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6 Cellular Respiration: Obtaining Energy from Food Why Cellular Respiration Matters About 20% of the energy produced by your body ▼ each day is used to sustain your brain....

6 Cellular Respiration: Obtaining Energy from Food Why Cellular Respiration Matters About 20% of the energy produced by your body ▼ each day is used to sustain your brain. You have something in common with a sports car: You both require an air ▼ intake system to burn fuel efficiently. ▲ Similar metabolic processes produce alcohol, pepperoni, soy sauce, rising bread, and acid in your muscles after a hard workout. 124 DESIGN SERVICES OF # 152561 Cust: Pearson Au: Simon Pg. No. 124 C/M/Y/K M06_SIMO2368_05_GE_CH06.indd 124 Title: EBP 5e Short / Normal S4CARLISLE Publishing Services 25/09/15 10:06 AM  CHAPTER CONTENTS CHAPTER THREAD Energy Flow and Chemical Cycling in the Biosphere 126 Exercise Science Cellular Respiration: Aerobic Harvest of Food Energy 128 BIOLOGY AND SOCIETY Getting the Most Out of Your Fermentation: Anaerobic Harvest of Food Energy 135 Muscles 125 THE PROCESS OF SCIENCE What Causes Muscle Burn? 136 EVOLUTION CONNECTION The Importance of Oxygen 137 Exercise Science BIOLOGY AND SOCIETY Getting the Most Out of Your Muscles Serious athletes train extensively to reach the peak of their physical potential. A key aspect of athletic condi- tioning involves increasing aerobic capacity, the ability of the heart and lungs to deliver oxygen to body cells. For many endurance athletes, such as long-distance runners or cyclists, the rate at which oxygen is provided to working muscles is the limiting factor in their performance. Why is oxygen so important? Whether you are exercising or just going about your daily tasks, your muscles need a continuous supply of energy to perform work. Muscle cells obtain this energy from the sugar glucose through a series of chemical reactions that depend upon a constant input of oxygen (O2). Therefore, to keep moving, your body needs a steady supply of O2. When there is enough oxygen reaching your cells to support their energy needs, metabolism is said to be aerobic. As your muscles work harder, you breathe faster and deeper to inhale more O2. If you con- tinue to pick up the pace, you will approach your aerobic capacity, the maximum rate at which O2 can be taken in and used by your muscle cells and therefore the most strenuous exercise that your body can maintain aerobically. Exercise physiologists (scientists who study how the body works during physical activity) there- fore use oxygen-monitoring equipment to precisely determine the maximum possible aerobic output for any given person. Such data allow a well-trained athlete to stay within aerobic limits, ensuring the maximum possible output—in other words, The science of exercise. Exercise physiologists can help his or her best effort. athletes perform their best by carefully monitoring the If you work even harder and exceed your aerobic capac- consumption of oxygen and the production of carbon dioxide. ity, the demand for oxygen in your muscles will outstrip your body’s ability to deliver it; metabolism then becomes anaerobic. With insufficient O2, your muscle cells switch to an “emergency mode” in which they break down glucose very inefficiently and produce lactic acid as a by- product. As lactic acid and other by-products accumulate, muscle activity is impaired. Your muscles can work under these conditions for only a few minutes before they give out. Every living organism depends on processes that provide energy. In fact, we need energy to walk, talk, and think—in short, to stay alive. The human body has trillions of cells, all hard at work, all demanding fuel continuously. In this chapter, you’ll learn how cells harvest food energy and put it to work with the help of oxygen. Along the way, we’ll consider the implications of how the body responds to exercise. 125 DESIGN SERVICES OF # 152561 Cust: Pearson Au: Simon Pg. No. 125 C/M/Y/K M06_SIMO2368_05_GE_CH06.indd Title: EBP 5e 125 Short / Normal S4CARLISLE Publishing Services 25/09/15 10:06 AM  Energy Flow and Chemical Cycling CHAPTER 6 CELLULAR RESPIRATION: OBTAINING ENERGY in the Biosphere FROM FOOD All life requires energy. In almost all ecosystems on heterotrophs (“other-feeders”), organisms that cannot Earth, this energy originates with the sun. During make organic molecules from inorganic ones. Therefore, photosynthesis, plants convert the energy of sunlight we must eat organic material to get our nutrients and to the chemical energy of sugars and other organic mol- provide energy for life’s processes. ecules (as we’ll discuss in Chapter 7). Humans and other Most ecosystems depend entirely on photosynthesis animals depend on this conversion for our food and for food. For this reason, biologists refer to plants and CHECKPOINT more. You’re probably wearing clothing made of a prod- other autotrophs as producers. Heterotrophs, in con- What chemical ingredients uct of photosynthesis—cotton. Most of our homes are trast, are consumers, because they obtain their food by do plants require from the environment to synthesize framed with lumber, which is wood produced by photo- eating plants or by eating animals that have eaten plants their own food? synthetic trees. Even textbooks are printed on a material (Figure 6.1). We animals and other heterotrophs depend Answer: CO2, H2O, and soil minerals (paper) that can be traced to photosynthesis in plants. on autotrophs for organic fuel and for the raw organic But from an animal’s point of view, photosynthesis is materials we need to build our cells and tissues. primarily about providing food. Chemical Cycling between Producers and Consumers Photosynthesis and Cellular Plants and other autotrophs (“self-feeders”) are organ- isms that make all their own organic matter—including Respiration carbohydrates, lipids, proteins, and nucleic acids—from The chemical ingredients for photosynthesis are carbon nutrients that are entirely inorganic: carbon dioxide dioxide (CO2), a gas that passes from the air into a plant from the air and water and minerals from the soil. In via tiny pores, and water (H2O), which is absorbed from other words, autotrophs make their own food; they the soil by the plant’s roots. Inside leaf cells, organelles don’t need to eat to gain energy to power their cellular called chloroplasts use light energy to rearrange the processes. In contrast, humans and other animals are atoms of these ingredients to produce sugars—most ▶ Figure 6.1 Producer and consumer. A giraffe (consumer) eating leaves produced by a photosynthetic plant (producer). 126 DESIGN SERVICES OF # 152561 Cust: Pearson Au: Simon Pg. No. 126 C/M/Y/K M06_SIMO2368_05_GE_CH06.indd 126 Title: EBP 5e Short / Normal S4CARLISLE Publishing Services 25/09/15 10:06 AM  importantly glucose (C6H12O6)—and other organic photosynthesis. Plants store chemical energy via photosyn- ENERGY FLOW AND molecules (Figure 6.2). You can think of chloroplasts as thesis and then harvest this energy via cellular respiration. CHEMICAL CYCLING IN THE BIOSPHERE tiny solar-powered sugar factories. A by-product of pho- (Note that plants perform both photosynthesis to produce tosynthesis is oxygen gas (O2) that is released through fuel molecules and cellular respiration to burn them, while pores into the atmosphere. animals perform only cellular respiration.) Plants usually Both animals and plants use the organic products of make more organic molecules than they need for fuel. This photosynthesis as sources of energy. A chemical process photosynthetic surplus provides material for the plant to called cellular respiration uses O2 to convert the energy grow or can be stored (as starch in potatoes, for example). stored in the chemical bonds of sugars to another source of Thus, when you consume a carrot, potato, or turnip, you are chemical energy called ATP. Cells expend ATP for almost eating the energy reservoir that plants (if unharvested) all their work. In both plants and animals, the production would have used to grow the following spring. of ATP during cellular respiration occurs mainly in the People have always taken advantage of CHECKPOINT organelles called mitochondria (see Figure 4.18). plants’ photosynthetic abilities by eating You might notice in Figure 6.2 that energy takes a them. More recently, engineers have man- What is misleading about the following statement? one-way trip through an ecosystem, entering as aged to tap into this energy reserve “Plants have chloroplasts sunlight and exiting as heat. Chemi- Sunlight energy to produce liquid biofuels, pri- that perform photosynthesis, enters ecosystem cals, in contrast, are recycled. marily ethanol (see Chapter 7 for whereas animals have Notice also in Figure 6.2 that a discussion of biofuels). But no mitochondria that perform the waste products of cellular matter the end product, you can cellular respiration.” respiration are CO2 and trace the energy and raw materials plants. It does. respiration does not also occur in H2O—the very same in- for growth back to solar-powered Answer: It implies that cellular gredients used as inputs for photosynthesis. ◀ Figure 6.2 Energy flow and chemical cycling in ecosystems. Energy flows Photosynthesis through an ecosystem, (in chloroplasts) converts light energy entering as sunlight and exiting to chemical energy C6H12O6 as heat. In contrast, chemical CO2 elements are recycled within Carbon dioxide Glucose an ecosystem. + + O2 H 2O Oxygen Water Cellular respiration (in mitochondria) harvests food energy to produce ATP ATP drives cellular work Heat energy exits ecosystem 127 DESIGN SERVICES OF # 152561 Cust: Pearson Au: Simon Pg. No. 127 C/M/Y/K M06_SIMO2368_05_GE_CH06.indd Title: EBP 5e 127 Short / Normal S4CARLISLE Publishing Services 25/09/15 10:06 AM  Cellular Respiration: CHAPTER 6 CELLULAR RESPIRATION: OBTAINING ENERGY Aerobic Harvest of Food Energy FROM FOOD We usually use the word respiration to mean breath- outside air. Oxygen present in the air you inhale ing. Although respiration on the organismal diffuses across the lining of your lungs and level should not be confused with cellular You have something into your bloodstream. And the CO2 in your respiration, the two processes are closely in common with a bloodstream diffuses into your lungs and CHECKPOINT related (Figure 6.3). Cellular respiration sports car: You both exits your body when you exhale. Every At both the organismal and requires a cell to exchange two gases with molecule of CO2 that you exhale was origi- require an air intake cellular levels, respiration involves taking in the gas its surroundings. The cell takes in oxygen system to burn fuel nally formed in one of the mitochondria of _________ and expelling the in the form of the gas O2. It gets rid of waste efficiently. your body’s cells. gas _________. in the form of the gas carbon dioxide, or Internal combustion engines, like the ones Answer: O2; CO2 CO2. Respiration, or breathing, results in the ex- found in cars, use O2 (via the air intakes) to change of these same gases between your blood and the break down gasoline. A cell also requires O2 to break down its fuel (see Figure 5.2). Cellular respiration—a ▼ Figure 6.3 How breathing is related to cellular respiration. biological version of internal combustion—is the main When you inhale, you breathe in O2. The O2 is delivered to your cells, way that chemical energy is harvested from food and where it is used in cellular respiration. Carbon dioxide, a waste product converted to ATP energy (see Figure 5.6). Cellular res- of cellular respiration, diffuses from your cells to your blood and travels piration is an aerobic process, which is just another to your lungs, where it is exhaled. way of saying that it requires oxygen. Putting all this together, we can now define cellular respiration as the aerobic harvesting of chemical energy from organic fuel molecules. O2 MAJOR THEMES IN BIOLOGY Interconnections Evolution Structure/Function Information Flow Energy Transformations within Systems Energy CO2 Evolution by natural selection is biology‘s core unifying theme and can be seen at every level in the The structure of an object, such as a molecule or a body part, provides insight into its function, and vice Within biological systems, information stored in DNA is transmitted and expressed. An Overview All biological systems depend on obtaining, converting, and releasing energy and matter. Transformations All biological systems, from molecules to ecosystems, depend on interactions between of Cellular Respiration hierarchy of life. versa. components. One of biology’s overarching themes is that all living Lungs organisms depends on transformations of energy and matter. We see examples of such transformations O2 CO2 throughout the study of life, but few are as important as the conversion of energy in fuel (food molecules) to a form that cells can use directly. Most often, the fuel molecule used by cells is glucose, a simple sugar (monosaccharide) with the formula C6H12O6 (see O2 Figure 3.6). (Less often, other organic molecules are CO2 used to gain energy.) This equation summarizes the transformation of glucose during cellular respiration: C6H12O6 + 6 O2 6 CO2 + 6 H 2O + approx. 32 ATP Muscle Cellular cells respiration The series of arrows in this formula represents the fact that cellular respiration consists of many chemical steps. A specific enzyme catalyzes each reaction—more than two dozen reactions in all—in the pathway. In fact, these reactions constitute one of the most impor- tant metabolic pathways for nearly every eukaryotic 128 DESIGN SERVICES OF # 152561 Cust: Pearson Au: Simon Pg. No. 128 C/M/Y/K M06_SIMO2368_05_GE_CH06.indd 128 Title:EBP 5e Short / Normal S4CARLISLE Publishing Services 25/09/15 10:06 AM  cell—those found in plants, fungi, protists, and animals. transfer forms a molecule called NADH (the H repre- CELLULAR RESPIRATION: This pathway provides the energy these cells need to sents the transfer of hydrogen along with the electrons) AEROBIC HARVEST OF FOOD ENERGY maintain the functions of life. that acts as a shuttle carrying high-energy electrons The many chemical reactions that make up cellu- from one area of the cell to another. The third stage of lar respiration can be grouped into three main stages: cellular respiration is electron transport. Electrons cap- glycolysis, the citric acid cycle, and electron transport. tured from food by the NADH formed in the first two Figure 6.4 is a road map that will help you follow the stages are stripped of their energy, a little bit at a time, three stages of respiration and see where each stage until they are finally combined with oxygen to form occurs in your cells. During glycolysis, a molecule of water. The proteins and other molecules that make up glucose is split into two molecules of a compound called electron transport chains are embedded within the inner pyruvic acid. The enzymes for glycolysis are located membrane of the mitochondria. The transport of elec- in the cytoplasm. The citric acid cycle (also called the trons from NADH to oxygen releases the energy your Krebs cycle) completes the breakdown of glucose all the cells use to make most of their ATP. way to CO2, which is then released as a waste product. The overall equation for cellular respiration shows CHECKPOINT The enzymes for the citric acid cycle are dissolved in that the atoms of the reactant molecules glucose and Which stages of cellular the fluid within mitochondria. Glycolysis and the citric oxygen are rearranged to form the products carbon di- respiration take place in the acid cycle generate a small amount of ATP directly. They oxide and water. But don’t lose track of why this process mitochondria? Which stage takes place outside the generate much more ATP indirectly, via reactions that occurs: The main function of cellular respiration is to mitochondria? transfer electrons from fuel molecules to a molecule generate ATP for cellular work. In fact, the process can electron transport; glycolysis called NAD+ (nicotinamide adenine dinucleotide) produce around 32 ATP molecules for each glucose Answer: the citric acid cycle and that cells make from niacin, a B vitamin. The electron molecule consumed. Mitochondria Cytoplasm Cytoplasm Animal cell Plant cell Cytoplasm Mitochondrion High-energy electrons via carrier molecules – Glycolysis – Citric 2 – Acid Electron Glucose Pyruvic Cycle Transport Chain acid ATP ATP ATP ▶ Figure 6.4 A road map for cellular respiration. 129 DESIGN SERVICES OF # 152561 Cust: Pearson Au: Simon Pg. No. 129 C/M/Y/K M06_SIMO2368_05_GE_CH06.indd Title: EBP 5e 129 Short / Normal S4CARLISLE Publishing Services 25/09/15 10:06 AM  CHAPTER 6 CELLULAR RESPIRATION: The Three Stages of Cellular from fuel molecules to ADP (Figure 6.6). Glycolysis thus produces a net of two molecules of ATP per molecule OBTAINING ENERGY FROM FOOD Respiration of glucose. (This fact will become important during our later discussion of fermentation.) What remains of the Now that you have a big-picture view of cellular respira- fractured glucose at the end of glycolysis are two mol- tion, let’s examine the process in more detail. A small ecules of pyruvic acid. The pyruvic acid still holds most version of Figure 6.4 will help you keep the overall of the energy of glucose, and that energy is harvested process of cellular respiration in plain view as we take a in the second stage of cellular respiration, the citric closer look at its three stages. acid cycle. Stage 1: Glycolysis The word glycolysis – ▼ Figure 6.6 ATP synthesis by direct phosphate transfer. Citric means “splitting of Glycolysis Acid – – Glycolysis generates ATP when enzymes transfer phosphate Cycle sugar” (Figure 6.5), groups directly from fuel molecules to ADP. – and that’s just what – – Electron happens. During ATP ATP Transport Chain glycolysis, a six-carbon Enzyme glucose molecule is broken in ATP half, forming two three-carbon mol- P ADP ATP ecules. Notice in Figure 6.5 that the initial split requires an energy investment of two ATP molecules per glucose. The three-carbon molecules then donate high-energy electrons to NAD+, forming NADH. In addition to NADH, glycolysis also makes four ATP mol- P P ecules directly when enzymes transfer phosphate groups ▼ Figure 6.5 Glycolysis. In glycolysis, a team of enzymes splits glucose, eventually forming two molecules of pyruvic acid. After investing 2 ATP at the start, glycolysis generates 4 ATP directly. More energy will be harvested later from high-energy electrons used to form NADH and from the two molecules of pyruvic acid. INPUT OUTPUT – – NADH P P NAD+ 2 ATP 2 ADP 2 3 P 2 ATP 2 ADP P 2 Pyruvic acid 1 P P P 2 3 Glucose 2 ADP NAD+ 2 ATP P Key – – Carbon atom NADH P Phosphate group – High-energy electron Energy investment phase Energy harvest phase 130 DESIGN SERVICES OF # 152561 Cust: Pearson Au: Simon Pg. No. 130 C/M/Y/K M06_SIMO2368_05_GE_CH06.indd 130 Title: EBP 5e Short / Normal S4CARLISLE Publishing Services 25/09/15 10:06 AM  INPUT OUTPUT CELLULAR RESPIRATION: 2 Breakdown of the fuel AEROBIC HARVEST (from glycolysis) generates NADH (to citric acid cycle) OF FOOD ENERGY – – NAD+ NADH CoA ◀ Figure 6.7 The link between glycolysis and the citric acid cycle: the conversion of pyruvic acid 1 Pyruvic acid loses Acetic acid 3 Acetic acid attaches to acetyl CoA. Remember that one a carbon as CO2 to coenzyme A Acetyl CoA molecule of glucose is split into two Pyruvic acid molecules of pyruvic acid. Therefore, CO2 Coenzyme A the process shown here occurs twice for each starting glucose molecule. Stage 2: The Citric the first reaction of the citric acid cycle. The CoA is then Acid Cycle stripped and recycled. The two molecules of Glycolysis Citric Acid – The citric acid cycle finishes extracting the energy Cycle – pyruvic acid, the fuel Electron – of sugar by dismantling the acetic acid molecules all that remains after gly- Transport Chain the way down to CO2 (Figure 6.8). Acetic acid joins colysis, are not quite ATP ATP a four-carbon acceptor molecule to form a six-carbon ready for the citric acid ATP product called citric acid (for which the cycle is named). cycle. The pyruvic acid For every acetic acid molecule that enters the cycle as must be “groomed”—converted to fuel, two CO2 molecules eventually exit as a waste a form the citric acid cycle can use (Figure 6.7). First, product. Along the way, the citric acid cycle harvests each pyruvic acid loses a carbon as CO2. This is the first energy from the fuel. Some of the energy is used to of this waste product we’ve seen so far in the break- produce ATP directly. However, the cycle captures much down of glucose. The remaining fuel molecules, each more energy in the form of NADH and a second, CHECKPOINT with only two carbons left, are called acetic acid (the closely related electron carrier called FADH2. All the Two molecules of what acid that’s in vinegar). Electrons are stripped from carbon atoms that entered the cycle as fuel are accounted compound are produced these molecules and transferred to another molecule of for as CO2 exhaust, and the four-carbon acceptor mol- by glycolysis? Does this molecule enter the citric NAD+, forming more NADH. Finally, each acetic ecule is recycled. We have tracked only one acetic acid acid cycle? acid is attached to a molecule called coenzyme A (CoA), molecule through the citric acid cycle here. But because converted to acetic acid. an enzyme derived from the B vitamin pantothenic acid, glycolysis splits glucose in two, the citric acid cycle oc- Answer: Pyruvic acid. No; it is first to form acetyl CoA. The CoA escorts the acetic acid into curs twice for each glucose molecule that fuels a cell. ▶ Figure 6.8 The INPUT OUTPUT Citric citric acid cycle. acid 1 Acetic acid 2 2 CO2 ADP + P ATP 3 Citric Acid Cycle – – 3 NAD+ 3 NADH 4 – – FAD FADH2 5 6 Acceptor molecule 131 DESIGN SERVICES OF # 152561 Cust: Pearson Au: Simon Pg. No. 131 C/M/Y/K M06_SIMO2368_05_GE_CH06.indd Title: EBP 5e 131 Short / Normal S4CARLISLE Publishing Services 25/09/15 10:06 AM  CHAPTER 6 Stage 3: Electron the electrons to oxygen. At the same time, oxygen picks CELLULAR RESPIRATION: up hydrogen, forming water. OBTAINING ENERGY Transport FROM FOOD Let’s take a closer look Citric Acid – The overall effect of all this transfer of electrons Glycolysis at the path that elec- Cycle – Electron – during cellular respiration is a “downward” trip for trons take on their way Transport Chain electrons from glucose to NADH to an electron trans- from glucose to oxygen port chain to oxygen. During the stepwise release of ATP ATP (Figure 6.9). During ATP chemical energy in the electron transport chain, our cellular respiration, the cells make most of their ATP. electrons gathered from food molecules It is actually oxygen, the “fall” in a stepwise cascade, losing energy at each step. “electron grabber,” at the end, In this way, cellular respiration unlocks chemical energy that makes it all possible. By in small amounts, bit by bit, that cells can put to pro- pulling electrons down the ductive use. The first stop in the path down the cascade transport chain from fuel is NAD+. The transfer of electrons from organic fuel molecules, oxygen functions (food) to NAD+ converts it to NADH. The electrons somewhat like gravity pulling have now taken one baby step down in their trip from objects downhill. This role glucose to oxygen. The rest of the cascade consists of an as a final electron acceptor is electron transport chain. how the oxygen we breathe Each link in an electron transport chain is actually a functions in our cells and molecule, usually a protein (shown as purple circles in why we cannot survive more Figure 6.9). In a series of reactions, each member of the than a few minutes without chain transfers electrons. With each transfer, the elec- it. Viewed this way, drowning trons give up a small amount of energy that can then be is deadly because it deprives used indirectly to generate ATP. The first molecule of cells of the final “electron the chain accepts electrons from NADH. Thus, NADH grabbers” (oxygen) needed to carries electrons from glucose and other fuel molecules drive cellular respiration. and deposits them at the top of an electron transport The molecules of electron chain. The electrons cascade down the chain, from transport chains are built molecule to molecule, like an electron bucket brigade. into the inner membranes The molecule at the bottom of the chain finally “drops” of mitochondria (see ▼ Figure 6.9 The role of oxygen in harvesting food energy. In cellular respiration, electrons “fall” in small steps from food to oxygen, producing water. NADH transfers electrons from food to an electron transport chain. The attraction of oxygen to electrons “pulls” the electrons down the chain. e– e– Electrons from food NAD+ e– e– NAD+ NADH Stepwise release ATP of energy used to make ATP 2 e– – – Electron transport chain 2 e– 1 2 O2 H 2O Hydrogens, electrons, 2 H+ and oxygen combine to form water 132 DESIGN SERVICES OF # 152561 Cust: Pearson Au: Simon Pg. No. 132 C/M/Y/K M06_SIMO2368_05_GE_CH06.indd 132 Title: EBP 5e Short / Normal S4CARLISLE Publishing Services 25/09/15 10:06 AM  Figure 4.18). Because these membranes are highly structures that act like turbines. Each of these minia- CELLULAR RESPIRATION: folded, their large surface area can accommodate thou- ture machines, called an ATP synthase, is constructed AEROBIC HARVEST OF FOOD ENERGY sands of copies of the electron transport chain—another from proteins built into the inner mitochondrial good example of how biological structure fits function. membrane, adjacent to the proteins of the electron Each chain acts as a chemical pump that uses the energy transport chains. Figure 6.10 shows a simplified view released by the “fall” of electrons to move hydrogen ions of how the energy previously stored in NADH and (H+) across the inner mitochondrial membrane. This FADH2 can now be used to generate ATP. NADH pumping causes ions to become more concentrated on and FADH2 transfer electrons to an electron trans- one side of the membrane than on the other. Such a dif- port chain. The electron transport chain uses this ference in concentration stores potential energy, similar energy supply to pump H+ across the inner mitochon- to the way water can be stored behind a dam. There drial membrane. Oxygen pulls electrons down is a tendency for the transport chain. The H+ concentrated on one hydrogen ions to side of the membrane rushes back “downhill” through gush back to where an ATP synthase. This action spins a component of the they are less con- ATP synthase, just as water turns the turbines in a dam. centrated, just as The rotation activates parts of the synthase mol- there is a tendency ecule that attach phosphate groups to ADP molecules for water to flow to generate ATP. downhill. The inner The poison cyanide produces its deadly effect by membrane tempo- binding to one of the protein complexes in the electron CHECKPOINT rarily “dams” hydro- transport chain (marked with a skull-and-crossbones What is the potential energy gen ions. symbol in Figure 6.10). When bound there, cyanide source that drives ATP production by ATP synthase? The energy of dammed water can be harnessed to blocks the passage of electrons to oxygen. This block- perform work. Gates in a dam allow the water to rush age is like clogging a dam. As a result, no H+ gradient is mitochondrion H+across the inner membrane of a downhill, turning giant turbines, and this work can be generated, and no ATP is made. Cells stop working, and Answer: a concentration gradient of used to generate electricity. Your mitochondria have the organism dies. ▼ Figure 6.10 How electron transport drives ATP synthase machines. Space H+ between membranes H+ H+ H+ H+ H+ H+ H+ Electron H+ carrier H+ H+ 3 H+ 5 H+ Protein complex Inner mitochondrial membrane – – FADH2 FAD Electron H+ flow 2 1 2 O2 + 2 H+ H 2O 6 – – 4 NADH NAD+ ADP + P ATP 1 + H+ H + H+ H H+ Matrix Electron transport chain ATP synthase 133 DESIGN SERVICES OF # 152561 Cust: Pearson Au: Simon Pg. No. 133 C/M/Y/K M06_SIMO2368_05_GE_CH06.indd Title: EBP 5e 133 Short / Normal S4CARLISLE Publishing Services 25/09/15 10:06 AM  CHAPTER 6 CELLULAR RESPIRATION: The Results of Cellular ▼ Figure 6.12 Energy from food. The monomers from OBTAINING ENERGY FROM FOOD Respiration carbohydrates (polysaccharides and sugars), fats, and proteins When taking cellular respiration apart to see how all can all serve as fuel for cellular respiration. the molecular nuts and bolts of its metabolic machin- Food ery work, it’s easy to lose sight of its overall function: to generate about 32 molecules of ATP per molecule of glu- cose (the actual number can vary by a few, depending on the organism and molecules involved). Figure 6.11 Carbohydrates Fats Proteins will help you keep track of the ATP molecules gener- ated. As we discussed, glycolysis and the citric acid cycle each contribute 2 ATP by directly making it. The rest of the ATP molecules are produced by ATP Sugars Glycerol Fatty acids Amino acids synthase, powered by the “fall” of electrons from food to oxygen. The electrons are carried from the organic fuel to electron transport chains by NADH and FADH2. Each electron pair “dropped” down a transport chain – Citric from NADH or FADH2 can power the synthesis of a few Glycolysis Acetyl Acid – CoA – ATP. You can visualize the process like this: Energy flows Cycle Electron from glucose to carrier molecules and ultimately to ATP. Transport Chain We have seen that glucose can provide the energy to make the ATP our cells use for all their work. All of the energy-consuming activities of your body—moving your ATP muscles, maintaining your heartbeat and temperature, and even the thinking that goes on within your brain—can be traced back to ATP and, before that, the glucose that But even though we have concentrated on was used to make it. The importance of glucose glucose as the fuel that is broken down during is underscored by the severity of diseases in About 20% of the cellular respiration, respiration is a versatile which glucose balance is disturbed. Diabetes, energy produced by metabolic furnace that can “burn” many CHECKPOINT which affects more than 20 million Ameri- your body each day is other kinds of food molecules. Figure 6.12 Which stage of cellular cans, is caused by an inability to properly regu- used to sustain diagrams some metabolic routes for the use respiration produces the late glucose levels in the blood due to problems your brain. of carbohydrates, fats, and proteins as fuel majority of ATP? with the hormone insulin. If left untreated, a glu- for cellular respiration. Taken together, all Answer: electron transport cose imbalance can lead to a variety of problems, of these food molecules make up your calorie- including cardiovascular disease, coma, and even death. burning metabolism. ▶ Figure 6.11 A summary of ATP yield during cellular Cytoplasm respiration. Mitochondrion – – – – – – 6 NADH 2 NADH 2 NADH – – 2 FADH2 – Glycolysis 2 – 2 Acetyl Citric – Glucose Pyruvic Electron CoA Acid acid Transport Chain Cycle Maximum per glucose: 2 2 About About ATP ATP 28 ATP 32 ATP by direct by direct by ATP synthesis synthesis synthase 134 DESIGN SERVICES OF # 152561 Cust: Pearson Au: Simon Pg. No. 134 C/M/Y/K M06_SIMO2368_05_GE_CH06.indd 134 Title: EBP 5e Short / Normal S4CARLISLE Publishing Services 25/09/15 10:06 AM  Fermentation: Anaerobic Harvest FERMENTATION: ANAEROBIC HARVEST OF FOOD ENERGY of Food Energy Although you must breathe to stay alive, some of your pyruvic acid. That isn’t very efficient compared with the cells can work for short periods without oxygen. This 32 or so ATP molecules each glucose molecule generates anaerobic (“without oxygen”) harvest of food energy is during cellular respiration, but it can energize muscles called fermentation. for a short burst of activity. However, in such situations your cells will have to consume more glucose fuel per second because so much less ATP per glucose molecule Fermentation in Human is generated under anaerobic conditions. To harvest food energy during glycolysis, NAD+ Muscle Cells must be present to receive electrons (see Figure 6.9). You know by now that as your muscles work, they This is no problem under aerobic conditions, because require a constant supply of ATP, which is generated by the cell regenerates NAD+ when NADH drops its cellular respiration. As long as your blood provides your electron cargo down electron transport chains to O2. muscle cells with enough O2 to keep electrons “falling” However, this recycling of NAD+ cannot occur under down transport chains in mitochondria, your muscles anaerobic conditions because there is no O2 to accept will work aerobically. the electrons. Instead, NADH disposes of electrons by But under strenuous conditions, your muscles can adding them to the pyruvic acid produced by glycolysis spend ATP faster than your bloodstream can deliver O2 ; (Figure 6.13). This restores NAD+ and keeps glycolysis when this happens, your muscle cells begin to work working. CHECKPOINT anaerobically. After functioning anaerobically for about The addition of electrons to pyruvic acid produces 15 seconds, muscle cells will begin to generate ATP by How many molecules of a waste product called lactic acid. The lactic acid by- ATP can be produced from the process of fermentation. Fermentation relies on product is eventually transported to the liver, where liver one molecule of glucose glycolysis, the first stage of cellular respiration. Gly- cells convert it back to pyruvic acid. Exercise physiolo- during fermentation? colysis does not require O2 but does produce two ATP gists have long speculated about the role that lactic acid Answer: two molecules for each glucose molecule broken down to plays in muscle fatigue, as you’ll see next. ▼ Figure 6.13 Fermentation: producing lactic acid. Glycolysis produces ATP even in the absence of O2. This process requires a continuous supply of NAD+ to accept electrons from glucose. The NAD+ is regenerated when NADH transfers the electrons it removed from food to pyruvic acid, thereby producing lactic acid (or other waste products, depending on the species of organism). INPUT OUTPUT 2 ADP 2 ATP +2 P Glycolysis – – – – 2 NAD+ 2 NADH 2 NADH 2 NAD+ 2 pyruvic acid + 2 H+ 2 lactic acid Glucose 135 DESIGN SERVICES OF # 152561 Cust: Pearson Au: Simon Pg. No. 135 C/M/Y/K M06_SIMO2368_05_GE_CH06.indd Title: EBP 5e 135 Short / Normal S4CARLISLE Publishing Services 25/09/15 10:06 AM  CHAPTER 6 CELLULAR RESPIRATION: Exercise Science THE PROCESS OF SCIENCE OBTAINING ENERGY FROM FOOD What Causes Muscle Burn? more rapidly, which is the opposite of what you would expect. Recent experiments have directly refuted Hill’s You may have heard that the burn you experience after hard conclusions. Research indicates that increased levels of exercise (“Feel the burn!”) is due to the buildup of lactic other ions may be to blame, and the role of lactic acid in acid in your muscles. This idea originated with the work of muscle fatigue remains a hotly debated topic. a British biologist named A.V. Hill. Considered one of the The changing view of lactic acid’s role in muscle fa- founders of the field of exercise physiology, Hill won a 1922 tigue illustrates an important point about the process Nobel Prize for his investigations of muscle contraction. of science: It is dynamic and subject to constant adjust- In 1929, Hill performed a classic experiment that ment as new evidence is uncovered. This would not have began with the observation that muscles produce surprised Hill, who himself observed that all scientific lactic acid under anaerobic conditions. Hill asked the hypotheses may become obsolete, and that changing question, Does the buildup of lactic acid cause muscle conclusions in light of new evidence is necessary for the fatigue? To find out, Hill developed a technique for advancement of science. electrically stimulating dissected frog muscles in a laboratory solution. He formed the hypothesis that a buildup of lactic acid would cause muscle ▼ Figure 6.14 A. V. Hill’s 1929 apparatus for measuring muscle fatigue. activity to stop. Hill’s experiment tested frog muscles under Battery Battery two different sets of conditions (Figure 6.14). First, he showed that muscle performance de- + – + – Force Force clined when lactic acid could not diffuse away measured measured from the muscle tissue. Next, he showed that when lactic acid was allowed to diffuse away, performance improved significantly. These results led Hill to the conclusion that the buildup of lactic acid is the primary cause of muscle failure under anaerobic conditions. Frog muscle Given his scientific stature (he was consid- stimulated by ered the world’s leading authority on muscle electric current activity), Hill’s conclusion went unchallenged for many decades. Gradually, however, evidence that contradicted Hill’s results began to accumulate. For example, the effect that Hill demonstrated did not appear to occur at human body tempera- Solution prevents Solution allows diffusion of lactic acid diffusion of lactic acid; ture. And certain individuals who are unable to muscle can work for accumulate lactic acid have muscles that fatigue twice as long Fermenta