the testis and to secrete seminal fluid, or semen, in which the sperms float. Prostate gland The sperm ducts from both sides join near the base of the urinary bladder, opening into a single tube called urethra (Figure 6.10). This junction occurs inside the prostate gland. The prostate gland adds its secretion to the seminal fluid. The urethra leads to the outside of the body through an organ called penis. It carries both urine and seminal fluid. Penis The penis is a muscular, tubular organ made up of loose tissue with spaces in between. This is called erectile tissue. On being stimulated, the erectile tissue fills with blood, making the penis erect and firm, so that it may enter the vagina of the female and discharge the sperms. Sperm The sperm is the male gamete. It has a head and a long tail, which helps it swim towards the ovum (egg). Female Reproductive System The female reproductive organs include the ovaries, Fallopian tubes, uterus and vagina. Ovary The ovaries are a pair of small, oval organs in the lower part of the abdominal cavity. They produce ova. At the time of birth, a female already has thousands of immature ova in her ovaries. Many of these degenerate during childhood. The ova start maturing when the female reaches puberty. Every 28 days, one of the ovaries releases an ovum. When an ovum is released from the ovary, it is taken up by a thin Fallopian tube (also called oviduct) through its funnel-shaped opening. The ovum is passed down the duct and into the uterus, which passes it out of the body through the vagina. The ovum is very small and, therefore, hardly noticeable. Fallopian tube The Fallopian tubes, or oviducts, are a pair of thin tubes that lead from the ovaries to the uterus. Each Fallopian tube has a funnel-shaped opening near the ovary. It is lined by cilia. The movement of the cilia helps conduct the ovum down the Fallopian tube and into the uterus.

Reproduction 59 Uterus Fig. 6.11 Human female sex organs Vagina The uterus (womb) is a hollow, pear-shaped, elastic muscular structure. Its upper portion, into which the Fallopian tubes enter, is broader. The narrow lower portion, called cervix, consists of a ring of muscles. The uterus opens into the vagina through the cervix. A fertilized ovum (zygote) develops into a baby inside the uterus. The vagina is a tube leading to the outside of the body through an opening called vulva. The vagina is the organ where the penis is inserted during intercourse for the discharge of sperms. It is also the passage through which the fully developed baby is born. Fertilization When semen is discharged in the vagina during sexual intercourse, the sperms begin moving up the vagina and uterus, finally reaching the Fallopian tubes. But only one sperm enters the ovum. Most of the sperms die while climbing up the Fallopian tubes. A sperm can remain alive in the Fallopian tube for about 12 hours. In this span of time, if it meets the ovum, it is likely to enter the ovum. This is called fertilization. Changes after Fertilization Implantation The fertilized egg (zygote) moves down the Fallopian tube and continuously undergoes cell division. Thus it forms a hollow ball of cells, called embryo. The embryo gets embedded in the wall of the uterus, which is thick and has muscles, glands and a large number of blood capillaries. This process is called implantation. Pregnancy The developing embryo at first derives nourishment directly from the mother’s blood flowing in the vessels lining the uterine wall. In about three weeks, it starts absorbing food and oxygen through an organ called placenta. The placenta is a disclike organ in the lining of the uterine wall. It has numerous villi, which are in direct contact with the mother’s blood flowing in the uterine wall. These villi provide a large surface area for glucose and oxygen to pass from the mother to the embryo and for wastes produced by the embryo to be passed into the mother’s blood. The embryo is connected to the placenta by a tube called the umbilical cord. By eight weeks, the embryo starts showing human features and is referred to as foetus. The total period of embryonic development, from the time of fertilization to birth, is called gestation period. It is around 280 days, or 9 months, in humans.

60 Foundation Science: Biology for Class 10 Birth The wall of the uterus develops a thick layer of muscles during pregnancy. At the time of birth, the uterine muscles contract rhythmically and powerfully, causing labour pains to the mother. Finally, the baby is expelled by the contraction of the uterine muscles. This is called birth or parturition. What happens when the egg is not fertilized If the ovum is not fertilized in the upper part of the oviduct, it keeps on descending and is finally passed out through the vagina. It remains in the body for about 24–72 hours. As an egg is released for fertilization every month, the uterus also prepares itself every month for the implantation of a fertilized egg. The uterus becomes thick-walled and spongy in order to nourish the embryo. If no fertilization takes place, the thick uterine wall is no longer needed. So, it gradually begins to shrink. This shrinkage ruptures its blood vessels. As a result, blood and mucus ooze out of the vagina. This period, which lasts for 3–5 days, is called the menstrual period, and the process is called menstruation. If the ovum is fertilized, it gets implanted in the uterus wall and embryonic development starts. In this case, the uterus continues to develop in order to hold the embryo. And in this case, there is no question of its shrinkage resulting in menstruation. REPRODUCTIVE HEALTH Reproductive health is concerned with healthy and safe sexual practices. Unhealthy practices can lead to the transmission of diseases from one partner to another and even to the offspring. Reproductive health also depends on healthy behaviour and outlook towards sex life. Sexual maturation and body growth are gradual processes. Even with some degree of sexual maturation, the body and mind are not mature enough for a sexual act, childbearing and bringing up children. Reproduction at an early age, say between 13 and 20 years, is not advisable as the uterus is not completely developed to hold the foetus for the entire gestation period of 9 months. There is a risk that the uterus may rupture or the foetus may be aborted. Also, sexual intercourse involves intimate physical contact between the male and female sex organs. This contact may transmit certain diseases from one partner to another. Such diseases are called sexually transmitted diseases (STDs). Sexually Transmitted Diseases Sexually transmitted diseases are caused by a variety of microorganisms (such as bacteria and viruses) that live in the warm and moist environments of the vagina, urethra, anus, etc. STDs occur mostly in individuals who are involved in sexual activity with many partners. Bacterial infections Gonorrhoea and syphilis are common sexually transmitted bacterial infections. These are caused by bacteria that infect the ureter in men and the cervix in women. Viral infections Viruses cause STDs such as herpes, genital warts and cervical cancer. AIDS (Acquired Immune Deficiency Syndrome) is caused by the human immunodeficiency virus (HIV), which attacks the immune system and kills people. The primary route of transmission of HIV is sexual, but it is also spread by the use of infected needles among intravenous drug users, by the transfusion of infected blood, and from an infected woman to her foetus during pregnancy. Prevention To prevent sexually transmitted diseases, the following precautions can be taken. 1. Using a protective covering called condom over the penis

Reproduction 61 2. Using disposable needles and syringes 3. Not sharing shaving blades or razors 4. Not having multiple sex partners 5. Testing and screening the blood for HIV before transfusing it FAMILY PLANNING The human population is growing rapidly and this is a major cause for concern. With the increase in population, the resources of the earth will deplete more rapidly. The environment will be adversely affected and it will be difficult to maintain the quality of life of the large population. It is, therefore, extremely important to control population growth. In many countries, as in ours, the population grows rapidly because birth rates are high and death rates are comparatively low. In such countries it is extremely important to have a small family. Contraception It is possible to limit the size of a family through various means. One is to prevent pregnancy. Fertilization of the ovum and its subsequent implantation is referred to as conception or pregnancy. Prevention of conception is called contraception. Conception can be prevented in the following ways. Mechanical barrier There are a number of methods of contraception that create a mechanical barrier between the sperms and the egg. One method is to use a fine rubber tube called condom. This is worn over the penis during sexual intercourse, so that semen is collected in this tube and not discharged in the vagina. This method also prevents the spread of AIDS and many other sexually transmitted diseases. A diaphragm or cap can be fitted in the cervix of a woman to prevent semen from reaching the Fallopian tube. An intrauterine contraceptive device (IUCD), or loop or copper-T, is another contraceptive device which can be used by a woman to prevent conception. An IUCD is made of plastic or stainless steel. It is inserted in the uterus. Its insertion causes irritation in the uterine lining. As a result, there is a lot of mucus secretion which prevents implantation of the embryo. Chemical methods Oral contraceptives can be taken to change the hormonal balance of the body so that the ovum is not released from the ovary. Since the ovum does not come into the Fallopian tube, it is not fertilized. Oral contraceptives are tablets which a woman has to take every day. These are also called birth control pills. Of all the contraceptive measures, oral contraceptive pills are the most effective. However, the change of hormonal balance caused by the intake of oral contraceptives occasionally has undesirable side effects. Surgical methods If the vas deferens, which carries the sperms to the urethra, is tied by a thread, the sperms cannot go past the tied point. The vas deferens can be exposed by a slight incision at the base of the scrotum. This incision and subsequent ligature (tying by thread) of the vas deferens by a surgeon is called vasectomy. In women, ligature of the Fallopian tube can prevent the passage of ova down the Fallopian tube. This is called tubectomy. Both vasectomy and tubectomy ensure that fertilization will not take place.

62 Foundation Science: Biology for Class 10 Surgery can also be used for aborting unwanted pregnancies. However, this is often misused for illegally aborting female foetuses. (The killing of a foetus is called foeticide.) To prevent female foeticide, prenatal sex determination has been prohibited by law. • POINTS TO REMEMBER • · Living organisms can reproduce by two · The testes produce the male gametes (sperms). methods: asexual and sexual. · The female reproductive system consists of the · Asexual reproduction can occur by binary fission, ovaries, Fallopian tubes, uterus and vagina. multiple fission, budding, fragmentation, sporulation, and vegetative propagation (in plants). · The ovaries produce the female gametes (ova). Each month, one ovum (egg) matures and is · Vegetative propagation in plants occurs by released from the ovary into the Fallopian tube. means of stems, roots and leaves. It is also done artificially (grafting, cutting, layering, tissue · Semen (containing sperms) is discharged into the culture) and is applied in horticulture and vagina during sexual intercourse. If a sperm agriculture. reaches the Fallopian tube and fuses with an ovum, fertilization occurs and a zygote is formed. · Sexual reproduction involves the fusion of male Fertilization is followed by the implantation of and female gametes. the embryo in the uterine wall. · Flowers bear the sexual parts of the plant— · The developing embryo in the uterus obtains androecium (male part) and gynoecium (female oxygen and food from the mother’s blood part). Flowers may be unisexual or bisexual. through the placenta. The umbilical cord links the embryo to the placenta. Childbirth takes place · The male gamete of plants is produced in the due to the contractions of the uterine walls. stamen and the female gamete is produced in the carpel. · If no fertilization takes place, the uterine wall disintegrates, resulting in menstruation. · The transfer of pollen grains from the anther of a stamen to the stigma of the pistil is called · Sexually transmitted diseases are contracted pollination. when a person engages in sexual acts with an infected person. STDs are caused by · The fusion of pollen and egg is called fertilization. microorganisms like bacteria and viruses. HIV infection causes AIDS. · In humans, the male reproductive system consists of the testes, seminal vesicles, penis, and · Contraception ensures population control. glands such as the prostate gland. • EXERCISES • A. Very-Short-Answer Questions 11. Name two sexually transmitted diseases caused by bacteria. 1. How do the following organisms reproduce? (a) Hydra (b) Yeast (c) Planaria (d) Rhizopus 12. Write the expanded form of AIDS. [CBSE] 2. Name the type of fission carried out by Amoeba. 13. Which virus is responsible for causing AIDS? [CBSE] [CBSE] 14. Write the full form of IUCD. [CBSE] 3. Name three methods of asexual reproduction. B. Short-Answer Questions 4. What is vegetative propagation? 5. Name a plant in which vegetative propagation 1. Mention two features of asexual reproduction. takes place by means of leaves. 2. What are the differences between binary fission and 6. Define sexual reproduction. multiple fission? 7. Give one example each of unisexual and bisexual 3. Explain the terms ‘fission’ and ‘regeneration’ as flowers. 8. Why do gametes have half the usual number of used in relation to reproduction. [CBSE] chromosomes? 4. Mention the various steps of budding in yeast. 9. Which organ enables the developing foetus to 5. What is tissue culture? obtain nourishment from the mother’s blood? 10. What is contraception? 6. Why is variation beneficial for the species, but not necessarily for the individual?

Reproduction 63 7. What is the importance of DNA copying in (a) layering (b) tissue culture reproduction? (c) cutting (d) grafting 8. What is the effect of inaccurate copying of DNA on 4. Vegetative propagation can take place by means of reproduction? [CBSE] (a) roots only (b) stem only 9. Mention two functions of seminal vesicles. (c) roots, stem and leaves (d) none of these 10. What is meant by implantation of the embryo? 5. Which of the following is formed during tissue 11. What is the vas deferens and what is its function? culture? 12. Differentiate between vasectomy and tubectomy. (a) Embryo (b) Callus (c) Cotyledon (d) Gametes 13. List any three ways in which AIDS is transmitted. 6. The fusion of a male and a female gamete results in Why is this disease considered so dreadful? [CBSE] the formation of 14. Name one sexually transmitted disease caused by a (a) egg (b) sperm bacterial infection and one by a viral infection. How (c) spore (d) zygote can these be prevented? [CBSE] 7. Which of these secretes seminal fluid? C. Long-Answer Questions (a) Seminal vesicle (b) Prostate gland 1. Explain vegetative propagation in plants. (c) Neither of these (d) Both of these 2. Describe the structures and functions of androecium 8. In which part of the female human reproductive and gynoecium. system is the ovum normally fertilized by a sperm? 3. What is pollination? Differentiate between self-pollination and cross-pollination. Name some (a) Ovary (b) Oviduct agents of pollination. (c) Uterus (d) Vagina 4. What are the changes seen in girls and boys at the time of puberty? 9. In human females the fertilized egg gets implanted 5. What happens when the ovum is not fertilized? in the 6. State the role of (a) placenta and (b) umbilical cord (a) uterus (b) vagina in the development of the foetus. (c) ovary (d) ureter 7. Give two reasons why a woman should avoid frequent pregnancies. Explain the following methods 10. The ovary releases an egg approximately every of contraception giving one example of each. [CBSE] (i) Barrier method (ii) Chemical method (a) 8 days (b) 14 days (iii) Surgical method (c) 21 days (d) 28 days 11. Which of the following is found in men? (a) Vas deferens (b) Fallopian tube (c) Ovum (d) Placenta 12. Which of these is not a sexually transmitted disease? (a) AIDS (b) Syphilis (c) Typhoid (d) Gonorrhoea D. Objective Questions II. Fill in the blanks. I. Pick the correct option. 1. Transfer of pollen from anther to stigma is 1. Which of these can undergo fragmentation or called _____. regeneration? 2. Buds are found in the notches of _____ in Bryophyllum. (a) Sponge (b) Flatworm (c) Spirogyra (d) All of these 3. Fruit is formed from _____ . 2. Which of the following does not happen in asexual 4. Ovules develop into _____. reproduction? 5. An embryo is formed by the repeated division of _____. (a) Binary fission (b) Multiple fission (c) Fertilization (d) Budding 6. The developing foetus obtains nourishment from the mother’s blood through the organ called _____. 3. The method commonly used to produce new rose plants is 7. The gestation period in humans is _____ days. Objective Questions 3. (c) • ANSWERS • 1. (d) 2. (c) 8. (b) 6. (d) 7. (d) 4. (c) 5. (b) 11. (a) 12. (c) 9. (a) 10. (d) v

7 Heredity and Evolution Heredity and Evolution VARIATIONS Any difference between individual organisms or groups of organisms of any species, caused either by genetic differences or by environmental factors, is called variation. Variations can be seen in physical appearance, metabolism, behaviour, learning ability, etc. Variations are due to differences that occasionally appear in genes during their duplication. Duplication of genes is essential for a cell to divide, and this is required for all kinds of reproduction. Hence reproduction is the cause of variations. You will not find much variation among vegetatively propagated potato tubers or Bougainvillea stem cuttings. However, you will find distinct variations among the offspring (progeny) of sexually reproducing animals including humans. Some variations are inherited and are important for evolution. Here we will learn about how variations are created and inherited. Types of Variations There are two types of variations—germinal and somatic. Germinal, or genetic, variations are caused either by differences in the numbers or structures of chromosomes or by differences in genes (units of heredity). Changes in genes are the primary sources of germinal variations. These variations are heritable. Height and eye colour are examples of germinal variations. Somatic variations may result from several factors such as climate, food supply, environment, and interactions with other organisms. These variations are not due to changes in genes or chromosomes. They are not transmittable to future generations. Hence, they are not significant in the process of evolution. Heritable Variation Heredity involves the transmission of characteristics from parents to their offspring. Among sexually reproducing organisms, the progeny are not exact duplicates of their parents. They usually vary in many traits. The reason why organisms resemble their parents lies in the precise copying of their genes, which carry hereditary characters from one generation to the next. On the other hand, no two offspring have exactly the same genes. This is because the offspring of sexually reproducing organisms receive varying combinations of genetic material from both parents. Such variations result from mutations (errors in DNA copying). Variations also result from genetic recombination during sexual reproduction. Let us see how heritable variation arises, and how it is passed on to the offspring and accumulated over a period of time to play a role in evolution. 64

Heredity and Evolution 65 Heritable variation is the result of changes in the arrangement of genes on the chromosomes and changes in the sequence of the bases that constitute DNA. Such changes occur spontaneously and randomly in the DNA, which undergoes duplication just before the cell divides. Duplication is done with great precision, but once in about ten million duplications there is an error as a different base may be put in place of the correct one. This mistake, or mutation, gets copied in subsequent cell divisions. Such spontaneous changes in the DNA are proportional to the number of duplications and cell divisions. With successive generations these variations go on accumulating in the descendants. Hence, those organisms that reproduce at a faster rate accumulate a greater number of variations. Accumulation of Variations Variations in an individual may be an Fig. 7.1 Diversity is created over successive advantage or a disadvantage for it. It may generations. The original organism gives rise to enable or disable it to cope with changes in the individuals which are similar in body design but environment. Advantageous variations are have slight variations. Each of them, in turn, gives selected by environmental factors. For rise to offspring in the next generation. The example, bacteria that tolerate low offspring too are slightly different from each other. temperatures can survive in a cold wave. Such Some differences are unique; others are inherited heritable variations lead to the evolution and from their respective parents formation of new species. An advantage of sexual reproduction is that the variations accumulated in the gametes of each sex are combined when they fuse to form the zygote. Hence an offspring produced from the zygote receives and carries the variations of both the parents. On the other hand, in asexual reproduction, there are minor differences among the offspring. These are due to small errors in DNA copying. As gametes and zygote formation are not involved, the asexually produced offspring are quite similar. They have fewer variations accumulated over generations. HEREDITY One of the most important functions of living organisms is that they breed their own kind. A frog breeds to produce a frog, and a mouse breeds to produce a mouse. Moreover, you must have noticed that children resemble their parents, and brothers and sisters resemble each other more closely than they resemble other people. This means the following: 1. The traits, or characteristics, of a species are passed down from one generation to another. 2. The closer the relationship, the greater is the similarity. This continuity of traits, which is maintained for all species and passed down to succeeding generations, is called heredity. However, along with the similarities between brothers and sisters, there are also some dissimilarities. No two individuals are exactly alike. The dissimilarities are called variations. The study of the pattern of transmission of characters from parents to their offspring is called genetics. What do parents give their children that make them resemble the parents? Both father and mother pass on genes, inherited from their own parents, to their children. Genes are stretches of DNA containing coded information for making proteins. These are found on the chromosomes of cells. Genes are the units of heredity.

66 Foundation Science: Biology for Class 10 The genetic constitution of an individual organism is called its genotype. The genotype determines the characteristic features of an individual (such as height, complexion, hair colour, etc.), which are slowly expressed during development. These characteristic features of an individual also depend on the environment. The sum of the activities of genes and the impact of the environment on them leads to the formation of visible traits, called phenotype. Observe the following features in your classmates. 1. Note their eye colour and find out how many in your class have black, brown, blue or grey eyes. 2. Also enquire about the eye colour of their parents. Find out if the brown-eyed friend’s parents are also brown-eyed. Record you observations. 3. Similarly, prepare a list of the types of ear lobes your classmates have. Record how many have free ear lobes and how many have attached ear lobes. Find out the types of ear lobes their parents have. 4. These physical features are heritable. Therefore, you will find that one or more of their parents and grandparents too have had these features. The inheritance patterns of eye colour and ear lobe follow certain rules of heredity. They depend on the genetic material contributed by the father and the mother to the child. Thus, the inheritance of each trait depends on paternal and maternal DNA. Fig. 7.2 The colour of the eye is a heritable variation found in humans. Mendel’s Laws for the Inheritance of Traits Gregor Johann Mendel (1822–84) was an Austrian monk and botanist. He is regarded as the father of genetics. He applied his knowledge of science and mathematics to his experiments on pea plants and established the principles of genetics. Though the results of his experiments were published in 1866, they remained virtually unknown until 1900. In 1857, Mendel began a series of experiments on the pea plant (Pisum sativum) to study the pattern of inheritance of various characters. He chose pea plants for three reasons. First, pea plants are self-pollinating. Second, they are easy to cultivate. Third, they have sharply defined characters. Mendel chose to study seven different characters in pea plants. Each of these characters such as height, seed shape, seed colour, etc., had two sharply defined and contrasting traits (e.g., tallness and dwarfness, round seed and wrinkled seed, yellow seed and green seed). He crossed a variety of pea plant carrying a particular trait (e.g., tallness) of a character (such as height) with another variety having a contrasting trait (e.g., dwarfness) of the same character. These two plants were considered as the parental generation (P). The generation that was produced by crossing these two was called the first filial generation (F1). When F1 plants were crossed among themselves, the generation that was produced was called the second filial generation (F2). The results of Mendel’s experiments showed the following: 1. Whenever two traits of a character were crossed, the F1 plants showed only one of the traits; the other trait never appeared. It did not matter whether the trait came from the pollen or the egg. 2. The trait that did not appear in F1 reappeared in F2, but in ¼ of the total number of plants. Mendel called the substance responsible for each trait a ‘factor’. He explained that each genetic character was represented or controlled by a pair of unit factors, or elements. (Later on,

Heredity and Evolution 67 the unit factors became known as alleles or allelomorphs. When the term ‘gene’ was coined and defined, the allele became synonymous with the gene.) One of the alleles came from one parent and the other from the other parent. The first-generation plants of Mendel’s experiment were all tall plants. But the allele representing dwarfness was neither destroyed nor altered. It could not be expressed in the first generation because it was dominated by the allele representing tallness. In other words, the allele for tallness was dominant and the allele for dwarfness was recessive. Notations used in Mendel’s experiments The dominant trait is usually represented by a capital letter. For example, tallness is represented by T and dwarfness is represented by the corresponding small letter t. If tallness is due to a pair of dominant alleles, it is written as TT. If tallness is due to only one dominant allele, it is written as Tt. If both the alleles are recessive, making the organism dwarf, then it is written as tt. A homozygous condition is one in which both the alleles are of the same nature, e.g., TT or tt. A heterozygous condition is one in which the alleles are of different nature, e.g., Tt. When two characters are taken into account, the notation for the homozygous dominant could be AABB, and for the homozygous recessive it could be aabb. Inheritance of one character When the tall plants in F1 were crossed Fig. 7.3 Inheritance of traits over two generations among themselves, the F2 generation had 75% tall plants and 25% dwarf plants. Thus, the phenotype ratio was 3 : 1 (see Figure 7.3). This led Mendel to conclude that the alleles representing dwarfness were intact and were neither lost nor altered. Mendel’s experiment with one character (monohybrid cross) led to the formulation of the law, or principle, of segregation. It states that although the alleles of a character remain together, they are separated in subsequent generations. Independent inheritance of two separate characters After studying the inheritance of the contrasting traits of one character, Mendel went on to perform an experiment with two characters (dihybrid cross). He crossed a plant having round and yellow seeds with a plant having wrinkled and green seeds. All the F1 plants had round and yellow seeds. When a certain number of F1 plants were crossed among themselves, they gave rise to four types of seeds. Of these, 315 seeds were round and yellow, 108 were round and green, 101 were wrinkled and yellow, and 32 were wrinkled and green. Hence, their phenotype ratio was about 9 : 3 : 3 : 1 (see Figure 7.4). What does this result indicate? It indicates that the chances for the pea seeds to be round or wrinkled do not depend on their chances to be yellow or green. Each pair of alleles is independent of the other pairs. This is the principle of independent assortment. Mendel’s studies provided a breakthrough in our knowledge of heredity. While formulating the principles of heredity, Mendel stated that the units of heredity (which he called ‘factors’) controlled the inheritance of characters. (This view is radically different from an earlier view held by some scientists. These scientists held that characters mixed like paints of different colours. When brought together in the zygote, they got mixed and could not be separated again. This blending of characters gave rise to intermediate characters.)

68 Foundation Science: Biology for Class 10 Fig. 7.4 Independent inheritance of two separate characters—seed colour and seed shape What Mendel could not determine was the nature of the ‘factors’. Now, scientists have not only come to know about the physical location of these hereditary units (genes) but have also discovered their molecular compositions. The unravelling of the physical basis of heredity is regarded as the most fascinating chapter in the history of modern biology. Mechanism of expression of traits A gene contains the information for making proteins in the cell. The proteins synthesized according to this information may be enzymes that catalyse biochemical reactions. Each trait is the outcome of several such biochemical reactions, each of which is controlled by a specific enzyme. If the enzyme is not produced because its gene is absent, that particular reaction will not occur and the trait resulting from its reaction will not appear phenotypically. Thus, each trait is controlled by a gene. Each parent contributes one copy of the gene for a particular character. Thus there are two genes for every character. In the gamete, however, only one copy is present because of reduction division. What Mendel perceived was that each gene (allele) is an independent unit which is neither linked with nor influenced by the other gene. Also, each allele can be separated in gametes. Determination of Sex Sex determination in humans In human beings, sex is determined by genetic inheritance. Genes inherited from the parents determine whether an offspring will be a boy or a girl. Genes for all the characters are linearly arranged on chromosomes. These include the genes for sexual characters. Generally, characters related to the reproductive system are called sexual characters and those that are not are called vegetative characters. The chromosomes that carry genes for sexual characters are called sex chromosomes, while those that carry genes for the vegetative characters are called autosomes.

Heredity and Evolution 69 A sex chromosome that carries the genes for male characters is called Y chromosome and one which carries the genes for female characters is called X chromosome. We have a total of 46 chromosomes. Half of them come from the mother and the rest, from the father. Out of these 46 chromosomes, 44 are autosomes and 2 are sex chromosomes. The sex chromosomes are not always a perfect pair. In females there are 44 autosomes and two X chromosomes. In males there are 44 autosomes, one X chromosome and one Y Fig. 7.5 X–Y system of sex determination in man chromosome. So the chromosomes in woman are 44 + XX, while the chromosomes in man are 44 + XY. Let us see the inheritance pattern of X and Y chromosomes. During gamete formation, the normal diploid chromosome number is halved. This is called the haploid condition. All the eggs of a female have 22 + X chromosomes. A male produces two types of sperms—one type bears the 22 + X composition and the other, 22 + Y. Therefore, in every 100 sperms, 50 have Y chromosomes and 50 have X chromosomes. Any one of the two types of sperms can fertilize the egg. If a Y-bearing sperm fertilizes the egg, the zygote has the 44 + XY composition, and the resulting embryo grows to be a boy. When an X-bearing sperm fertilizes the egg, the resulting zygote has the 44 + XX composition. This embryo develops into a girl. All the children inherit one X chromosome from the mother. Therefore, sex is always determined by the other sex chromosome that they inherit from the father. One who inherits the X chromosome of the father is a girl, while one who inherits the Y chromosome of the father is a boy. Role of environment in sex determination Environmental conditions such as temperature around the developing embryo may determine sex in some animals. Such conditions may override the genetic basis. Some animals such as snails can even change their sex, showing that their sex is not genetically determined. Incubation of the eggs of the turtle Chrysema picta at a high temperature produces females. But the incubation of the eggs of the lizard Agama agama at a high temperature produces males. EVOLUTION Evolution refers to the process by which early organisms of the earth diversified into various new forms through slow but continuous variations. Ever since the appearance of the first living beings on the earth some 3.5 billion years ago, new forms have continuously originated. And, the different forms have undergone modifications and given rise to new forms. The newer forms are sufficiently different to be recognized as new species. They breed amongst their own members and not with the ancestral forms or any other forms. The newly formed species may give rise to still newer species over a period of time . This process is called descent with modification. This is the main theme of evolution. Evolution occurs due to the survival of advantageous variations produced in reproduction. Sources of Genetic Variation You know that there are two main types of variations—somatic and germinal (genetic). Genetic variation arises due to mutation and it can account for the creation of a new species. Mutation is any change in the structure of a gene. Mutation may lead to a change in the expression of a gene. Such a change may even produce harmful effects in the organism.

70 Foundation Science: Biology for Class 10 Another source of variation is genetic recombination. It is a natural process due to which the arrangement of genes in the progeny is in a combination that differs from that of the parents. This is because the offspring receive genes from both the parents, and this ensures the transmission of some genetic variability from the parents to the offspring. Mutation and genetic recombination may give rise to new characters due to change in genes. These new characters may help the individuals to adapt to their environment. Sometimes the new characters may not help individuals to adapt. Disease, competition, etc., can eliminate those less well-adapted individuals. The survivors pass on their advantageous characters to their offspring. This enables the offspring to adapt well to their environment. Thus nature selects new characters by favouring some of them and eliminating others. In this way natural selection may lead to the evolution of a new species with new characters. Let us see how variations in a population lead to evolution. Frequency of Genes in a Population The proportion of a particular allele in a population is called gene frequency. How does gene frequency in a population change? Let us consider the genes of a particular species. All the genes in a population of a species at a given time form its gene pool. The frequency of certain genes in the population of an area can change due to certain environmental factors. Fig. 7.6 Variations in a population Let us take an example and observe the results in different situations (see Figure 7.6). Suppose there is a population of red beetles living in some bushes in a particular area of a forest. Let us assume that they can generate heritable variations during their sexual reproduction. In the first situation, a heritable colour variation occurs so that a green offspring is born to its red-coloured parents. The green beetle then passes its green trait on to its offspring. These green beetles living in the green leaves of the bushes escape the notice of crows, while the red beetles, because of their bright colour, are easily spotted and eaten by the crows. As a result, the red beetles are soon eaten up by the crows, while the green beetles survive, reproduce and increase in number. In the second situation, a blue-colour variation arises during the reproduction in red beetles. The blue beetle also gives birth to more and more blue beetles. This change in colour, however, gives no survival advantage over the red variety since the crows easily find and eat both blue and red beetles. Now, suppose a bush fire occurs suddenly and kills a large number of beetles, and all the surviving beetles are, by chance, blue. The progeny of the blue beetles are also blue. The survival of these blue beetles is, however, not a case of natural selection, unlike the survival of the

Heredity and Evolution 71 green beetles in the first situation. This is a case of genetic drift, that is, a random change in the gene frequency. In the third situation, many of the bushes dry up due to a prolonged dry period. The bushes that survive the drought have smaller leaves. This leads to a food shortage for the red beetles. Incidentally, some beetles in the population are smaller in size on account of a heritable variation. They manage to survive, as they require less food, while most of the large beetles die of starvation. Some young beetles of the large variety survive, but these cannot grow to their full Fig. 7.7 A population of beetles size due to undernourishment. Hence, there are only small beetles. When the drought ends and there is enough food for all the beetles, large beetles reappear, and there are both large and small beetles in the population. The genetically small beetles remain small even when they have more food, but the undernourished beetles, which did not undergo any genetic change, grow to their normal size. In the above situations, we can examine where genetic drift has occurred. In the first situation, natural selection played a part in preserving a certain heritable variation. In the second situation, an accident caused genetic drift. In the third situation, an environmental factor led to the production of smaller beetles although some were also produced due to heritable variation. Acquired and Inherited Traits From very early times scientists have been trying to explain the origin, evolution and diversity of life forms. In the nineteenth century, however, the idea that complex animals and plants developed by gradual change from simpler forms began to be taken seriously. The mechanism of the origin of new species from the existing species was explained first by Jean Baptiste Pierre Antoine de Monet Lamarck (1744–1829), a French biologist, and then by Charles Robert Darwin (1809–82), a British naturalist. Acquired traits Having accepted the fact that new species have arisen from pre-existing species with modifications, a number of scientists have tried to explain the mechanism by which this might have occurred. The first scientific theory concerning this came from Lamarck. His ideas, written in his book Philosophie Zoologique (meaning ‘philosophical zoology’), published in 1809, are known as Lamarckism. Lamarck observed the changes and adaptations in certain organs in animals. He suggested that favourable changes appear due to the use or disuse of organs over a long period of time. For example, some organs develop in size if they are in continuous use, while their disuse has an opposite effect. He concluded that such characters acquired by an organism during its lifetime are transmitted to the next generation. This inheritance of acquired characters results in the evolution of one or more new species. However, most scientists disagree with this, as it has not been supported by experiments. For example, the offspring of a couple of mice whose tails have been cut off are not born tailless. This was demonstrated by an experiment performed by August Friedrich Leopold Weismann (1834–1914), a German biologist. In sexually reproducing organisms, germ cells are produced in the reproductive organs, while the rest of the body has somatic cells. Changes in somatic cells due to environmental factors are not transmitted to the offspring. This is because a change in a somatic organ caused by a

72 Foundation Science: Biology for Class 10 physiological response by the body does not bring about a corresponding change in reproductive organs. For example, in the earlier illustration, if beetles starve, their size will get reduced. But if they reproduce, their progeny may not have reduced body size if they get enough food. This means that starvation of the parent beetles does not alter the DNA sequence of their germ cells so as to bring about a variation in the next generation. Even if the reproductive cells suffer from starvation, this does not lead to any change in the DNA. The son of a wrestler is, therefore, not born muscular. Similarly, cutting off the tails of mice do not change the genes of their germ cells. Inherited traits Darwinism is the first modern theory that attempted to explain the origin of new species. Charles Darwin made an extensive study of the flora and fauna of the Galápagos Islands in South America. He came to certain conclusions, which he explained in his book The Origin of Species, published in 1859. Darwin proposed that new species arise by the slow Fig. 7.8 Charles Robert Darwin accumulation of advantageous variations over a period of time. Though he did not say how these variations arise, he said that variations are so common in nature that no two individuals are alike. Darwin’s second observation was that although the power of reproduction of organisms is enormous, the population size of any species always remains within a limit. He explained it by saying that overpopulation results in a competition for food and shelter, ultimately leading to a struggle for existence among the members of a species. In such a struggle, those that survive must have some favourable qualities that enable them to overcome the difficult situation. These qualities are advantageous variations. The surviving organisms repeat the process of reproduction. Biologically, a species that can reproduce and leave a large number of offspring is considered successful. When the new generation with advantageous characters begins to reproduce, the situation of overproduction and inevitable struggle is repeated. The survivors will have more advantageous characters that help them to compete and survive. All these new features might make them considerably different from the original forms. These differences, or variations, when accumulated over a long period of time lead to the origin of new species. Thus, selection of advantageous variations by nature leads to the origin of new species Charles Darwin explained the mechanism of origin of new species by natural selection. But his theory fell short of explaining the mechanism or the source of heritable variations. This was explained by Hugo de Vries (1848–1935), a Dutch botanist. According to him, heritable variations arise when there is a change in the genes of the germplasm (protoplasm of a germ cell). He called it mutation. The manner in which heritable variations are passed on to the succeeding generations was explained by Gregor Mendel after he performed his pea-plant experiments. If a particular trait spreads in the population, it means that it is favoured by natural selection. On the other hand, an acquired trait is not transmitted to the offspring. Those animals that do not show enough variations are likely to be wiped out, as they cannot cope with changing circumstances. Genetic variability gives an ability to adapt and adds to the chances of survival of a species. A small population of any species would have fewer mutations, resulting in lesser variability and diminished ability to adapt. For example, the small number of tigers surviving in the world do not have enough variations to adapt well to changes in the environment and hence may become extinct.

Heredity and Evolution 73 Origin of Life on the Earth Darwin explained the evolution of life from simple to complex forms. Mendel tried to explain the inheritance of traits in living beings. But we are still trying to explain the origin of life on the earth. We can speculate whether life began as a cell or from an inorganic molecule. Probably some of the chemical reactions involved in metabolism began when no form of life existed. Small carbon-containing molecules such as acetic acid and citric acid may have been abundant on the young earth. They took part in chemical reactions that produced larger molecules like amino acids, lipids, sugars and RNA. Gradually, these molecules may have interacted and produced the biomolecules essential for life. The above account of the origin of life is based on the general concepts outlined by Aleksandr Ivanovich Oparin (1924), John Burdon Sanderson Haldane (1928), Stanley Lloyd Miller and Harold Clayton Urey (1953), and Sidney Walter Fox (1965). They believed that life arose on the early earth some 3.5 billion years ago by a series of chemical reactions in the seas. The conditions on the early earth were different from those of the present. Elements such as carbon and nitrogen were not present as their oxides (e.g., CO2 and NO2 ), as they are today, but were present as CH4 (methane) and NH3 (ammonia). Oxygen was not available as there were no photosynthesizing organisms. Hence the atmosphere was a reducing (electron-adding) one and the synthesis of organic molecules could occur easily. In their experiments, Miller and Urey simulated the prebiotic conditions (that is, when there was no life on the earth) and produced some amino acids (units of proteins) and other organic compounds. Their experiments suggested how these organic molecules were produced on the early earth. In these experiments, a mixture of methane, ammonia, water vapour and hydrogen was circulated through water at a temperature just below 100°C, and sparks were passed through the gaseous mixture to simulate the lightning flashes on the early earth. After several days of experimentation, the colour of the solution changed. The analysis of the liquid showed the presence of simple carbon compounds like several types of amino acids that make up protein molecules essential for life. However, when the experiment was carried out in oxidizing conditions, no amino acids were formed. This suggested that reducing conditions were essential for any prebiotic synthesis of organic molecules. Therefore, we can assume that life cannot arise again on the earth because the reducing atmosphere that was conducive to the formation of biomolecules is no longer present. The life forms present today are diverse. They have been changing and evolving since the time they originated. Let us learn how new species originated from existing ones. SPECIATION Individuals of a species are similar and they can breed among themselves. At the same time, there are some small, but significant, differences (variations) between the individuals of a species. Heritable variations are transmitted to the offspring. These variations are important as they produce changes in the characters of that particular species. This leads to microevolution, or evolution on a small scale with the emergence of new varieties or new subspecies. To understand how such small variations lead to the formation of a new species, let us take the beetle’s example again. Suppose there are beetles spread over a large area. If the population of beetles gets divided into two subpopulations by a barrier (say, a river or a mountain) then it will be difficult for the members of one subpopulation to go to the other side for mating. Therefore, exchange of genetic material, or gene flow, between them will decrease. They will be restricted to mate within their own subpopulations. In other words, they will be forced to inbreed, or mate with closely related individuals in their own isolated subpopulations. In this process, the recessive mutant genes of each parent have a much greater chance of coming together. The genes will now be expressed giving benefit or harm to the offspring. These new characters, or variations, may be selected by nature and may lead to the formation of a new species. The new generations differ so much from the original population that they can no longer

74 Foundation Science: Biology for Class 10 interbreed to produce fertile offspring. This leads to speciation, that is, the formation of one or more species from an existing species. After a few years, if a male beetle from one isolated area and a female from another area are brought together, they may or may not mate with each other. If they mate but are unable to reproduce, then they have become two different species. If they are able to reproduce, then they are still the same species. Over many generations, different variations are accumulated in each subpopulation. Suppose, for example, in one area with a beetle subpopulation, crows are scarce due to the presence of eagles. And in another area, crows are present in large numbers. Natural selection will not select the green variety of beetles in the first area as there are no crows to eat the beetles. But the green variety will be selected in the second area as the crows will eat the other beetles there. Thus, natural selection may operate differently on the same variations in subpopulations of different areas. Nature will select those variations that help to adapt better in a particular environment. Over a period of time, the processes of genetic drift and natural selection will cause the two isolated subpopulations to become more and more different from each other. Microevolution is generally a consequence of gene mutation. But larger changes in the genetic make-up, like change in the number of chromosomes, may not allow the germ cells of two subpopulations to fuse together. This prevents interbreeding and causes the emergence of new species. Speciation due to inbreeding, genetic drift and natural selection will be applicable to all sexually reproducing organisms. Geographical isolation does not play any role in the speciation of asexually reproducing organisms. It also does not play any major role in the speciation of self-pollinating plants. EVOLUTION AND CLASSIFICATION The classification of organisms into a hierarchy of groups, namely, kingdoms, phyla (or divisions), classes, orders, families, genera and species, is based on their similarities and differences. For example, the genus Rana of frogs has several species like clamitans, catesbeiana, tigrina, etc. Although they belong to the same genus, they show subtle variations. Similarities suggest that some similar characters are present in the groups. These characters must have come from common genes, which in turn have come from a common ancestor. To interpret similarities and differences between groups on the basis of their evolution we go down the hierarchy of classification. Fig. 7.9 The genus Rana of frogs has species like (a) clamitans and (b) catesbeiana.

Heredity and Evolution 75 Classification shows how closely organisms are related with respect to evolution. It is based on the assumption that each organism has descended from its ancestral type with some modification. There is a hierarchy of characteristics that helped taxonomists to form classification groups. For example, the first level of classification depends on whether the cell has a nucleus or not. Except the members of the kingdom Monera (e.g., bacteria) all other organisms have a true, or well-organized, nucleus. The next level of classification depends on whether the nucleated organisms are unicellular or multicellular. The basis for classifying the multicellular organisms is whether they are capable of photosynthesis or not. The basis for classifying nonphotosynthetic organisms is the presence or absence of the vertebral column in the body. In this way, organisms can be arranged in groups based on their physiological, biochemical, anatomical or evolutionary relationships. Tracing Evolutionary Relationships In order to find out the evolutionary relationships among organisms, we have to look for their common features. Different organisms would have common features if they are inherited from a common ancestor. Comparative anatomy The study of body parts of animals of a particular group shows how apparently dissimilar animals have quite similar anatomical structures. For example, the forelimbs of man, cat, whale and bat are made up of the same skeletal elements. They have been modified to suit the environmental conditions in which these animals live. These organs are functionally dissimilar but structurally similar. Such organs are called homologous organs. The anatomical similarity points to the existence of a common ancestor from which these organisms have evolved. However, though the wings of a bat and the wings of a bird look similar, they are anatomically dissimilar. Such organs are called analogous organs. Fig. 7.10 (a) Homologous organs—forelimbs of some mammals (b) Analogous organs—wings

76 Foundation Science: Biology for Class 10 Comparative embryology A comparative study of the stages of the embryonic development of animals reveals that in their early stages they were very similar. These embryonic stages reflect their ancestry. The embryological stages of an organism give us an idea about the stages of its evolution. For example, when we study the human embryo, we find that at a certain stage it has gills. This suggests that fish is one of the earliest ancestors in the evolution of mammals including human beings. Fossils Fossils are the remains or traces of organisms that lived in the past. Usually the hard parts of organisms (e.g., bones, shells and teeth) turn to stonelike fossils. Sometimes fossils also include remains like skeletons, the preserved impressions of tracks left by organisms on rocks, and so on. Fossil records are a proof of the changes in and the relationships between various groups of organisms. Palaeontology is the study of fossils. A comparison of fossils and present-day organisms gives the evidence of evolution. It shows how one species gives rise to another species with certain modifications. Palaeontologists have found the fossils of dinosaurs that lived in the Jurassic period (about 200 million years ago). These have some features similar to the reptiles present today. The fossil records of the animal Archaeopteryx that lived about 150 million years ago show that it had some features like birds (e.g., feathers) and some features like reptiles (e.g., claws on wings, and a bony tail). This supports the hypothesis that birds evolved from reptiles. Fig. 7.11 Fossils of some ancient animals How are fossils studied? Fossils are found in sedimentary rock. This type of rock is formed by the slow deposition and hardening of sand, stones, clay, etc., over a period of time. The lower layers of the rock are older, while the upper layers are recent. We usually use two ways for dating fossils. One way is to assume that animals that lived and died in recent years would be found buried in the top layers of soil. For example, fossils of horselike animals would be exposed on digging these layers. As we dig deeper, we will find older fossils. We may, for example, find fossils of dinosaurs. Fossils found in the bottom layers are the oldest. These may be fossils of invertebrates that lived in very ancient times. The second way of detecting the age of a fossil is by finding out the age of the

Heredity and Evolution 77 various layers of rock. This is determined by Fig. 7.12 Fossils form layer by layer. radiocarbon dating. We can calculate the age of a Fig. 7.13 Planaria has simple eyes, or eyespots, fossil by finding the ratios of different isotopes of the same element in the fossil material. that just detect light. Evolution Is a Gradual Process Fig. 7.14 Incipient feathers in Archaeopteryx Evolution is a gradual process—no organism evolved suddenly. Complex organs evolved in organisms gradually over generations. But what would have been the advantage of the incipient (primitive) organs evolved in the early stages of evolution? How was each intermediate stage selected during its evolution? For example, how did our eye evolve to its present complex form? And, why were its incipient forms (e.g., eyespots in Planaria) selected by nature? Again, wings in birds are an important adaptation for flight. But the featherlike structures initially formed in primitive birds would have had no advantage in flying. Then why did nature select the early feathers? The answer to these questions is that these features would have had some marginal advantages like providing insulation against cold and hence they were selected by nature. Artificial selection also plays an important role in creating diversity, as the following example shows. Humans have cultivated the wild cabbage plant for thousands of years. By carefully examining variations in these cabbage plants, crossing them and artificially selecting their traits, they got a variety of plants like cauliflower, broccoli, kohlrabi and kale. Thus, through artificial selection, all these different-looking plants were developed from the same ancestor. In nature too, structures that look very dissimilar have evolved from a common ancestral design. A study of comparative anatomy and fossils suggests that although the wings of a bat, the flippers of a whale, and the arms of a man look different, they are anatomically quite similar. Comparing the DNA of different species can directly show us how much the DNA has changed during the evolution of these species. Evolution Is Not Necessarily Progressive We can compare the evolution of different species by studying their evolutionary family tree. In the evolutionary family tree of species, many branches (species) can arise at any stage of evolution. A species may not be eliminated for a new species to emerge from it. Are the new species better than the older ones? Evolution does not always mean that the newly evolved forms are better than those that existed in the past. Evolution simply means the generation of diversity and selection by nature.

78 Foundation Science: Biology for Class 10 Natural selection and genetic drift lead to the formation of a population that cannot breed with the original one any more, as in case of the evolution of humans and chimpanzees from a common ancestor. Scientists have suggested that chimpanzees are our nearest evolutionary cousins. About 99 per cent of our 25,000-odd genes are identical to those of chimpanzees. Scientists have discovered our genetic differences with chimpanzees recently. A gene-by-gene comparison shows that these genetic differences are relatively subtle. But even a difference of one per cent would mean 250 possible mutations. A single change in the DNA bases can produce a dramatic change, so 250 mutations will bring about many variations. Humans are very different from chimpanzees and have not evolved from chimpanzees. Rather, humans and chimpanzees evolved from a common ancestor, and each had a separate course of growth and adaptation. In evolution, the new forms evolved are more complex than their ancestors. It is the adaptability of a species to the environment that supports its survival, not the complexity of the species. Had it not been th e case, simpler forms would not have been living today. For example, bacteria are very simple in form, yet they are surviving today. They are found in hot springs, in deep-sea thermal vents, and in ice sheets. This is because of their adaptability to the changing environment. Each species, whether complex or simple, is subject to natural selection. Each species has to go through the process of natural selection to survive and reproduce. In evolutionary terms, we cannot say that a particular species has a better design than another. Each species is well suited and adapted to its environment and hence is good enough to live and reproduce. Molecular phylogeny The ancestors of different organisms, including humans, can be traced by studying the changes in their DNA. A change in the DNA means a change in its protein sequence. The ancestry, or phylogeny, determined by a comparative study of DNA sequences is called molecular phylogeny. How do we trace a human being’s ancestry by studying DNA sequences? Mutation means any change in the bases of DNA. It occurs when there is any duplication of bases, inversion of bases, deletion of bases, addition of bases, and/or substitution of one base with another. The study of these mutations and the pattern of their inheritance allow us to find out how closely one species is related to another and how and when one form diverged from another. The number of mutations in a particular gene over a period of time enables us to calculate how far back in time one species diverged from another. This is because mutations would accumulate over generations. These mutations can be traced backwards in time to find out at which stage each species diverged from another. The more distantly related the organisms are, the greater is the difference in their DNA. In order to trace the ancestry from the fossil of an organism, its DNA is first obtained. Even one drop of frozen (or preserved) blood is enough to get DNA for the purpose. The sequence of this DNA is determined. A large number of fossils are collected from different geological periods and from different regions. The specific DNA sequence of each is determined and compared with the others. The comparative study enables us to find out what the original sequence was, when it underwent a change, in which form the change was passed down, and so on. In this way we can go back in time and determine the ancestor of an organism. Studies in molecular phylogeny help in the classification of organisms. Human migration A mutation in a person’s DNA is passed down to all the descendants of that person. This means that if the same mutation is present in two individuals, both of them share the same ancestor. These mutations can be hidden when the genes are all mixed up during the formation of gametes and zygote. However, the DNA in mitochondria is passed down intact from mother to child. Moreover, the Y chromosome, which determines maleness, is also passed down intact from father to son. Therefore, by comparing the mutations in mitochondrial DNA and Y chomosomes of individuals all over the world, we can trace when and where those people parted and migrated around the world. A study of the changes in the DNA of human beings from all over the world and a study of human fossils scattered all over the globe have suggested that the first human beings originated

Heredity and Evolution 79 in Africa. The earliest fossils of modern humans were found in Ethiopia in Africa. These humans were hunters. They lived about 200,000 years ago. Some of them left Africa and migrated to West Asia. From there they migrated to Central Asia, South Asia and East Asia. One group of their descendants migrated down to the islands of Indonesia and to the Philippines and Australia, while another group travelled to Europe. Another group migrated to northern Asia, while yet another group went to North America and onward to South America. These early humans migrated in all directions and mixed with one another. Scientists have come to this conclusion by studying human DNA sequences and Y chromosomes, and by analysing the mutations that occurred in humans over a period of time. Today, you can give your blood to a laboratory and get your ancestry traced. You can also find out in which part of the world your ancestors lived. All this is possible by studying the chromosome and mitochondrial DNA. For example, the earliest known mutation spread outside Africa with the migration of humans some 50,000 years ago. If the Y chromosome of an American contains this particular mutation along with various other mutations, it proves his African ancestry. Thus genetic mutations act as markers, tracing ancestry through time. All human beings of the world, whether they are African or American, share the same gene pool and hence all modern humans belong to the same species—Homo sapiens. There are, however, a large number of genes in the gene pool that serve as the source of individual variations. It is for this reason that no two individuals are identical in looks, abilities, behaviour, etc. Therefore, there is great diversity in human features such as skin colour, height, hair colour, and so on. But there is no biological basis for assuming that humans with different features belong to different races. • POINTS TO REMEMBER • · The study of the pattern of inheritance of · Life originated about 3.5 billion years ago by a series characters from parents to offspring is called of chemical reactions in the seas leading to the genetics. The hereditary units are called genes. information of amino acids and other biomolecules. · The genetic constitution of an individual organism · Evolution refers to the process by which early is called its genotype, while the physical appearance organisms of the earth diversified into various is referred to as the phenotype of the individual. forms through a slow but continuous process. · Differences among individual organisms or · Darwin’s theory of natural selection proposed that groups of organisms, caused either by genetic new species arise by the slow accumulation of differences or by environmental factors, are advantageous variations over a period of time. called variations. Darwin explained that in a struggle for existence, nature selects those organisms that have · Changes in somatic, or nonreproductive, cells are advantageous variations and eliminates those not heritable. that do not have these. · Mendel’s experiments on pea plants established · The sources of variations are mutation and the principles of genetics. He called the substance genetic recombination. responsible for each trait a ‘factor’. He postulated the following principles: · Sex is determined by different factors in different species. In humans, sex is determined by the · Life originated about 3.5 billion years ago by a inheritance of the paternal sex chromosome. series of chemical reactions in the seas leading to Inheritance of a paternal X chromosome produces the formatio a female, while inheritance of a Y chromosome produces a male. (a) In a cross between two traits of a character, only the dominant trait appears in the F1 · Variations may or may not bring survival generation. The recessive trait reappers in the advantages. F2 generation. · Speciation takes place when geographically (b) Each pair of alleles gets separated during isolated populations have variations. Natural gamete formation. This is the law of segregation. selection and genetic drift take part in speciation. (c) In a cross involving two or more pairs of · Evolutionary relationships are taken into account contrasting traits each pair of alleles is in the classification of organisms. separated independently of the other pair. This is the law of independent assortment.

80 Foundation Science: Biology for Class 10 · Evolutionary relationships can be traced by · Different organs adapt to new functions in the studying homologous and analogous characteristics course of their evolution. Feathers may of different species. have evolved for giving warmth and later been adapted for flight. · Fossils help in the study of extinct species. · Humans originated in Africa and migrated to · Evolution of complex organs like the eyes takes different parts of the world. Human ancestry can place due to the survival advantages of the be traced by tracing the changes in DNA intermediate stages of such organs. backwards in time. • EXERCISES • A. Very-Short-Answer Questions 5. Briefly state Mendel’s findings with respect to (a) dominant and recessive characters, (b) the law 1. Define heredity. of segregation, and (c) the law of independent assortment. 2. What is a gene? 3. Where are genes located? 6. Explain the usefulness of comparative anatomy with reference to (a) homologous organs and 4. How are germinal variations caused? (b) analogous organs. 5. Who was Gregor Johann Mendel? 6. What is a dominant trait? 7. Who proposed the theory of natural selection? 7. What is the difference between dominant and Explain this theory briefly. [CBSE] recessive traits? 8. What is the difference between acquired and 8. Which scientist gave the first modern theory of the inherited traits? origin of new species from existing ones? 9. Does geographical isolation play a major role in the 9. Mention one similarity between a human embryo speciation of a sexually reproducing organism? and fish. Explain. 10. Mention two sources of variations. 10. Are acquired traits of an individual inherited? Explain. 11. Define phenotype. 11. Why is the small number of surviving tigers a cause of worry? 12. Who first proposed the theory of inheritance of 12. Describe the different ways in which individuals acquired traits? [CBSE] with a particular trait may increase in a population. 13. Is geographical isolation an important factor in the 13. How is the equal genetic contribution of father and speciation of a self-pollinating plant species? mother to their offspring ensured? B. Short-Answer Questions 14. Explain how advantageous variations help an organism survive? 1. Distinguish between heredity and variations. 2. What is the difference between genotype and 15. Explain how sexual reproduction gives rise to more phenotype? variations than asexual reproduction does. How does it affect the evolution of sexually reproducing 3. What are the similarities between Mendel’s ‘factors’ organisms? and the genes as we know today. 4. Define homologous organs. 16. Explain whether eye colour is genetically inherited. 5. How do embryological studies provide evidences D. Objective Questions for evolution? I. Pick the correct option. 6. What factors lead to the origin of new species? 1. The genetic constitution of an individual organism 7. What are fossils? How do they help us learn about is called the process of evolution? [CBSE] (a) its genotype (b) its phenotype C. Long-Answer Questions (c) heredity (d) gene 1. Explain whether a bacterium or a chimpanzee has a 2. The hereditary units are stretches of DNA called better body design. (a) chromosomes (b) traits 2. Trace human migration from the time it began in Africa. (c) characters (d) genes 3. Explain the role of fossils in tracing evolutionary 3. Which of the following plants did Mendel choose relationships. for his experiments? (a) Pisum sativum 4. What are variations? Distinguish between germinal (b) Hibiscus rosa-sinensis and somatic variations. (c) Mirabilis jalapa (d) None of these

Heredity and Evolution 81 4. The science dealing with heredity and variations 9. The forelimbs of man, cat, bat and whale are is called (a) analogous organs (b) homologous organs (a) palaeontology (b) genetics (c) missing links (d) fossils (c) embryology (d) phylogeny 10. Which of these is not a part of Darwinism? (a) Struggle for existence 5. Mendel’s contribution to genetics was the (b) Overpopulation (a) principle of mutation (c) Natural selection (b) theory of natural selection (d) Use and disuse of organs (c) law of independent assortment of factors (d) principle of genetic recombination 6. Which of the following is not one of Mendel’s laws? II. Fill in the blanks. (a) Law of dominance (b) Law of segregation 1. The study of the pattern of inheritance of (c) Law of independent assortment chromosomes from parents to offspring is (d) Law of incomplete dominance called _____. 7. What happened when Mendel crossed two traits of a 2. The number of X chromosomes in a human ovum character (say, tallness and dwarfness) in pea plants? is _____. (a) Both the traits appeared in equal numbers in F1 . 3. In Mendel’s experiment, the trait which did not (b) The offspring showed a blend of the two traits. appear in the F1 generation was said to be _____. (c) Only the dominant trait appeared in F1 . 4. The number of autosomes in the human zygote (d) Only the recessive trait appeared in F1 . is _____. 8. The Japanese are genetically the closest to 5. Darwin made an extensive study of the flora and fauna of the _____ Islands in South America. (a) Indian schoolboys (b) chimpanzees 6. The Origin of Species was written by _____ . (c) gorillas (d) monkeys • ANSWERS • Objective Questions 3. (a) 4. (b) 5. (c) 1. (a) 2. (d) 8. (a) 9. (b) 10. (d) 6. (d) 7. (c) v

8 Our Environment The OenuvriEronnvmiroenntmoefnatn organism means the physical and biological conditions in which it lives. The physical conditions include soil, light, temperature, etc. And the biological conditions include the other plants, animals and microorganisms around it. A change in any of these conditions can affect the organism. To understand how, we need to look at the different ways in which an organism interacts with others and with its surroundings. In this chapter we will look at the interactions between the organisms of an ecosystem. ECOSYSTEM An organism cannot live in isolation. It needs other organisms, nutrients from its environment, and so on, to survive. So, nature has provided functional units in which different organisms of a given area can live and interact among themselves and with their surroundings. An ecosystem is a functional unit consisting of all the living beings of an area and the nonliving components of their environment, interacting to form a stable system. There are different kinds of ecosystems. They can be natural ecosystems such as deserts, grasslands, forests and lakes, or man-made ecosystems such as gardens, aquariums and crop fields. An ecosystem may be as small as an aquarium or as big as an ocean. A pond is an example of an aquatic ecosystem. All the algae, plants, insects, microorganisms and fish in the pond, and the water and soil of the pond are part of this ecosystem. The organisms of the pond get everything they need from the pond itself. And they help to keep its water and soil in good condition, replenishing the nutrients they take from them. This makes the ecosystem self-sustaining. Now let us look at the ecosystem of a garden. In a garden you will find different plants and animals such as bees, butterflies, earthworms, frogs and birds. They depend on each other and on the nonliving things like the soil, air and water. For example, the earthworms gets nutrition from the soil. In turn, they keep the soil fertile. So do certain kinds of bacteria living in the soil. Birds, bees and butterflies get food from the plants in the garden. They help to keep the ecosystem working by helping in the pollination of the plants. Stability in Ecosystems All ecosystems are stable systems. This means that they maintain a natural balance. An ecosystem involves the flows of nutrients and energy (in the form of food). If the organisms living in an ecosystem use up nutrients, like nitrogen, from their environment, without replenishing them, soon the system will collapse. However, a balance is maintained between the availability and use of nutrients by recycling them through natural processes. You already know how things like nitrogen and carbon are recycled in nature. A balance is also required to provide different amounts of energy (from food) needed by different organisms. As we shall see, the numbers of different organisms in an ecosystem are balanced in such a way that each organism gets the required amount of food. For example, in a forest ecosystem, the numbers of the prey (like rabbits) are always more than the numbers of the predator (like foxes), to ensure adequate food for the predator. 82

Our Environment 83 Structure of an Ecosystem An ecosystem consists of two components—the abiotic component (nonliving component) and the biotic component (living component). Fig. 8.1 Components of an ecosystem Abiotic component The abiotic, or nonliving, component consists of the physical environment, nutrients and climatic factors. The physical environment consists of soil, water and air. Inorganic substances such as carbon dioxide, oxygen, nitrogen, water, phosphorus, sulphur, sodium, potassium and calcium constitute nutrients. Things like sunlight, rainfall, temperature, humidity and atmospheric pressure constitute the climatic factors. Biotic component The biotic, or living, component of an ecosystem can be classified on the basis of how the organisms get their food, i.e., whether they are producers, consumers or decomposers. Producers Organisms which make their own food are called producers. They are also called autotrophs. (In Greek, autos = self, trophe = nutrition.) All green plants and certain blue-green algae act as food producers in ecosystems. Consumers Organisms that depend on other organisms for food are called consumers or heterotrophs. (In Greek, heteros = other.) All animals which eat plants or other animals are consumers. Bacteria and fungi that depend on dead plants and animals for food are also in a way consumers. Consumers can be classified as herbivores, carnivores and omnivores. Herbivores eat only plants and plant products. Cows, deer and rabbits are herbivores. Carnivores eat only the flesh of other animals. Tigers, snakes and hawks are carnivores. Omnivores eat plants as well as the flesh of other animals. Man and crow are examples of omnivores. Sometimes it is useful to classify the consumers in an ecosystem on the basis of ‘who eats whom’. Primary consumers are those who feed directly on the producers (plants). In other words, herbivores are primary consumers. Carnivores who feed on plant-eating animals (herbivores) are secondary consumers. For example, a grasshopper that feeds on plants is a primary consumer, and the frog that eats the grasshopper is a secondary consumer. The frog could be eaten by a larger carnivore like a snake. A carnivore that feeds on smaller carnivores is called a tertiary consumer. This consumer may be eaten by the largest carnivore, or the top carnivore, of the ecosystem. The top carnivore is not killed and eaten by other animals of the ecosystem. The top carnivore belongs to a higher order of consumers. For example, a hawk could be the top carnivore of an ecosystem. Other examples of top carnivores are tigers and lions. (Primary, secondary and tertiary consumers are also called consumers of the first, second and third order respectively.) Decomposers Organisms which feed on dead plants and animals are called decomposers. Decomposers are also called saprotrophs or saprophytes (in Greek, sapros = rotten). They include bacteria, fungi and worms. Decomposers break down (decompose) the compounds present in dead plants and animals into simpler substances and obtain nutrition from them. The substances formed in decomposition are released into the soil and the atmosphere. Thus, decomposers play

84 Foundation Science: Biology for Class 10 an important role in the recycling of materials, replenishment of the soil’s nutrients, etc. They also clean up our surroundings by decomposing dead organisms and wastes from animals and plants. Take a large glass bowl or jar and put some soil and aquatic plants in it. Fill three-fourths of the bowl with water and place it near a window through which sunlight comes in. Put some fish in the bowl. You will need to put some fish food in the bowl from time to time. The oxygen needed by the fish will be liberated by the aquatic plants through photosynthesis. After a few days the water in your aquarium will become dirty. This is because of the waste generated by the fish and the plants. We do not need to clean natural aquatic ecosystems like ponds and lakes. In these, wastes are consumed by decomposers. Food Chain For an ecosystem to work, there has to be a flow of energy within it. The organisms of the ecosystem need energy in the form of food. The ultimate source of this energy is the sun. Producers like green plants trap solar energy and convert it into the chemical energy of food. When a primary consumer eats the producer, a part of this energy is passed on to it. The primary consumer is then eaten by a secondary consumer. And the secondary consumer may be eaten by a tertiary consumer, and so on. In this way energy gets transferred from one consumer to the next higher level of consumer. A series of organisms through which food energy flows in an ecosystem is called a food chain. It may also be defined as follows. A food chain in an ecosystem is a series of organisms in which each organism feeds on the one below it in the series. In a forest ecosystem, grass is eaten by a deer, which in turn is eaten by a tiger. The grass, deer and tiger form a food chain (Figure 8.2). In this food chain, energy flows from the grass (producer) to the deer (primary consumer) to the tiger (secondary consumer). Fig. 8.2 A food chain in a forest ecosystem A food chain in a grassland ecosystem may consist of grasses and other plants, grasshoppers, frogs, snakes and hawks (Figure 8.3). Fig. 8.3 A food chain in a grassland ecosystem In a freshwater aquatic ecosystem like a pond, the organisms in the food chain include algae, small animals, insects and their larvae, small fish, big fish and a fish-eating bird or animal (Figure 8.4).

Our Environment 85 Fig. 8.4 A food chain in a freshwater pond A food chain always begins with producers. Herbivores (plant-eaters) come next in the chain. They are consumed by carnivores (flesh-eaters). A few food chains can be long and may extend to the fourth, fifth or even sixth order of consumers. Some common food chains are mentioned below. Plants ® Deer ® Lion Plants ® Worm ® Bird ® Cat Plants ® Grasshopper ® Frog ® Snake ® Hawk Algae ® Small animal ® Small fish ® Big fish ® Bird Food Web A food chain is a linear arrangement of animals. It does not give a complete picture of the feeding relationships among the different organisms of an ecosystem because many of them eat more than one kind of food. For example, a snake does not eat only mice. It may eat insects, frogs, small birds, etc. Snakes, in turn may be eaten by hawks, eagles, peacocks, owls, etc. Thus, an organism can be a part of many food chains. Different kinds of linked food chains exist in an ecosystem. These food chains form a network of interconnected food chains, called a food web. Fig. 8.5 A simple food web of a grassland ecosystem

86 Foundation Science: Biology for Class 10 A food web is a series of interconnected food chains representing the feeding relationships of the organisms within an ecosystem. A food web of a grassland ecosystem is shown in Figure 8.5. Unlike a food chain, a food web has several alternative pathways for the flow of energy. In the food web of a grassland ecosystem shown in Figure 8.5, there are five food chains. 1. Plants ® Grasshopper ® Hawk 2. Plants ® Grasshopper ® Lizard ® Hawk 3. Plants ® Rabbit ® Hawk 4. Plants ® Mouse ® Snake ® Hawk 5. Plants ® Mouse ® Hawk Trophic Levels Sometimes it is useful to study an ecosystem by grouping its organisms by their positions or levels in food chains. A level or position in a food chain is called a trophic level. A particular trophic level consists of the organisms that occupy the same position (say, of primary consumers) in food chains. Energy and materials are transferred from one trophic level to another. The producers in a food chain are at the first trophic level. The herbivores, which feed upon plants, are at the second trophic level. The carnivores, which feed upon herbivores, are at the third trophic level, and so on. So, each trophic level supports the one above it in terms of food. Fig. 8.6 Trophic levels In a simple food chain of a grassland ecosystem, there are three trophic levels. Grass (producer) is at the first trophic level. Deer (herbivore) is at the second trophic level and lion (carnivore) is at the third trophic level. What would happen if all the organisms of a trophic level are removed? The natural balance would be disturbed and the results would be disastrous. For example, in a grassland ecosystem, if all the carnivores (like lions) at the third trophic level are removed, the numbers of herbivores (like deer) in the trophic level below would go on increasing. Their numbers would soon be more than that can be supported by the plants of the region. They would eat up all the plants and turn the area into a desert. Flow of Energy You know that there is a flow of energy in the form of food within an ecosystem. The flow of this energy is unidirectional, i.e., it flows in one direction—from the producers to the consumers at successively higher trophic levels. This energy cannot flow back because a higher-level consumer such as a snake cannot be food for a lower-level consumer such as a rabbit. Let us now look at the flow of energy a bit more closely.

Our Environment 87 Fig. 8.7 Flow of energy in an ecosystem Green plants absorb a very small fraction (about 1%) of the solar energy reaching the outer part of the atmosphere. Through photosynthesis they convert this energy into chemical energy, which is stored as food (carbohydrates). A part of the trapped energy is used by plants in metabolic activities like the growth of new tissues, and a part of it is lost into the surroundings as heat. The remaining energy is available as food to primary consumers. Thus we see that only a fraction of the energy absorbed by plants is finally available to the next trophic level. When primary consumers like deer eat plants, they get the available energy in plants. Some of this energy is used for activities like moving, digesting, etc., and some of it is lost as heat. Only about 10% of the available energy in the food gets transformed into new tissues (flesh) of the deer. This is available to the carnivores (secondary consumers) at the next trophic level. At this level too, the usage, loss and storage of energy follow the same pattern. And this continues at every trophic level. Apart from this, energy from dead plants and animals is transferred to the decomposers. We find that when energy flows from the producers to the consumers at different levels, there is a loss of energy at each trophic level. It has been found that only about 10% of the energy available to a trophic level is transferred to the next higher level. This is called the ten per cent law. Let us look at an example. If 10,000 kilocalories of energy are available to grass (producers), 1,000 kilocalories of energy would be available to grasshoppers (primary consumers), 100 kilocalories would be available to frogs (secondary consumers) and only 10 kilocalories would be available to snakes (consumers of the third order). After this, very little energy would be left for the next level. So, food chains generally have up to three or four trophic levels. Fig. 8.8 Ten per cent law of transfer of energy Now, the organisms at a trophic level are food for the organisms at the next higher trophic level. But there is a loss of energy as one goes from a lower to a higher trophic level. Therefore, the organisms at the higher level need to eat a large amount of food to fulfil their requirement of energy. So, the number of organisms at a lower trophic level is usually more than that at the next higher trophic level. If the numbers of organisms at different trophic levels are represented graphically, a pyramid is formed, which is called the pyramid of numbers.

88 Foundation Science: Biology for Class 10 Fig. 8.9 Pyramid of numbers Biological Magnification Producers like plants and algae take in nutrients from their surroundings. Sometimes they also take in harmful chemicals along with the other nutrients. These harmful chemicals include compounds of mercury, and chemicals like DDT, which is widely used to kill mosquitoes. These chemicals cannot be broken down by plants and other organisms. So, they accumulate in their bodies. Since organisms at a higher trophic level eat many lower-level organisms, the amount of chemicals entering their bodies increases. Thus, as the chemicals pass on from one level to the next in the food chain, the amount of chemicals accumulated in organisms at each level increases. The progressive increase in the accumulation of a harmful substance in organisms at successively higher trophic levels is called biological magnification or biomagnification. How does biological magnification harm different animals? It has been observed that birds in which DDT has accumulated lay eggs with very thin shells that break easily, even before chicks are born. This affects the growth of their population. In Japan, a mercury compound from a factory was dumped in the waters of Minamata Bay. The mercury found its way to humans who ate fish in which mercury had accumulated. More than a thousand people died as a result. Many countries have banned the use of DDT and the use of mercury in things like batteries. HOW WE AFFECT THE ENVIRONMENT Human activities can have a harmful effect on the environment. A large number of environmental problems arise due to the wastes we generate and the substances, like CFCs (chlorofluorocarbons), that we release into the environment. Human-generated Waste A thing which is of no use to us or which we throw away is called waste. Waste includes sewage and garbage. Used water and the waste materials produced by human bodies are carried away through underground pipes as sewage. Garbage refers to the solid household waste that we throw away regularly.

Our Environment 89 Garbage may have food scraps, paper, plastic materials, polythene, packing materials, wood, glass, and a host of other items. As our economic and social conditions are improving, we are generating more waste. For example, we are using more paper for newspapers, coupons, posters, and so on. This results in more waste paper. We are also buying more appliances, clothes, etc. Each of these comes with packing materials, which we are throwing out as waste. Biodegradable and nonbiodegradable materials Waste materials can be classified on the basis of whether they can be degraded, or broken down, by biological processes. Materials that get broken down by living organisms are called biodegradable materials. These are substances of plant or animal origin such as paper, wood, cotton, food materials and sewage. So, biodegradable wastes can be broken down into simpler, natural substances by decomposers (saprophytes) such as bacteria and fungi. These substances are harmless and nontoxic. Materials that cannot be broken down by living organisms are called nonbiodegradable materials. Metals, plastics, DDT, detergents, certain dyes, etc., are some examples of nonbiodegradable substances. Nonbiodegradable materials are major pollutants. They may be chemically less active. So they remain unchanged in the environment for a long time. You have already seen how harmful materials like DDT and mercury can be. Plastic bags, bottles, etc., often find their way to drains, etc. Since they do not decompose, they keep on adding up and finally, they choke the drains. Nonbiodegradable detergents, dyes, etc., get into the soil and water bodies and harm the organisms living there. Fig. 8.10 Nonbiodegradable wastes Dig a pit in your garden. Put in alternate layers of waste and soil in the pit. The layers of waste can consist of shredded vegetable peels, used tea leaves, stale food, etc. Also add layers of waste paper, torn clothes, leaves, polythene bags, broken glass, plastic bottles, broken wooden articles, and so on. Spray this layered pit with water and cover it with at least 15 cm of soil. Dig up the pit after 15 days. You will find that decomposition has started in the kitchen wastes, waste paper, cotton clothes and wooden articles, as they are biodegradable. Polythene bags, broken glass, plastic bottles, etc., remain unchanged since they are nonbiodegradable. In general, we should try to use biodegradable materials wherever possible to reduce the harmful effects of nonbiodegradable materials. For certain things, we have a choice between biodegradable and nonbiodegradable materials. For example, disposable cups can be made from nonbiodegradable plastics or from paper (biodegradable) or clay. We should weigh the environmental impact of using a certain material. Although paper is biodegradable, using more paper means cutting more trees. Some may say that good quality clay can be put to better uses like agriculture. But since nonbiodegradable plastic is not a good choice, we have to choose

90 Foundation Science: Biology for Class 10 between clay and paper. In future we can perhaps use biodegradable plastics. Such plastics are already being used to make disposable cutlery, capsule covers, etc. Remember that some nonbiodegradable materials like glass, metals and certain plastics can be recycled or reused. This lessens their harmful effect on the environment. Management of garbage and sewage Management of garbage and sewage involves collecting, transporting and disposing waste. Garbage is collected by municipal bodies or private agencies. In many cities, garbage-disposal bins are provided in each neighbourhood. There may be separate bins for biodegradable kitchen wastes and for recyclable materials like paper, glass, metals and plastics. When separate bins are not provided, ragpickers pick out plastics, paper, etc., from the garbage and sell them to people who recycle the material. Fig. 8.11 Garbage bins for different types of waste Collected garbage is transported to disposal sites by special trucks or tractors. Recyclable solid waste is sent for recycling. The rest is used to fill up land at landfill sites. Sometimes, the solid waste is burnt. This is especially so for hospital wastes and wastes that can pollute the soil. Sewage is carried by pipes to sewage-treatment plants. Here the sewage is filtered and the organic material in the sewage is allowed to settle down and decompose in large tanks. The water coming out of these tanks is clean and is released into water bodies. In some places, sewage is used to produce biogas used for cooking and electricity generation. Fig. 8.12 Sewage carried by pipes entering treatment tanks, at left

Our Environment 91 CFCs and the Ozone Hole High above the surface of the earth, there is a layer in the atmosphere that is rich in ozone (O3). Ozone, which is made up of three atoms of oxygen, affects our respiratory system. However, the layer of ozone in the atmosphere protects us by absorbing the ultraviolet (UV) radiation from the sun. This prevents the harmful UV radiation from reaching the surface of the earth. Ultraviolet radiation can cause skin cancer and cataract. It also affects plants. Ozone is formed at the higher levels of the atmosphere. When UV radiation falls on oxygen molecules, the high energy of UV radiation splits molecular oxygen (O2) into free oxygen (O) atoms. These atoms then combine with molecular oxygen to form ozone. O2 ¾¾U¾V ¾® O+ O radiation O + O2 ¾® O3 (ozone) Unfortunately, the amount of ozone in the ozone layer is being depleted due to the release of certain chemicals into the atmosphere. Among these chemicals are chlorofluorocarbons (CFCs), which are made of carbon, fluorine and chlorine. CFCs are used in air-conditioners, refrigerators, aerosol sprays, shaving foam, fire extinguishers, etc. These compounds rise up high in the atmosphere and break down to form chlorine atoms. These chlorine atoms react with the ozone to form oxygen. Cl + O3 ® ClO + O2 chlorine atom ozone chlorine monoxide oxygen Fig. 8.13 Formation of the ozone hole

92 Foundation Science: Biology for Class 10 As ozone gets converted to oxygen, the amount of ozone in the ozone layer decreases. Over time this has created an ozone-deficient area in the atmosphere. This area is called the ozone hole. This is most noticeable over Antarctica. To stop the damage to the ozone layer, the United Nations Environment Programme (UNEP) brought forward an agreement in 1987 to freeze the CFC production at the 1986 level. Most countries have signed this agreement. As a result, there has been a slight decrease in the rate of depletion of the ozone layer. • POINTS TO REMEMBER • · An ecosystem is a functional unit consisting of all · A food web is a series of interconnected food the living beings of an area and the nonliving chains representing the feeding relationship of components of their environment, interacting to the organisms within an ecosystem. form a stable system. · For an ecosystem to work, there has to be a flow · An ecosystem consists of abiotic (nonliving) and of energy and a cycling of nutrients. The flow of biotic (living) components. The biotic component energy is unidirectional, from a lower to a higher includes plants and animals, whereas the abiotic trophic level. Only about 10% of the energy component includes the physical environment, available to a trophic level is transferred to the nutrients and climatic factors. next higher level. · Organisms which make their own food are called · The progressive increase in the accumulation of a producers or autotrophs. Green plants and certain harmful substance in organisms at successively blue-green algae are producers. Organisms that higher trophic levels is called biological depend on other organisms for food are called magnification or biomagnification. consumers or heterotrophs. Consumers can be classified as herbivores (plant-eaters), carnivores · Materials that get broken down by living (flesh-eaters) and omnivores (eat both plants and organisms are called biodegradable materials. flesh). Organisms which feed on dead plants and These are substances of plant or animal origin. animals are called decomposers. Decomposers Materials that cannot be broken down by living include bacteria, fungi and worms. organisms are called nonbiodegradable materials, e.g., metals, plastics, DDT. Nonbiodegradable · A food chain in an ecosystem is a series of materials are major pollutants. organisms in which each organism feeds on the one below it in the series. A level or position in a · Chemicals like CFCs have damaged the protective food chain is called a trophic level. ozone layer, which absorbs the harmful ultraviolet radiation. • EXERCISES • A. Very-Short-Answer Questions 9. One hundred kilocalories of energy are available to a trophic level. Approximately how much energy 1. What is an ecosystem? can be transferred to the next higher trophic level? 2. Categorize these as primary, secondary and tertiary 10. Food chains usually do not have more than four consumers: grasshopper, snake, hawk, owl, rat and trophic levels. Why? rabbit. 11. What is the main difference between biodegradable 3. Give a scientific term for each of the following. (a) The green plants which synthesize food and nonbiodegradable wastes? [CBSE] (b) Organisms which feed on plants or plant products (c) Organisms that can convert dead and decaying 12. Name one category of chemicals that are organic matter into simpler form responsible for the ozone hole. 4. What is a food chain? B. Short-Answer Questions 5. Give one example of a simple food chain. 1. What are the components of an ecosystem? 6. Which category of organisms forms the starting 2. How are heterotrophs different from autotrophs? point of a food chain? 3. What is the importance of decomposers in an 7. Make a flow chart of a food chain commonly found ecosystem? on land. 4. Why do you need to clean an aquarium periodically 8. Define food web. while ponds do not need to be cleaned? 5. What do you understand by a trophic level?

Our Environment 93 6. What will happen if all the organisms at a trophic 5. The third trophic level of a grassland food chain can level die? have 7. In what direction does the energy flow in a food (a) grass (b) grasshoppers chain? Why is it unidirectional? (c) hawks (d) frogs 8. Usually, why do you have a greater number of 6. In a food chain, herbivores constitute the organisms at the lower trophic levels? (a) first trophic level (b) second trophic level 9. Construct a food web that has rabbits, plants, (c) third trophic level herbivorous insects, hawks, sparrows, wolves, (d) fourth trophic level spiders, frogs and snakes. 10. Where is the ozone layer found? What is the 7. Which of the following constitute a food chain? (a) Plant, apple, butterfly, man importance of ozone layer? [CBSE] (b) Grass, spider, bee, buffalo (c) Plant, insect, toad, snake 11. How is ozone formed in the upper atmosphere? (d) Algae, insect larvae, fish, cow Why is damage to the ozone layer a cause of concern to us. What causes this damage? [CBSE] 8. What happens in biological magnification? 12. What are the problems caused by nonbiodegradable (a) There is progressive increase in the level of wastes? harmful substances through trophic levels. C. Long-Answer Questions (b) There is a progressive increase in the body weight through trophic levels. 1. All heterotrophs are consumers. How would you classify them? (c) There is a progressive increase in the number of organisms through trophic levels. 2. Explain the flow and loss of energy in an ecosystem. (d) There is progressive increase in biological 3. What is biological magnification? Explain how the activities through trophic levels. level of accumulation of harmful substances varies with trophic levels. 9. The flow of energy in an ecosystem is 4. What is the ozone hole? How was it formed? What (a) unidirectional (b) bidirectional is being done to control it? (c) multidirectional (d) cyclic D. Objective Questions 10. Which of the following contain only I. Pick the correct option. nonbiodegradable things? (a) Leaves, wood, plastics 1. In an ecosystem, there is a flow of (a) energy only (b) Polythene, aluminium can, mercury (b) nutrients only (c) DDT, cow dung, fruit peels (c) water only (d) Kitchen waste, sewage, pen (d) energy and nutrients 2. The ultimate source of energy in an ecosystem is the II. Fill in the blanks. 1. The physical and biological conditions in which an (a) producer (b) consumer organism lives is its _____. 2. In a food chain, each position is known as a _____ (c) sun (d) decomposer level. 3. Those who feed directly on the producers in an 3. The biotic component of an ecosystem consists of ecosystem are called _____ consumers. (a) plants and animals 4. All green plants are _____ , whereas animals are consumers. (b) green plants and algae 5. If the energy available at a trophic level is 100 units, the energy available at the level just below it will be (c) producers, consumers and decomposers about _____ units. 6. The ozone layer protects us from _____ radiation. (d) air, water and soil 4. Out of the following, which is a linear arrangement of organisms? (a) Trophic level (b) Ecosystem (c) Food chain (d) Food web • ANSWERS • Objective Questions 3. (c) 4. (c) 5. (d) 1. (d) 2. (c) 8. (a) 9. (a) 10. (b) 6. (b) 7. (c) v

9 Practicals FOR CLASS 9 1. PREPARATION OF TEMPORARY MOUNTS Experiment 1.1 Objective To prepare a stained temporary mount of an onion peel and to record observations and draw labelled diagrams Apparatus and materials required An onion, glass slide, watch glass, coverslip, forceps, needles, brush, blade, filter paper, safranin, glycerine, dropper, water, and a compound microscope Theory All living organisms are made up of cells. The shape, size and the number of these units vary in organisms. The three major components of a cell are the cell membrane, cytoplasm and nucleus. In a plant cell, a cell wall surrounds the cell membrane. Procedure 1. Take an onion and remove its outermost peel. 2. Now cut a small part from an inner scale leaf with the help of a blade. 3. Separate a thin, transparent peel from the convex surface of the scale leaf with the help of forceps. 4. Keep this peel in a watch glass containing water. 5. Add two drops of safranin stain in the watch glass to stain the peel. 6. Take a clean slide and put a drop of glycerine in the centre of the slide. 7. With the help of a brush and needle transfer the peel on the slide. Glycerine prevents the peel from drying up. 8. Carefully cover it with a coverslip and avoid any air bubble from entering under the coverslip. 9. Remove any excessive glycerine with a filter paper. 10. Observe the prepared mount of the peel under the low and high magnification of a compound microscope. Observations A large number of rectangular cells are visible. These cells lie close to each other with intercellular spaces between them. These cells are surrounded by distinct cell walls. These cells have a dark stained nucleus and a large vacuole in the centre. 94

Practicals 95 Fig. 9.1 (a)–(b) Methods of separating an onion peel (c) Structure of onion cells as seen under a microscope (450 ´) Precautions 1. Overstaining and understaining should be avoided. 2. Folding of the peel should be avoided. 3. Clean and dry glass slide and coverslip should be used. 4. Coverslip should be put carefully avoiding any air bubbles. Experiment 1.2 Objective To prepare a stained temporary mount of human cheek cells and to record observations and draw labelled diagrams Apparatus and materials required Toothpick, slide, coverslip, filter paper, needles, brush, watch glass, methylene blue, dropper, glycerine, water and a compound microscope Theory Animal cells are usually irregular in shape. They do not have a cell wall. They are surrounded by a cell membrane and contain cytoplasm and nucleus. Procedure 1. With the help of the flat end of a washed toothpick gently scrape the inside of your cheek. 2. Place the scrapings in the centre of a clean glass slide.

96 Foundation Science: Biology for Class 10 3. Add a drop of water and a drop of methylene blue. 4. After one minute remove the extra water mixed with methylene blue by slightly tilting the slide. 5. Put a drop of glycerine over the stained scrapings and cover it gently with a coverslip. 6. Remove the excessive glycerine using filter paper. 7. Observe the scrapings under the low and high magnifications of a microscope. Observations Many flat, oval or irregular cells are seen. The cell membrane encloses hyaline cytoplasm and an oval, dense nucleus. The cell wall is absent as in all animal cells. Fig. 9.2 (a) Removing epithelial cells from (b) Cheeck cells as seen under a microscape the buccal cavity using a toothpick Precautions 1. The cheeks should be scraped gently avoiding any injury. 2. Overstaining and understaining of the cells should be avoided. 3. Coverslip should be placed carefully avoiding the entry of air bubbles. 4. A dry and clean glass slide and coverslip should be used. 5. The cheek cells should be spread properly to avoid their folding. VIVA VOCE 1. Why are plant cells regular in shape? Plant cells are regular as they are surrounded by a thick and rigid cell wall. 2. Why do we use glycerine for mounting onion peel or cheek cells? We use glycerine as it does not allow the onion peel or the cheek cells to dry quickly. 3. Why can’t we see mitochondria and other cytoplasmic organelles in the cells of the mount? It is because we observe it under a light microscope with low magnification and low resolution. The higher magnification of an electron microscope and proper staining are required to observe mitochondria and other cytoplasmic organelles in the cell. 4. What are the three main parts of a cell? The three main parts of a cell are membrane, cytoplasm and nucleus. 5. What is the visible difference between an onion peel cell and a cheek cell? An onion peel cell has a thick cell wall, while a human cheek cell does not have cell wall. 6. Which stain is used for staining plant cells? Safranin.

Practicals 97 7. Name the stain used for staining animal cells. Methylene blue. 8. What is the main constituent of cell walls? Cellulose. 2. TISSUES Experiment 2.1 Objective To identify parenchyma and sclerenchyma tissues in plants from prepared slides and to draw their labelled diagrams Apparatus and materials required Permanent slides of parenchyma, sclerenchyma, and a compound microscope Theory A group of cells of the same size and shape, or of a mixed type, having a common origin and performing an identical function is called tissue. Plant tissues are of two types—meristematic and permanent. Meristematic tissue cells are capable of dividing, while permanent tissue cells are not. Parenchyma, collenchyma, and sclerenchyma are the three types of simple permanent tissues. Procedure 1. Take a permanent slide of parenchyma and study under the low magnification and then under the high magnification of microscope. 2. Similarly place and study the other permanent slides of sclerenchyma. Observations The first slide of parenchymatous cells reveals the following features. Characters of Parenchyma 1. The cells are generally oval or spherical in shape. 2. These cells are large and are not packed closely, i.e., intercellular spaces are present. 3. Each cell has a large central vacuole and a peripheral cytoplasm with a prominent nucleus. 4. These living cells are found in the soft parts of the plants, i.e., root, stem, leaves, flowers, and fruits. 5. The important functions of these cells are storage of food, filling up spaces between other tissues and providing support to the plant. When they contain chloroplasts as in leaves, they help in the synthesis of food. The slides of sclerenchymatous cells show the following identifying features. Characters of Sclerenchyma 1. Cells are thick-walled, hard and contain little or no protoplasm. 2. The cells are oval, polygonal and are of different shapes. 3. The cells are dead and the nucleus is absent. 4. These cells are packed closely, i.e., intercellular spaces are absent. 5. The cell wall is evenly thickened with lignin and perforated with pits. 6. They provide strength and rigidity to the plant parts with hardness.