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Chapter Contents The metabolic and Chemical Cycling in the Biosphere 92 processes that produce in Cellular Respiration: Aerobic Harvest of 94 your muscles after : Anaerobic Harvest of Food Energy 101 a hard workout are similar to the processes that produce , Why Cellular Respiration Matters soy sauce, , and bread.

You can survive for weeks without and days without drinking, but you can only live for minutes without . Why? Because every in your body relies on the energy created by using to break down during the process of cellular respiration.

You have something in common with a sports car: You both require an air intake system to burn The cells of your brain efficiently. burn through a quarter pound of glucose each day.

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SIMO1751_06_C06_PR4.indd 90 7/28/17 7:03 AM Chapter Thread Exercise Science

Biology and Society Getting the Most Out of Your Muscles 91

The Process of Science What Causes Muscle Burn? 102

Evolution Connection The Importance of Oxygen 103

Biology and Society Exercise Science

The science of exercise. Endurance athletes must Getting the Most Out of Your Muscles carefully monitor their efforts so that they maintain Serious athletes train extensively to reach the peak of their physical potential. A key aspect of ath- an aerobic pace over the letic conditioning involves increasing aerobic capacity, the ability of the heart and lungs to deliver long term. 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 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, is said to be aerobic. As your muscles

work harder, you breathe faster and more deeply to inhale O2. If you continue 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 scientists can 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, his or her best effort. If you work even harder and exceed your aerobic capacity, the demand for oxygen in your muscles will outstrip your body’s ability to deliver it; metabolism then becomes anaerobic. With insuffi-

cient O2, your muscle cells switch to an “emergency mode” in which they break down glucose very inefficiently and produce as a by-product. As lactic acid and other wastes accumulate, muscle activity is impaired. Your muscles can work under these conditions for only a few minutes before they give out. When this happens (sometimes called “hitting the wall”), your muscles cannot ­function, and you will likely collapse, unable to even stand. Every living depends on processes that provide energy. In fact, we need energy to walk, talk, and think—in short, to stay alive. The 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.

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SIMO1751_06_C06_PR4.indd 91 7/28/17 7:03 AM 6 Cellular Respiration: Obtaining Energy Energy Flow and Chemical Cycling from Food in the Biosphere

All requires energy. In almost all ecosystems on Earth, ones. Therefore, we must eat organic material to get our this energy originates with the sun. During ­ , and provide energy for life’s processes. the energy of is converted to the chemical energy Most ecosystems depend entirely on photosynthesis of and other organic (as we’ll ­discuss in for food. For this reason, biologists refer to and Checkpoint Chapter 7). Photosynthesis takes place in the other autotrophs as producers. , in con- What chemical ingredients of plants and algae, as well as in some . All trast, are consumers because they obtain their food by do plants require from the environment to synthesize depend on this conversion for food and more. eating plants or by eating animals that have eaten plants

their own food? You’re probably wearing clothing made of a product of ­(Figure 6.1). We animals and other heterotrophs depend

minerals photosynthesis—cotton. Most of our homes are framed on autotrophs for organic fuel and for the raw organic

◾ 2 2 O, and soil and O, H ,

Answer: CO Answer: with lumber, which is wood produced by photosynthetic materials we need to build our cells and tissues. trees. Even textbooks are printed on a material (paper) that can be traced to photosynthesis in plants. But from an ’s point of view, photosynthesis is ­primarily about Chemical Cycling between providing food. Photosynthesis and Cellular Respiration Producers and Consumers Within a , the chemical ingredients for photosynthesis

Plants and other autotrophs (“self-feeders”) are are (CO2), a gas that passes from the air into

that make all their own organic matter—including carbohy- a plant through tiny pores, and (H2O), absorbed from drates, , , and nucleic —from nutrients the soil by the plant’s roots. Inside cells, organelles that are entirely inorganic: carbon dioxide from the air and called chloroplasts use light energy to rearrange the water and minerals from the soil. In other words, autotrophs of these ingredients to produce sugars—most importantly

make their own food; they don’t need to eat to gain energy glucose (C6H12O6)—and other organic molecules (Figure 6.2). to power their cellular processes. In contrast, and You can think of chloroplasts as tiny solar-powered sugar

other animals are heterotrophs (“other-feeders”), organ- factories. A by-product of photosynthesis is oxygen gas (O2) isms that cannot make organic molecules from inorganic that is released through pores into the atmosphere.

▶ Figure 6.1 Producer and consumer. A koala ­(consumer) eating produced by a photosyn- thetic plant (producer).

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SIMO1751_06_C06_PR4.indd 92 7/28/17 7:03 AM A chemical process called cellular respiration uses and cellular respiration to burn them, while animals per- Energy Flow and

O2 to convert the energy stored in the chemical bonds form only cellular respiration.) Plants usually make more Chemical Cycling in the of sugars to another source of chemical energy called organic molecules than they need for fuel. This photosyn- Biosphere ATP. Cells expend ATP for almost all their work. In both thetic surplus provides material for the plant to grow or plants and animals, the production of ATP during cel- can be stored (as starch in potatoes, for example). Thus, lular ­respiration occurs mainly in the organelles called when you consume a , , or , you are ­mitochondria (see Figure 4.19). eating the energy reservoir that plants (if unharvested) You might notice in Figure 6.2 that energy takes a would have used to grow the following spring. one-way trip through an ecosystem, entering as sunlight People have always taken advantage of plants’ and exiting as heat. Chemicals, in contrast, are recycled. ­photosynthetic abilities by eating them. More Notice also in Figure 6.2 that the waste products of recently, engineers have managed to tap into Checkpoint

cellular respiration are CO2 and H2O—the very same this energy reserve to produce liquid biofuels, What is misleading about the ingredients used as inputs for photosynthesis. Plants primarily (see Chapter 7 for a dis- following statement? “Plants store chemical energy through photosynthesis cussion of biofuels). But no matter the end perform photosynthesis, whereas animals perform and then harvest this energy through cellu- product, you can trace the energy cellular respiration.”

lar respiration. (Note that plants and raw materials for growth

Sunlight energy does. life all fact in when

perform both photosynthesis enters ecosystem back to solar-powered photo- respiration, cellular perform mals

◾ -

to produce fuel molecules synthesis. ani only that implies It Answer:

◀ Figure 6.2 Energy flow and chemical cycling in ecosystems. Energy flows Photosynthesis (in chloroplasts) through an ecosystem, enter- converts light energy ing as sunlight and exiting as to chemical energy heat. In contrast, chemical C6H12O6 CO2 elements are recycled within Carbon dioxide Glucose an ecosystem. + + O2 H2O Oxygen Water

Figure Walkthrough Mastering Biology goo.gl/SvLydh

Cellular respiration (in mitochondria) harvests food energy to produce ATP

ATP drives cellular work

Heat energy exits ecosystem

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SIMO1751_06_C06_PR4.indd 93 7/28/17 7:03 AM 6 Cellular Respiration: Obtaining Energy Cellular Respiration: from Food Aerobic Harvest of Food Energy

We usually use the word respiration to mean breathing. between your blood and the outside air. Oxygen present in Although respiration on the organismal level should not be the air you inhale ­diffuses across the lining of your lungs

confused with cellular respiration, the two processes are and into your bloodstream.­ And the CO2 in your blood- closely related (Figure 6.3). Cellular respiration requires stream diffuses into your lungs and exits your body when

Checkpoint a cell to exchange two gases with you exhale. Every of CO2 that You have At both the organismal and its surroundings. The cell takes in you exhale was originally formed in something in cellular levels, respiration oxygen in the form of the gas O2. It one of the mitochondria of your body’s involves taking in the gas gets rid of waste in the form of the common with a cells. Internal engines, like ______and expelling the gas sports car: You gas carbon dioxide, or CO2. Res- the ones found in cars, use O2 (through

______. piration, or breathing, results in both require an the air intakes) to break down gaso-

◾ 2 2

; CO ; Answer: O Answer: air intake system the exchange of these same gases line. A cell also requires O2 to break to burn fuel down its fuel (see Figure 5.2). Cellular efficiently. respiration—a biological version of ▼ Figure 6.3 How breathing is related to cellular respiration. internal ­combustion—is the main way When you inhale, you breathe in O2. The O2 is delivered to your cells, where it is used in cellular respiration. Carbon dioxide, a waste that chemical energy is harvested from food and converted ­product of cellular respiration, diffuses from your cells to your blood to ATP energy (see Figure 5.6). Cellular respiration is an and travels to your lungs, where it is exhaled. aerobic process, which is just another 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

CO 2 An Overview of Cellular Respiration All living organisms depend on transformations of Lungs energy and matter. We see examples of such transforma- tions throughout the study of life, but few are as impor- tant as the conversion of energy in fuel (food molecules) O2 CO2 to a form that cells can use directly. Most often, the fuel molecule used by cells is glucose, a simple sugar (mono-

saccharide) with the formula C6H12O6 (see Figure 3.6). (Less often, other organic molecules are used to gain O energy.) This equation summarizes the transformation 2 CO 2 of glucose during cellular respiration:

Many steps C6H12O6 + 6 O2 6 CO2 6+ H2O + approx. 32ATP + heat

Muscle Cellular cells The series of arrows in this formula represents the fact respiration that cellular respiration consists of many chemical steps. A specific catalyzes each reaction in the pathway, more than two dozen reactions in all. In fact, these reactions constitute one of the most important metabolic pathways for nearly every eukaryotic cell: those found in plants, fungi, protists, and animals. This pathway provides the energy these cells need to maintain the functions of life.

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SIMO1751_06_C06_PR4.indd 94 7/28/17 7:03 AM The many chemical reactions that make up cellular hydrogen along with the electrons) that acts as a Cellular Respiration: respiration can be grouped into three main stages: shuttle carrying high-energy electrons from one area Aerobic Harvest ­, the cycle, and electron transport. of the cell to another. The third stage of cellular res- of Food Energy Figure 6.4 is a road map that will help you follow the piration is electron transport. Electrons captured three stages of respiration and see where each stage from food by the NADH formed in the first two stages occurs in your cells. During glycolysis, a molecule are stripped of their energy, a little bit at a time, of glucose is split into two molecules of a compound until they are finally combined with oxygen to form called . The for glycolysis are water. The proteins and other molecules that make up located in the . The (also ­electron transport chains are embedded within the called the Krebs cycle) completes the breakdown of inner membrane of the mitochondria. The transport

glucose all the way to CO2, which is then released as of electrons from NADH to oxygen releases the energy a waste product. The enzymes for the citric acid cycle your cells use to make most of their ATP. are dissolved in the fluid within mitochondria. Glycol- The overall equation for cellular respiration shows Checkpoint ysis and the citric acid cycle generate a small amount that the atoms of the reactant molecules glucose and oxy- of ATP directly. They generate much more ATP indi- gen are rearranged to form the products carbon dioxide Which stages of cellular respiration take place in the rectly, by reactions that transfer electrons from fuel and water. But don’t lose track of why this process occurs: mitochondria? Which stage + molecules to a molecule called NAD (nicotinamide The main function of cellular respiration is to generate takes place outside the

dinucleotide) that cells make from niacin, a ATP for cellular work. In fact, the process can produce mitochondria?

B . The electron transfer forms a molecule around 32 ATP molecules for each glucose molecule glycolysis transport; electron ◾

called NADH (the H represents the transfer of ­consumed. and cycle acid citric the Answer:

▶ Figure 6.4 A road map Mitochondria Cytoplasm for cellular respiration.

Cytoplasm

Animal cell Plant cell

Cytoplasm

High-energy electrons via carrier molecules

– Glycolysis – Citric – 2 Acid Electron Glucose Pyruvic Cycle Transport Chain acid

ATP ATP ATP

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SIMO1751_06_C06_PR4.indd 95 7/28/17 7:03 AM 6 half, forming two three-carbon molecules. Notice Cellular Respiration: The Three Stages of in Figure 6.5 that the initial split requires an energy Obtaining Energy “investment” of two ATP molecules per glucose. from Food Cellular Respiration The three-carbon molecules then donate ­high- Now that you have a big-picture view of cellular respira- energy electrons to NAD+, forming NADH. In tion, let’s examine the process in more detail. A small ver- addition to NADH, glycolysis also “banks” four ATP sion of Figure 6.4 will help you keep the overall process of ­molecules directly when enzymes transfer cellular respiration in plain view as we take a closer look groups from fuel molecules to ADP. Glycolysis thus at its three stages. produces a “profit” of two molecules of ATP per Stage 1: Glycolysis molecule of glucose (two invested, but four banked; this fact will become important during our discussion – The word glycolysis Citric Acid – of fermentation later). What remains of the ­fractured means “splitting of Glycolysis – Cycle glucose at the end of glycolysis are two molecules of sugar” (Figure 6.5), – – pyruvic acid. The pyruvic acid still holds most of the and that’s just what – Electron energy of glucose, and that energy is harvested happens. During ATP ATP Transport Chain in the second stage of cellular respiration, the citric glycolysis, a six-carbon acid cycle. ­glucose molecule is broken in ATP

▼ 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 + 2 ATP NAD P Key –– Carbon NADH

P Phosphate group

– High-energy electron Energy investment phase: Energy harvest phase: spends 2 ATP produces 4 ATP

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SIMO1751_06_C06_PR4.indd 96 7/28/17 7:03 AM INPUT OUTPUT Cellular Respiration: 2 Breakdown of the fuel Aerobic Harvest (from glycolysis) (to citric acid cycle) generates NADH of Food Energy –– NAD+ NADH CoA

◀ Figure 6.6 The link between glycolysis and the citric acid cycle: the conversion 1 Pyruvic acid loses Acetic acid 3 Acetic acid attaches of pyruvic acid to acetyl CoA. Remember a carbon as CO2 to Acetyl CoA that one molecule of glucose is split into two Pyruvic acid CO Coenzyme A molecules of pyruvic acid. Therefore, the 2 process shown here occurs twice for each starting glucose molecule.

Stage 2: CoA escorts the acetic acid into the first reaction of the The Citric Acid Cycle citric acid cycle. The CoA is then stripped and recycled. During glycolysis, one Citric The citric acid cycle finishes extracting the energy of Glycolysis Acid – molecule of glucose is Cycle – sugar by dismantling the acetic acid molecules all the way Electron – split into two molecules Transport Chain down to CO2 (Figure 6.7). Acetic acid joins a four-carbon of pyruvic acid. But acceptor molecule to form a six-carbon product called citric ATP ATP before pyruvic acid can be ATP acid (for which the cycle is named). For every acetic acid Checkpoint used by the citric acid cycle, molecule that enters the cycle as fuel, two CO2 mol- 1. Two molecules of what it must be “groomed”—converted to a form the citric acid ecules eventually exit as a waste product. Along the way, compound are produced cycle can use (Figure 6.6). First, each pyruvic acid the citric acid cycle harvests energy from the fuel. Some by glycolysis? Does this loses a carbon as CO2. This is the first of this waste prod- of the energy is used to produce ATP directly. However, the molecule enter the citric uct we’ve seen so far in the breakdown of glucose. The cycle captures much more energy in the form of NADH acid cycle? remaining fuel molecules, each with only two carbons and a second, closely related electron carrier called 2. Pyruvic acid must travel from the ______, where left, are called acetic acid (the acid that’s in ). FADH2. All the carbon atoms that entered the cycle as Electrons are stripped from these molecules and fuel are accounted for as CO exhaust, and the four-carbon glycolysis takes place, to 2 the ______, where the ­citric

transferred to another molecule of NAD+, forming more acceptor molecule is recycled. We have tracked only one

acid cycle takes place.

2. NADH. Finally, each acetic acid is attached to a mol- acetic acid molecule through the citric acid cycle here. But mitochondria cytoplasm;

ecule called coenzyme A (CoA), an enzyme derived from because glycolysis splits glucose in two, the citric acid cycle acid. acetic to converted first is

◾ 1. Pyruvic acid. No; it it No; acid. Pyruvic

the B vitamin pantothenic acid, to form acetyl CoA. The occurs twice for each glucose molecule that a cell. Answers:

▶ Figure 6.7 INPUT OUTPUT The citric acid cycle. Citric acid

1 Acetic acid 2 2 CO2

ADP + P ATP 3 Citric Acid Cycle –– 3 NAD+ NADH3 4

–– FAD FADH2 5 6 Acceptor molecule

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SIMO1751_06_C06_PR4.indd 97 7/28/17 7:03 AM 6 Stage 3: top of an . The molecule at the Cellular Respiration: Electron Transport bottom of the chain finally “drops” the electrons to Obtaining Energy During cellular respira- Citric oxygen. At the same time, oxygen picks up hydrogen, from Food Glycolysis Acid – tion, the electrons gath- Cycle – forming water. Electron – ered from food molecules Transport Chain The overall effect of all this transfer of electrons gradually “fall,” losing during cellular respiration is a “downward” trip for ATP ATP energy at each step. In this ATP electrons from glucose to NADH to an electron trans- way, cellular respiration port chain to oxygen. During the unlocks chemical energy in small amounts, bit by stepwise release of chemical bit, that cells can put to productive use. energy in the electron transport Electrons are transferred from glucose in food mol- chain, our cells make most of ecules to NAD+. This electron transfer converts NAD+ to their ATP. It is actually oxy- NADH. Then NADH releases two electrons that enter an gen, the “electron grabber,” electron transport chain, a series of electron carrier at the end, that makes it all molecules. This chain is like a bucket brigade, with each possible. By pulling elec- molecule passing an electron to the next molecule. With trons down the transport each exchange, the electron gives up a bit of energy. This chain from fuel molecules, downward cascade releases energy from the electron and oxygen functions some- uses it to make ATP (Figure 6.8). what like gravity pulling Let’s take a closer look at the path that electrons objects downhill. Because take (Figure 6.9). Each link in an electron transport oxygen is the final electron chain is a molecule, usually a (shown as acceptor, we cannot survive purple circles in Figure 6.9). In a series of reactions, more than a few minutes each member of the chain transfers electrons. With without breathing. Viewed each transfer, the electrons give up a small amount this way, drowning is deadly of energy that can then be used indirectly to generate because it deprives cells of ATP. The first molecule of the chain accepts electrons the final “electron grabbers” from NADH. Thus, NADH carries electrons from glu- ­(oxygen) needed to drive cellular cose and other fuel molecules and deposits them at the respiration.

▶ Figure 6.8 Cellular respiration ▼ Figure 6.9 The role of oxygen in harvesting food energy. In cellular respira- illustrated using a hard-hat ­analogy. tion, electrons “fall” in small steps from food to oxygen, producing water. NADH Splitting the bonds of a molecule transfers electrons from food to an electron transport chain. The attraction of of glucose provides the energy that Electron released oxygen to electrons “pulls” the electrons down the chain. from NADH boosts an electron from a low-energy ATP – – state to a high-energy state. e e Electrons from food NAD+ e– e– e– NAD+ NADH

e– ATP – e– e

e– – Electron e transport e– chain Electron transport chain – e 2 e– 1 O 2 2

Glucose H2O

Hydrogens, electrons, + 2 H and oxygen combine to form water

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SIMO1751_06_C06_PR4.indd 98 7/28/17 7:03 AM The molecules of electron transport chains are downhill, turning giant turbines, and this work can be Cellular Respiration: built into the inner membranes of mitochondria (see used to generate electricity. Your mitochondria have Aerobic Harvest ­Figure 4.19). Because these membranes are highly structures that act like turbines. Each of these miniature of Food Energy folded, their large surface area can accommodate machines, called an ATP synthase, is constructed from thousands of copies of the electron transport chain— proteins built into the inner mitochondrial membrane, a good example of how biological structure fits adjacent to the proteins of the electron transport chains. ­function. Each chain acts as a chemical pump that uses Figure 6.10 shows a simplified view of how the energy

the energy released by the “fall” of electrons to move previously stored in NADH and FADH2 can now be used + hydrogen ions (H ) across the inner mitochondrial mem- to generate ATP. NADH and FADH2 transfer elec- brane. This pumping causes ions to become more concen- trons to an electron transport chain. The electron trated on one side of transport chain uses this energy supply to pump H+ the membrane than across the inner mitochondrial membrane. Oxygen on the other. Such a pulls electrons down the transport chain. The H+ difference in concen- concentrated on one side of the membrane rushes back tration stores poten- “downhill” through an ATP synthase. This action spins a tial energy, similar component of the ATP synthase, just as water turns the to the way water can turbines in a dam. The rotation activates parts of the be stored behind a synthase molecule that attach phosphate groups to ADP dam. There is a ten- molecules to generate ATP. dency for hydrogen The produces its deadly effect by ions to gush back to ­binding to one of the protein complexes in the electron Checkpoint where they are less transport chain (marked with a skull-and-crossbones concentrated, just as symbol in Figure 6.10). When bound there, cyanide What is the potential energy source that drives ATP there is a tendency for water to flow downhill. The inner blocks the passage of electrons to oxygen. This blockage is

­production by ATP synthase?

membrane temporarily “dams” hydrogen ions. like ­clogging the outflow channel of a dam. As a result, no mitochondrion a

across the inner membrane of of membrane inner the across + H of

The energy of dammed water can be harnessed to H gradient is generated, and no ATP is made. Cells stop + ◾

perform work. Gates in a dam allow the water to rush working, and the organism dies. gradient concentration a Answer:

▼ Figure 6.10 How electron transport drives ATP synthase machines. Mitochondrial membranes 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 Mitochondrion 1 O + H OH O 2 2 + 2 H 2 6 –– 4 NADH NAD+ ADP + P ATP 1 H+ H+ H+ H+ H+

Matrix Electron transport chain ATP synthase

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SIMO1751_06_C06_PR4.indd 99 7/28/17 7:03 AM 6 ▼ Figure 6.12 Energy from Cellular Respiration: The Results of food. The monomers from Obtaining Energy (polysaccharides from Food Cellular Respiration and sugars), , 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- ery work, it’s easy to lose sight of its overall function: to Food 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 will help you keep track of the ATP molecules gener- Carbohydrates Fats Proteins 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 syn- thase, powered by the “fall” of electrons from food to Sugars Fatty acids Amino acids 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 from

NADH or FADH2 can power the synthesis of a few ATP. – Acetyl Citric – You can visualize the process like this: Energy flows from Glycolysis Acid CoA – glucose to carrier molecules and ultimately to ATP. Cycle Electron We have seen that glucose can provide the energy Transport Chain to make the ATP our cells use for all their work. All of the energy-consuming activities of your body—­ The cells of moving your muscles, maintaining your heart- ATP your brain beat and temperature, and even the thinking burn through a that goes on within your brain—can be traced But even though we have concentrated on glucose as quarter pound back to ATP and, before that, the glucose that the fuel that is broken down during cellular respiration, of glucose was used to make it. The importance of glucose respiration is a versatile metabolic furnace that can “burn” each day. is underscored by the severity of diseases in many other kinds of food molecules. Figure 6.12 diagrams which glucose balance is disturbed. Diabetes, some metabolic routes for the use of carbohydrates, fats, which affects more than 20 million Americans, is caused and proteins as fuel for cellular respiration. Taken together, Checkpoint by an inability to properly regulate glucose levels in the all of these food molecules make up your -burning Which stage of cellular blood due to problems with the hormone insulin. If left metabolism. The interplay between these pathways ­respiration produces the untreated, a glucose imbalance can lead to a variety of provides a clear example of the theme of system inter­

majority of ATP? problems, including cardiovascular disease, coma, and actions; in this case, all of these interactions contribute ◾

Answer: electron transport electron Answer: even death. to maintaining a balanced 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 Citric Glucose Pyruvic Acetyl – Acid Electron acid CoA Cycle Transport Chain Maximum per glucose:

2 2 About About ATP ATP 28 ATP 32 ATP

by direct by direct by ATP synthesis synthesis synthase 100

SIMO1751_06_C06_PR4.indd 100 7/28/17 7:03 AM Fermentation: Fermentation: Anaerobic Harvest of Food Energy Anaerobic Harvest of Food Energy

Although you must breathe to stay alive, some of your compared with the 32 or so ATP molecules each glucose cells can work for short periods without oxygen. This molecule generates during cellular respiration, but it anaerobic (“without oxygen”) harvest of food energy is can energize muscles for a short burst of activity. How- called fermentation. ever, in such situations your cells will have to consume more glucose fuel per second because so much less ATP per glucose molecule is generated under anaerobic con- Fermentation in ditions. Fermentation in muscle cells may be a vestige of evolution (see the Evolution Connection section) Human Muscle Cells and provides a survival advantage even today. You know by now that as your muscles work, they require To harvest food energy during glycolysis, NAD+ must a constant supply of ATP, which is generated by cellular be present to receive electrons (see Figure 6.9). This is respiration. As long as your blood provides your muscle no problem under aerobic conditions because the cell + cells with enough O2 to keep electrons “falling” down regenerates NAD when NADH drops its electron cargo

transport chains in mitochondria, your muscles will work down electron transport chains to O2. However, this aerobically. recycling of NAD+ cannot occur under anaerobic con-

But under strenuous conditions, your muscles can ditions because there is no O2 to accept the electrons. spend ATP faster than your bloodstream can deliver Instead, NADH disposes of electrons by adding them to

O2 ; when this happens, your muscle cells begin to the pyruvic acid produced by glycolysis (Figure 6.13). work anaerobically. After functioning anaerobically This restores NAD+ and keeps glycolysis working. for about 15 seconds, muscle cells will begin to gener- The addition of electrons to pyruvic acid produces Checkpoint ate ATP by the process of fermentation. ­Fermentation a waste product called lactic acid. The lactic acid by-­ How many molecules of ATP relies on glycolysis, the first stage of cellular respira- product is eventually transported to the , where liver can be produced from one

tion. Glycolysis does not require O2 but does produce cells convert it back to pyruvic acid. Exercise scientists molecule of glucose during

two ATP molecules for each glucose molecule bro- have long speculated about the role that lactic acid plays fermentation? ◾

ken down to pyruvic acid. That isn’t very efficient in muscle fatigue, as you’ll see next. two Answer:

▼ 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+ NADH2 NADH2 2 NAD+

2 Pyruvic acid + + 2 H 2 Lactic acid Glucose

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SIMO1751_06_C06_PR4.indd 101 7/28/17 7:03 AM 6 Cellular Respiration: The Process of Science Exercise Science Obtaining Energy from Food What Causes Muscle Burn?

Background fatigue more rapidly, which is the opposite of what you You probably know that your muscles burn after hard would expect. Recent experiments have directly refuted exercise (“Feel the burn!”). But what causes the burn? Hill’s conclusions. Some research indicates that increased This question was investigated by one of the founders levels of other ions may be to blame. Therefore, the cause of the field of exercise science, a British biologist named of muscle fatigue remains hotly debated. A.V. Hill. In fact, Hill won a 1922 Nobel Prize for his The changing view of lactic acid’s role in muscle ­investigations of muscle contraction. fatigue illustrates an important point about the process Hill knew that muscles produce lactic acid under of science: Scientific belief is dynamic and subject to anaerobic conditions. In 1929, Hill developed a technique constant adjustment as new evidence is uncovered. This for electrically stimulating dissected frog muscles in a would not have surprised Hill, who himself observed that laboratory solution. He wondered if a buildup of lactic all scientific hypotheses may become obsolete, and that acid would cause muscle activity to stop. changing conclusions in light of new evidence is neces- sary for the advancement of science. Method Hill’s experiment tested frog muscles under two ▼ Figure 6.14 A. V. Hill’s 1929 apparatus for measuring muscle fatigue. different sets of conditions (Figure 6.14). First, he showed that muscle performance declined Battery Battery

when lactic acid could not diffuse away from the + – + – . Next, he showed that when lactic Force Force measured measured 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 anaer- obic conditions.

Results Thinking Like a Scientist Given his scientific stature—he was ­considered Frog muscle stimulated by The explanation for the the world’s leading authority on muscle electric current cause of muscle burn has ­activity—Hill’s conclusion went unchallenged changed over time as new for many decades. Gradually, however, evidence evidence has accumulated. that contradicted Hill’s results began to accu- Does this represent a mulate. For example, the effect that Hill demon- failure of the scientific strated did not appear to occur at human body Solution prevents Solution allows temperature. And certain people who are unable diffusion of lactic acid diffusion of lactic acid; method? muscle can work for For the answer, see Appendix D. to accumulate lactic acid have muscles that twice as long

Fermentation in Our muscles cannot rely on for sharp or sour mainly to lactic acid. The very long. However, the two ATP molecules produced per also uses fermentation to produce soy sauce from , glucose molecule during fermentation are enough to sustain to pickle , , and , and to produce many microorganisms. products like sausage, pepperoni, and salami. The metabolic processes that We have domesticated , a microscopic fungus, is capable of both cel- produce acid in your muscles after such microbes to trans- lular respiration and fermentation. When kept in an a hard workout are similar to the form milk into cheese, anaerobic environment, yeast cells ferment sugars and processes that produce pepperoni, sour cream, and yogurt. other to stay alive. As they do, the yeast produce soy sauce, yogurt, and bread. These foods owe their ethyl as a waste product instead of lactic acid

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SIMO1751_06_C06_PR4.indd 102 7/28/17 7:03 AM (Figure 6.15). This alcoholic fermentation also releases the yeast to ferment glucose into ethanol. And as every Fermentation:

CO2. For thousands of years, people have put yeast to work baker knows, the CO2 bubbles from fermenting yeast also Anaerobic Harvest producing alcoholic beverages such as beer and using cause bread dough to rise. (The alcohol produced in fer- of Food Energy airtight barrels and vats, where the lack of oxygen forces menting bread is burned off during baking.) Checkpoint

▼ Figure 6.15 Fermentation: producing ethyl alcohol. The alcohol What kind of acid builds produced by yeast as bread rises is burned off during baking. up in human muscle during

strenuous activity?

◾ INPUT OUTPUT acid lactic Answer: 2 ADP 2 ATP + 2 P P 2 CO2 released Glycolysis

–– –– 2 NAD+ NADH2 NADH2 2 NAD+ 2 Ethyl alcohol 2 Pyruvic acid + 2 H+ Glucose

Evolution Connection Exercise Science

The Importance of Oxygen In the Biology and Society and the Process of Science sec- the pathway does not require any of the membrane-enclosed tions, we were reminded of the important role that oxygen organelles of the eukaryotic cell, which evolved about a bil- plays during . But in the last section on lion years after the prokaryotic cell. Glycolysis is a metabolic fermentation, we learned that exercise can continue on a heirloom from early cells that continues to function in fer- limited basis even under anaerobic (oxygen-free) condi- mentation and as the first stage in the breakdown of organic tions. Both aerobic and start with molecules by cellular respiration. The ability of our muscles glycolysis, the splitting of glucose to form pyruvic acid. to function anaerobically can therefore be viewed as a ves- Glycolysis is thus the universal energy-harvesting process tige of our ancient ancestors, who relied exclusively on this of life. If you looked inside a bacterial cell, one of your body . cells, or any other living cell, you would find the metabolic machinery of glycolysis. Atmospheric The universality of glycolysis in life on Earth has an oxygen First ­evolutionary basis. Ancient prokaryotes probably used reaches 10% eukaryotic ▼ Figure 6.16 A time line of of modern levels organisms ­glycolysis to make ATP long before oxygen and life on Earth. oxygen was present in Earth’s Oldest Atmospheric Origin of prokaryotic oxygen first atmosphere. The oldest known fos- Earth fossils appears sils of date back more than 3.5 billion years, and they resem- ble some types of photosynthetic 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 bacteria still found today. Evidence Billions of years ago

indicates, however, that significant levels of O2, formed as a by-product of bacterial photosynthesis, did not accumu- late in the atmosphere until about 2.7 billion years ago. (Figure 6.16). For almost 1 billion years, prokaryotes must have generated ATP exclusively from glycoly- sis, a process that does not require oxygen. The fact that glycolysis occurs in almost all organisms suggests that it evolved very early in ancestors common to all the domains of life. The location of glycolysis within the cell also implies great antiquity; 103

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Summary of Key Concepts The Results of Cellular Respiration You can follow the flow of molecules through the process of cellular ­respiration in the diagram. Notice that the first two stages primarily produce Energy Flow and Chemical Cycling high-energy electrons carried by NADH and that it is the final stage that in the Biosphere uses these high-energy electrons to produce the bulk of the ATP molecules ­produced during cellular respiration. Producers and Consumers Autotrophs (producers) make organic molecules from inorganic nutrients by Mitochondrion photosynthesis. Heterotrophs (consumers) must consume organic material and obtain energy by cellular respiration. O2

Chemical Cycling between Photosynthesis –– – – – – and Cellular Respiration 6 NADH 2 NADH 2 NADH The molecular outputs of cellular respiration—CO and H O—are the –– 2 2 2 FADH molecular inputs of photosynthesis, and vice versa. While these chemicals 2 cycle through an ecosystem, energy flows through, entering as sunlight and Glycolysis 2 exiting as heat. 2 Citric Acetyl – Pyruvic Acid Glucose CoA – acid Cycle – Heat Electron C H O 6 12 6 Transport Chain + 2 CO 4 CO 2 2 H2O Sunlight O2 ATP 2 by direct by direct 2 by ATP About ATP synthesis synthesis ATP synthase Cellular 28 ATP Photosynthesis respiration

CO2 + H2O

About 32 ATP Cellular Respiration: Aerobic Harvest of Food Energy Fermentation: An Overview of Cellular Respiration Anaerobic Harvest of Food Energy The overall equation of cellular respiration simplifies a great many chemical steps into one formula: Fermentation in Human Muscle Cells

When muscle cells consume ATP faster than O2 can be supplied for cellular respiration, the conditions become anaerobic, and muscle cells will begin to regenerate ATP by fermentation. The waste product under these anaerobic C6H12O6 + 6 O2 6 CO2 6+H2O + Approx. 32 ATP conditions is lactic acid. The ATP yield per glucose is much lower during ­fermentation (2 ATP) than during cellular respiration (about 32 ATP).

The Three Stages of Cellular Respiration Fermentation in Microorganisms Cellular respiration occurs in three stages. During glycolysis, a molecule of Yeast and some other organisms can survive with or without O2. Wastes glucose is split into two molecules of pyruvic acid, producing two molecules from fermentation can be ethyl alcohol, lactic acid, or other compounds, of ATP and two high-energy electrons stored in NADH. During the citric depending on the species.

acid cycle, what remains of glucose is completely broken down to CO2, producing a bit of ATP and a lot of high-energy electrons stored in NADH

and FADH2. The electron transport chain uses the high-energy electrons Mastering Biology to pump H+ across the inner mitochondrial membrane, eventually hand- For practice quizzes, BioFlix animations, MP3 tutorials, video tutors, and more + ing them off to O2, producing H2O. Backflow of H across the membrane study tools designed for this textbook, go to Mastering Biology™ ­powers the ATP synthases, which produce ATP from ADP.

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Self-Quiz The Process of Science

1. Which of the following statements is a correct distinction between 14. Your body makes NAD+ from the niacin and . The autotrophs and heterotrophs? recommended daily allowances are 20 mg for niacin and 1.7 mg for a. Only heterotrophs require chemical compounds from the riboflavin, thousands of times less than the amount of glucose needed environment. each day. How many NAD+ molecules are needed to break down each b. Cellular respiration is unique to heterotrophs. glucose molecule? Why are the requirements for niacin and riboflavin c. Only heterotrophs have mitochondria. so small? d. Only autotrophs can live on nutrients that are entirely inorganic. 15. Interpreting Data (BMR) is the amount of energy 2. Why are plants called producers? Why are animals called consumers? that must be consumed by a person at rest to maintain his or her body 3. How is your breathing related to your cellular respiration? weight. BMR depends on several factors, including sex, age, height, and 4. Of the three stages of cellular respiration, which produces the most ATP weight. The following graph shows the BMR for a 6r0s 45-year-old male. molecules per glucose? For this person, how does BMR correlate with weight? How many more must a 250-pound man consume to maintain his weight than a 5. The final of electron transport chains in mitochondria 200-pound man? Why does BMR depend on weight? is ______. 6. The poison cyanide acts by blocking a key step in the electron transport BMR by weight chain. Knowing this, explain why cyanide kills so quickly. 7. Cells can harvest the most chemical energy from which of the following? 2100 2050 a. an NADH molecule c. six CO2 molecules b. a glucose molecule d. two pyruvic acid molecules 2000 8. ______is a metabolic pathway common to both fermentation and 1950 cellular respiration. 1900 1850 9. Exercise scientists at an Olympic training center want to monitor 1800 athletes to determine at what point their muscles are functioning BMR (calories) 1750 anaerobically. They can do this by checking for a buildup of 1700 a. ADP. c. carbon dioxide. 1650 b. lactic acid. d. oxygen. 1600 10. A glucose-fed yeast cell is moved from an aerobic environment to an 150 160 170 180 190 200 210 220 230 240 250 anaerobic one. For the cell to continue to generate ATP at the same rate, Pounds approximately how much glucose must it consume in the anaerobic environment compared with the aerobic environment? For answers to the Self-Quiz, see Appendix D. Biology and Society

Identifying Major Themes 16. Delivery of oxygen to muscles is the limiting factor for many athletes. Some try blood doping (injecting oxygenated blood). Others train at high For each statement, identify which major theme is evident (the relationship of altitude. Both approaches increase the number of red blood cells. Why is structure to function, information flow, pathways that transform energy and doping considered cheating? How would you enforce antidoping rules? matter, interactions within biological systems, or evolution) and explain how the 17. The technology of fermentation dates back to the earliest civilizations. statement relates to the theme. If necessary, review the themes (see Chapter 1) Suggest a hypothesis for how it was discovered. and review the examples highlighted in blue in this chapter. 18. Alcohol consumption during pregnancy can cause a series of birth defects 11. The highly folded membranes of the mitochondria make these called fetal alcohol syndrome (FAS). Symptoms include head and facial organelles well suited to carry out the huge number of chemical irregularities, heart defects, intellectual disability, and behavioral problems. reactions required for cellular respiration to proceed. The U.S. Surgeon General’s Office recommends that pregnant women 12. Cellular respiration and photosynthesis are linked, with each process abstain from alcohol. Also, liquor bottles have a warning label. If you were using inputs created by the other. a server in a restaurant and a pregnant woman ordered a daiquiri, how 13. Your body uses many different intersecting chemical pathways that, all would you respond? Is it her right to make those decisions about her unborn together, constitute your metabolism. child’s health? Do you bear any responsibility? Is a restaurant responsible for For answers to Identifying Major Themes, see Appendix D. monitoring dietary habits of customers?

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