HUMAN CELL STRUCTURE

HUMAN CELL STRUCTURE Anatomy & physiology

Cell • Cell is the basic unit of life. • Cells are the smallest structures capable of basic life processes, such as taking in nutrients, expelling waste, and reproducing. • All living things are composed of cells. • Some microscopic organisms, such as bacteria and protozoa, are unicellular, meaning they consist of a single cell.

Cell • An animal cell typically contains several types of membrane-bound organs, or organelles. • The nucleus directs activities of the cell and carries genetic information from generation to generation. • The mitochondria generate energy for the cell. • Proteins are manufactured by ribosomes, which are bound to the rough endoplasmic reticulum or float free in the cytoplasm. • The Golgi apparatus modifies, packages, and distributes proteins while lysosomes store enzymes for digesting food. • The entire cell is wrapped in a lipid membrane that selectively permits materials to pass in and out of the cytoplasm

Plasma membrane • The plasma membrane that surrounds eukaryotic cells is a dynamic structure composed of two layers of phospholipid molecules interspersed with cholesterol and proteins. • Phospholipids are composed of a hydrophilic, or water-loving, head and two tails, which are hydrophobic, or water-hating. • The two phospholipid layers face each other in the membrane, with the heads directed outward and the tails pointing inward. • The water-attracting heads anchor the membrane to the cytoplasm, the watery fluid inside the cell, and also to the water surrounding the cell. • The water-hating tails block large water-soluble molecules from passing through the membrane while permitting fat-soluble molecules, including medications such as tranquilizers and sleeping pills, to freely cross the membrane. • Proteins embedded in the plasma membrane carry out a variety of functions, including transport of large water soluble molecules such as sugars and certain amino acids

Cell organelles Nucleus • The nucleus is surrounded by a double- layered membrane that protects the DNA from potentially damaging chemical reactions that occur in the cytoplasm. • Messages pass between the cytoplasm and the nucleus through nuclear pores, which are holes in the membrane of the nucleus. • In each nuclear pore, molecular signals flash back and forth as often as ten times per second. • For example, a signal to activate a specific gene comes in to the nucleus and instructions for production of the necessary protein go out to the cytoplasm.

Mitochondria • Mitochondria, minute sausage- shaped structures found in the clear cytoplasm of the cell, are responsible for energy production. • Mitochondria contain enzymes that help convert food material into adenosine triphosphate (ATP), which can be used directly by the cell as an energy source. • Mitochondria tend to be concentrated near cellular structures that require large inputs of energy, such as the flagellum, which is responsible for movement in sperm cells and single-celled plants and animals.

Ribosome • Ribosome • On the surface of the rough endoplasmic reticulum are numerous small, dark structures called ribosomes. • Ribosomes, which are also found floating free in the cytoplasm, are the sites of protein synthesis.

Endoplasmic reticulum (ER) • Endoplasmic Reticulum (ER), an extensive network of tubes that manufacture, process, and transport materials within nucleated cells. • The ER consists of a continuous membrane in the form of branching tubules and flattened sacs that extend throughout the cytoplasm (the cell’s contents outside of the nucleus) and connect to the double membrane that surrounds the nucleus. • There are two types of ER: rough and smooth.

Golgi Apparatus The Golgi apparatus, a minute cellular inclusion in the cytoplasm, is a series of smooth, stacked membranous sacs. The Golgi apparatus modifies proteins after they are produced by the ribosomes.

LYSOSOME • Lysosome, membrane- bound sac found in nucleated cells that contains digestive enzymes that break down complex molecules in the body. • Lysosomes are numerous in disease-fighting cells, such as white blood cells, that destroy harmful invaders or cell debris

Cytoskeleton • The cytoskeleton, a network of protein fibers, crisscrosses the cytoplasm of eukaryotic cells, providing shape and mechanical support. • The cytoskeleton also functions as a monorail to transport substances around the cell. • A cell such as an amoeba changes shape by dismantling parts of the cytoskeleton and reassembling them in other locations.

CENTROSOME • This directs the microtubules in the cell • It consists of a pair of centrioles ad plays an important role I the cell division

Cell extensions • These project from the plasma membrane and their main components are microtubules which allow movement . 1.Cilia 2. flagella

CELL DIVISION

MITOSIS Mitosis occurs in five stages: interphase, prophase, metaphase, anaphase, and telophase. During interphase, the start of mitosis, the DNA of each chromosome replicates. Each chromosome then reorganizes into paired structures called sister chromatids, with each member of the pair containing a full copy of the DNA sequence. During prophase, the sister chromatids condense, thickening until they appear joined at a single site, known as the centromere. The sister chromatids line up in the middle of the cell during metaphase. In anaphase, the chromatid pairs split apart at the centromere, and each half of the pair then moves toward opposite poles of the cells. In telophase, the final stage of mitosis, a nuclear membrane forms around the chromosomes at each pole of the cell. Mitosis ends with the formation of two new cells, each with a matching full set of chromosomes as well as an identical complement of cellular structures.

Transport of substances across membrane 1. Passive transport 2. Diffusion 3. Facilitated diffusion 4. Osmosis 5. Active transport 6. Sodium potassium pump 7. Bulk transport

Cell death • Necrosis • Apoptosis

• Necrosis or Mass death or Gangrene of individual cells or tissues of a living organism. • The immediate physiological cause of gangrene is blockage of arterial blood supply. Among the symptoms of gangrene are loss of sensation and function. • The affected part, usually a limb, becomes cold, turns progressively darker in color, and finally undergoes decomposition. • Gangrene may take two forms, dry gangrene or moist gangrene. • Dry, or chronic, gangrene occurs when the arteries are blocked gradually and the tissues are bloodless. It may be a result of arteriosclerosis • It may also follow frostbite, injury, or the vascular collapse that often accompanies diabetes

Apoptosis • Is an important area in cell biology concerns programmed cell death. • Millions of times per second in the human body, cells commit suicide as an essential part of the normal cycle of cellular replacement. • This also seems to be a check against disease: When mutations build up within a cell, the cell will usually self-destruct. • If this fails to occur, the cell may divide and give rise to mutated daughter cells, which continue to divide and spread, gradually forming a growth called a tumor. • This unregulated growth by cells can be benign, or harmless, or cancerous, which may threaten healthy tissue. • The study of apoptosis is one avenue that scientists explore in an effort to understand how cells become cancerous

• Write an essay about the structure and functions of human cell.