Essentials-of-Biology

82 PART ONE The Cell 5.2 ATP: Energy for Cells Learning Outcomes Upon completion of this section, you should be able to 1. Summarize the role of adenosine triphosphate (ATP) in a cell. 2. Describe the phases of the ATP cycle. 3. Describe the low of energy between photosynthesis and cellular respiration. adenine ribose triphosphate (ATP) Adenosine triphosphate (ATP) is the energy currency of cells. Just as you use coins to purchase all sorts of products, a cell uses ATP to carry out nearly all of PPP its activities, including synthesizing proteins, transporting ions across the plasma membranes, and causing organelles and flagella to move. Cells use the readily Figure 5.3 ATP. accessible energy supplied by ATP to provide energy wherever it is needed. ATP, the universal energy currency of cells, is composed of the Structure of ATP nucleotide adenine, the sugar ribose, and three linked phosphate groups (called a triphosphate). ATP is a nucleotide (see Section 3.2), the type of molecule that serves as a mono- mer for the construction of DNA and RNA. ATP’s name, adenosine triphosphate, means that it contains the sugar ribose, the nitrogen-containing base adenine, and three phosphate groups (Fig. 5.3). The three phosphate groups (shown as P in diagrams and formulas) are negatively charged and repel one another. Like trying to push together two negative ends of a battery, it takes energy to overcome the repulsion of the phosphate groups and link them by chemical bonds. This is why the bonds between the phosphate groups are high-energy bonds. However, these linked phosphate groups also make the molecule unstable. ATP easily loses the phosphate group at the end of the chain because the break- down products, ADP (adenosine diphosphate) and the separate phosphate group, are more stable than ATP. This reaction is written as ATP → ADP + P . Energy is released as ATP breaks down. ADP can also lose another phos- phate group to become AMP (adenosine monophosphate). Use and Production of ATP The continual breakdown and regeneration of ATP is known as the ATP cycle (Fig. 5.4). ATP holds energy for only a short period of time before it is used in a reaction that requires energy. Then ATP is rebuilt from ADP + P . Each ATP molecule undergoes about 10,000 cycles of synthesis and breakdown every day. Our bodies use some 45 kg (about 99 lb) of ATP daily (assuming minimal activity!), and the amount on hand at any one moment is sufficiently high to meet current metabolic needs for only about 1 minute. ATP’s instability, the very feature that makes it an effective energy car- rier, keeps it from being an energy storage molecule. Instead, carbohydrates and fats are the preferred energy storage molecules of cells due to their large number of H⏤C bonds. Their energy is extracted during cellular respiration and used to rebuild ATP, mostly within mitochondria. You will learn in Chapter 7 that the breakdown of one molecule of glucose permits the building of around 38 molecules of ATP. During cellular respiration, only 39% of the potential energy of glucose is converted to the potential energy of ATP; the rest is lost as heat.

CHAPTER 5 The Dynamic Cell 83 adenosine triphosphate Figure 5.4 The ATP cycle. PPP When ATP is used as an energy source, a phosphate group is removed by hydrolysis. ATP is primarily regenerated in the adenine mitochondria by cellular respiration. ribose ATP Energy from cellular respiration +ADP P Energy for cellular activity (e.g., protein synthesis, nerve conduction, muscle contraction) P P+ P adenosine diphosphate + phosphate The production of ATP is still worthwhile for the cell for the following reasons: 1. ATP releases energy quickly, which facilitates the speed of enzymatic reactions. 2. When ATP becomes ADP + P , the amount of energy released is usu- ally just enough for a biological purpose. Breaking down an entire carbo- hydrate or fat molecule would be wasteful, since it would release much more energy than is needed. 3. The structure of ATP allows its breakdown to be easily coupled to an energy-requiring reaction, as described next. Coupled Reactions Many metabolic reactions require energy. Energy can be supplied when a reaction that requires energy (e.g., building a protein) occurs in the vicinity of a reaction that gives up energy (e.g., ATP breakdown). These are called coupled reactions, and they allow the energy-releasing reaction to provide the energy needed to start the energy-requiring reaction. Usually, the energy-releasing reaction is the break- down of ATP, which generally releases more energy (and heat) than the amount consumed by the energy-requiring reaction. This increases entropy, but both reac- tions will proceed. The simplest way to represent a coupled reaction is like this: ATP ADP + P C+D A+B Coupling

84 PART ONE The Cell Figure 5.5 Coupled reaction. Muscle cell contains Muscle contraction occurs only when it is coupled to actin filaments and ATP breakdown. Myosin combines with ATP prior to its myosin filaments. breakdown. Release of ADP + P causes myosin to change position and pull on an actin ilament. (photo): © CNRI/Science Source 580× actin P ADP ATP myosin Connections: Health This reaction tells you that coupling occurs, but it does not show how coupling is achieved. Typically, the reaction transfers a phosphate group from ATP to one of What is creatine, and is it safe? the molecules in the reaction. This may either cause the molecule to change shape (and start a new function) or energize the molecule. In either case, the molecule In humans, creatine is found in muscle cells as has the ability to perform a function within the cell as a result of the coupled reaction. For example, when polypeptide synthesis occurs at a ribosome, an creatine phosphate (sometimes called phos- enzyme transfers a phosphate group from ATP to each amino acid in turn, and this transfer activates the amino acid, causing it to bond with another amino acid. phocreatine). Its function is to provide a brief, Figure 5.5 shows how ATP breakdown provides the energy necessary quick recharge of ATP molecules in muscle for muscle contraction. During muscle contraction, myosin filaments pull actin filaments to the center of the cell, and the muscle shortens. First, myosin com- cells. Over the past several years, creatine © McGraw- bines with ATP, and only then does ATP break down to ADP + P . The supplements have gained popularity with ath- Hill Education release of ADP + P from the molecule causes myosin to change shape and pull on the actin filament. letes for muscle-building as a performance The Flow of Energy enhancement. While there is some evidence that creatine In the biosphere, the activities of chloroplasts and mitochondria enable energy supplements may increase performance for some individu- to flow from the sun to the majority of life on the planet (the exception may be some organisms that live near deep-sea vents). During photosynthesis, the als, there have been no long-term FDA studies on the poten- chloroplasts in plants capture solar energy and use it to convert water and carbon dioxide to carbohydrates, which serve as food for themselves and for tial detrimental efects of creatine use on body organs—such other organisms. During cellular respiration, mitochondria complete the break- down of carbohydrates and use the released energy to build ATP molecules. as the kidneys, which are responsible for the secretion of Notice in Figure 5.6 that cellular respiration requires oxygen and produces excess creatine in the urine. In addition, creatine supple- carbon dioxide and water, the very molecules taken up by chloroplasts. It is actu- ally the cycling of molecules between chloroplasts and mitochondria that allows ments can produce dangerous interactions when used with over-the-counter drugs, such as acetaminophen and some- times even cafeine. Additional studies are needed to deter- mine if creatine supplements are safe.

CHAPTER 5 The Dynamic Cell 85 a flow of energy from the sun through all living organisms. This flow Energy Conversions heat of energy maintains the levels of biological organization from mole- cules to organisms to ecosystems. In keeping with the energy laws, solar energy useful energy is lost with each chemical transformation, and eventually the solar energy captured by plants is lost in the form of heat. In this way, living organisms are dependent upon an input of solar energy. Like all life, humans are also involved in this cycle. We inhale chloroplast oxygen and eat plants and their stored carbohydrates or other animals that have eaten plants. Oxygen and nutrient molecules enter our mito- chondria, which produce ATP and release carbon dioxide and water. Without a supply of energy- CONNECTING THE CONCEPTS rich foods, we could not pro- O2 5.2 The energy currency of the cell is duce the ATP molecules Chemical energy (carbohydrate) ATP, which is used by cells to power needed to maintain our bodies CO2 and H2O their cellular functions. and carry on activities. Check Your Progress 5.2 mitochondrion 1. Describe how ATP is produced, and heat explain why ATP cannot be used as an Chemical work energy storage molecule. ATP Transport work Mechanical work 2. Illustrate a coupled reaction, and explain the role of ATP in a coupled reaction. Figure 5.6 Flow of energy. 3. Describe how cellular respiration and Chloroplasts convert solar energy to the chemical energy stored in photosynthesis are connected. nutrient molecules. Mitochondria convert this chemical energy to ATP molecules, which cells use to perform chemical, transport, and 5.3 Metabolic Pathways and Enzymes mechanical work. (leaves): © Comstock/PunchStock RF; (woman): © Karl Weatherly/Getty RF Learning Outcomes Upon completion of this section, you should be able to 1. Illustrate how metabolic reactions are catalyzed by speciic enzymes. 2. Identify the role that enzymes play in metabolic pathways. 3. Explain the induced it model of enzymatic action. 4. Detail the processes that inhibit enzyme activity. 5. Relate the role of enzymes in lowering the energy of activation needed for a reaction. Life is a series of controlled chemical reactions. Just as a cell phone is built in a series of steps in a factory, the chemical reactions in a cell are linked to occur in a particular order. In the pathway, one reaction leads to the next reaction, which leads to the next reaction, and so forth, in an organized, highly struc- tured manner. This is called a metabolic, or biochemical, pathway. Metabolic pathways begin with a particular reactant and terminate with an end product. For example, in cells, glucose is broken down by a metabolic pathway, called cellular respiration (see Chapter 7), which consists of a series of reactions that produces the end products of energy (ATP), CO2, and H2O.

86 PART ONE The Cell Another example of a metabolic pathway is shown below. In this dia- gram, the letters A–F are reactants, and the letters B–G are products. Notice how the product from the previous reaction becomes the reactant of the next reaction. In the first reaction, A is the reactant and B is the product. Then B becomes the reactant in the next reaction of the pathway, and C is the product. This process continues until the final product (G) forms. e1 e2 e3 e4 e5 e6 ABCDE FG In this diagram, the letters e1–e6 represent enzymes, which are usually protein molecules that function as organic catalysts to speed chemical reac- tions. Enzymes can only speed reactions that are possible to begin with. In the cell, an enzyme is similar to a mutual friend who causes two people to meet and interact, because an enzyme brings together particular molecules and causes them to react with one another. The reactant molecules that the enzyme acts on are called its substrates. An enzyme converts substrates into products. The substrates and products of an enzymatic reaction vary greatly. Many enzymes facilitate the breakdown of a substrate into multiple products. Or an enzyme may convert a single substrate into a single product. Still others may combine two or more substrates into a single product. An Enzyme’s Active Site In most instances, only one small part of the enzyme, called the active site, accommodates the substrate(s) (Fig. 5.7). At the active site, the substrate fits into the enzyme seemingly as a key fits a lock; thus, most enzymes can fit only one substrate. However, the active site undergoes a slight change in shape in order to accommodate the substrate(s). This mechanism of enzyme action is called the induced fit model because the enzyme is induced (caused) to undergo a slight alteration to achieve optimal fit . The change in the shape of the active site facilitates the reaction that occurs next. After the reaction has been completed, the products are released, and the active site returns to its original state, ready to bind to another substrate molecule. Only a very small amount of each enzyme is needed in a cell because enzymes are not used up by the reactions. Figure 5.7 Enzymatic action. substrate products enzyme An enzyme has an active site where active the substrates and enzyme it site together in such a way that the substrates are oriented to react. Following the reaction, the products are released, and the enzyme is free to act again. enzyme reaction occurs

CHAPTER 5 The Dynamic Cell 87 enzyme Enzyme Inhibition active E1 other site Enzyme inhibition occurs when an active enzyme is prevented from combin- site E1 E2 E3 ing with its substrate. Enzyme inhibitors are often poisonous to certain organisms. Cyanide, for example, is an inhibitor of the enzyme cytochrome c E4 oxidase, which performs a vital function in cells because it is involved in making ATP. Cyanide is a poison because it binds the enzyme, blocking its S P activity. But some enzyme inhibitors are useful drugs. In another example, metabolic pathway end product penicillin is a poison for bacteria, but not humans, because it blocks the active site of an enzyme unique to bacteria. Many other antibiotic drugs also first substrate act as enzyme inhibitors. a. Active enzyme and active pathway The activity of almost every enzyme in a cell is regulated by feedback inhibition. In the simplest case, when a product is in abundance, it competes P E1 P E1 with the substrate for the enzyme’s active site. As the product is used up, inhi- P bition is reduced, and then more product can be produced. In this way, the concentration of the product always stays within a certain range. P b. Feedback inhibition Most metabolic pathways are regulated by more complex types of feed- back inhibition (Fig. 5.8). In these instances, when the end product is plentiful, S E1 P it binds to a site other than the active site of the first enzyme in the pathway. This binding changes the shape of the active site, preventing the enzyme from c. Inactive enzyme and inactive pathway binding to its substrate. Without the activity of the first enzyme, the entire pathway shuts down. Figure 5.8 Feedback inhibition. Energy of Activation a. This type of feedback inhibition occurs when the end product (P) of an active enzyme pathway is plentiful and (b) binds to the irst enzyme Molecules frequently do not react with one another unless they are activated in (E1) of the pathway at a site other than the active site. This changes the some way. In the lab, activation is very often achieved by heating a mixture to shape of the active site, so that (c) the substrate (S) can no longer bind increase the number of effective collisions between molecules. to the enzyme. Now the entire pathway becomes inactive. The energy needed to cause molecules to react with one another is called without the energy of activation (Ea). The energy of activation acts as a metabolic enzyme speed bump; it limits how fast a reaction can proceed from reactants to prod- ucts. Enzymes lower the amount of activation energy needed in a reaction with energy of activation (Fig. 5.9). In this way they act as catalysts that speed up the overall rate of the enzyme (more needed) reaction. Enzymes do not change the amount of energy in the products or reac- tants, they simply alter the rate of the reaction. Potential Energy reactant energy of activation CONNECTING THE CONCEPTS (less needed) 5.3 Metabolic pathways are organized sets of chemical reactions in a cell that are regulated by enzymes. product Check Your Progress 5.3 Reaction 1. Explain the beneit of metabolic pathways in cells.  Figure 5.9 Energy of activation (Ea). 2. Describe how the induced it model explains the binding of a Enzymes speed the rate of reactions because they lower the amount substrate to an enzyme’s active site.  of energy required for the reactants to react. Even reactions like this 3. Summarize the beneit of using feedback inhibition to control one, in which the energy of the product is less than the energy of the reactant, speed up when an enzyme is present. metabolic pathways. 

88 PART ONE The Cell 5.4 Cell Transport Learning Outcomes Upon completion of this section, you should be able to 1. Categorize the various ways in which materials can move across plasma membranes. 2. List the types of passive transport that can be used by cells. 3. Explain osmosis and the efect it has on cells in environments of diferent tonicities. 4. Describe how active transport is accomplished. 5. Deine the various forms of bulk transport that can move materials into or out of a cell. Connections: Health The plasma membrane regulates the passage of molecules into and out of the cell. This function is crucial because the life of the cell depends on the mainte- What causes cystic ibrosis? nance of its normal composition. The plasma membrane can carry out this function because it is selectively permeable, meaning that certain substances In 1989, scientists determined that defects in a gene on can freely pass through the membrane, some are transported across, and others chromosome 7 cause cystic ibrosis (CF). This gene, called are prohibited from entering or leaving. CFTR (cystic ibrosis conductance transmembrane regula- tor), codes for a protein that is responsible for the movement Basically, substances enter a cell in one of three ways: passive transport, of chloride ions across the membranes of cells that produce active transport, or bulk transport. Although there are different types of passive mucus, sweat, and saliva. Defects in this gene cause an im- transport, in all of them substances move from an area of higher concentration proper water-salt balance in the excretions of these cells, to an area of lower concentration, and no energy is required. Active transport which in turn leads to the symptoms of CF. Currently, there moves substances against a concentration gradient (from low to high concen- are over 1,400 known mutations in the CF gene. This tre- tration) and requires both a transport protein and lots of ATP. Bulk transport mendous amount of variation accounts for the diferences in requires energy, but movement of the large substances involved is independent the severity of the symptoms in CF patients. By knowing the of concentration gradients. precise gene that causes the disease, scientists have been able to develop new treatment options for people with CF. Passive Transport: No Energy Required At one time, people with CF rarely saw their 20th birthday; now it is routine for them to live into their 30s and 40s. New Difusion treatments, such as gene therapy, are being explored for patients with CF. One form of passive transport is diffusion. During diffusion, molecules move down their concentration gradient until equilibrium is achieved and they are distributed equally. Diffusion does not need to occur across a membrane; it oc- curs because molecules are in motion, but it is a passive form of transport be- cause energy is not expended. In cells, diffusion may occur across a plasma membrane. Some small, noncharged molecules, such as oxygen and carbon dioxide, are able to slip between the phospholipid molecules making up the plasma membrane. Diffusion is a physical process that can be observed with any type of molecule. For example, when a crystal of dye is placed in water (Fig. 5.10), the dye and water molecules move in various directions, but their net movement, which is the sum of their motions, is toward the region of lower concentration. Eventually, the dye is dispersed, with the dye particles being equally distrib- uted on either side of the membrane, and there is no net movement of dye in either direction. A solution contains both a solute and a solvent. In this case, the dye is called the solute, and the water is called the solvent. Solutes are usually solids or gases, and solvents are usually liquids.

CHAPTER 5 The Dynamic Cell 89 time time crystal dye a. Crystal of dye is placed in the water b. Di usion of water and dye molecules c. Equal distribution of molecules results Figure 5.10 Simple difusion demonstration. Difusion is spontaneous, and no chemical energy is required to bring it about. a. When a dye crystal is placed in water, it is concentrated in one area. b. The dye dissolves in the water, and there is a net movement of dye molecules from a higher to a lower concentration. There is also a net movement of water molecules from a higher to a lower concentration. c. Eventually, the water and the dye molecules are equally distributed throughout the container. Facilitated Difusion Facilitated diffusion (Fig. 5.11) occurs when an ion or a molecule diffuses across a membrane with assistance of a channel protein or carrier protein. While water may diffuse across a membrane because of its size, it is a polar molecule, so to move water across the plasma membrane cells use channel proteins called aquaporins. This is why water can cross the membrane much more quickly than expected. Glucose and amino acids are assisted across the plasma membrane by carrier proteins that change shape as they pass through. Channel proteins and carrier proteins are very specific to the molecule they assist across the membrane. solute plasma membrane carrier protein Outside Inside Figure 5.11 Facilitated difusion. In facilitated difusion, a carrier protein in the plasma membrane allows molecules to move from areas of high concentration to areas of low concentration.