Chapter 7

The Working Cell: Energy From Food

Concept 7.1
Concept 7.2
Concept 7.3
Concept 7.4
Concept 7.5
Concept 7.6
Chapter 7 Review

Nectar provides a hummingbird with the energy it needs to fly, breathe, build a nest, and carry out other life processes. In this chapter, you will learn how organisms release the energy stored in food. Before you start the chapter, go online to compare different daily calorie intakes and activity levels in the WebQuest. What is the connection between the food you eat and the energy you burn?

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Concept 7.1: Sunlight Powers Life


  • Trace the path of energy from sunlight to your cells.
  • Explain the relationship between photosynthesis and cellular respiration.


  • photosynthesis
  • cellular respiration


    Concept 7.1
    Watch sun power in action.

    You can trace the energy stored in foods, such as the juicy apples in the orchard below, all the way back to the sun. Humans and other animals depend on plants to convert the energy of sunlight into a chemical form of energy they can use – food molecules such as sugars.

    Light Energy to Food Energy

    Look up in the daytime sky, and you will see the star we call the sun burning brightly almost 150 million kilometers away. A tiny portion of the light energy radiated from the sun reaches Earth. And a tiny fraction of the sunlight shining on Earth powers life. The leaves of the apple trees and grasses in Figure 7.1 trap some of the energy of sunlight and use it to power the production of sugars and other food molecules. This process is called photosynthesis. Photo means "light," and in photosynthesis photo refers to light energy from the sun. To synthesize means "to make something." In photosynthesis, it is food that is made. Putting this all together, photosynthesis uses light energy to make food.

    Our Dependence on Plants

    On land, photosynthesis occurs mainly in green cells within the leaves of plants. The chemical ingredients for photosynthesis are water and carbon dioxide. Water enters the plant when it is absorbed from damp soil by the roots. The plant's veins then transport the water from the roots to the leaves. Carbon dioxide is a gas found in the surrounding air. It enters the plant through tiny pores in the plant's leaves. The green cells within the leaves rearrange the atoms of these ingredients to produce a sugar called glucose and oxygen. Energy is required for this process, and sunlight provides that energy.

    A plant uses some of the food it produces as its own fuel supply. However, food molecules cannot directly power cells. A process called cellular respiration harvests the energy stored in food to make molecules called adenosine triphosphate (ATP). Working cells then use this ATP as their main power supply.

    Notice in Figure 7.2 that the products of cellular respiration are carbon dioxide and water. They are also the ingredients for photosynthesis. Thus, plants recycle chemicals as they store (through photosynthesis) and harvest (through respiration) food energy. However, plants usually make more food than they need for fuel. This surplus food provides the organic material needed for the plant to grow. It is also the source of food for humans and other animals.

    Plants perform both photosynthesis and cellular respiration. Animals perform cellular respiration, but not photosynthesis. Humans and other animals must obtain their food by eating plants or by eating animals that ate plants. Animals depend on food not only for fuel, but also for the raw building materials used to make cells and tissues. If you analyze almost any food chain, the energy and raw materials for growth can be traced back to photosynthesis. You could say that life on Earth is solar-powered.

    Online Concept 7.1

    Watch sun power in action.

    Explore the animated apple tree to see how sunlight powers life. How are photosynthesis and cellular respiration related?

    Concept Check 7.1

    1. What is photosynthesis?
    2. In what way is your body solar-powered?
    3. What chemical ingredients does a plant use to make food?
    4. What are the products of cellular respiration?
    5. Why is the following statement misleading? "Plants do photosynthesis, but animals do cellular respiration."

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    Concept 7.2 Food stores chemical energy.


  • Give examples of energy being converted from one form to another.
  • Use calories as units of energy.


  • energy
  • kinetic energy
  • potential energy
  • chemical energy
  • calorie


    Concept 7.2
    Burn a marshmallow.

    Lab 7.1
    Investigating Chemical Energy Stored in Food

    You know from the personal experience of feeling hungry that you need food for energy. But what exactly is energy? How is it stored in the chemicals that make up food?

    Introduction to Energy

    Energy is defined as the capacity to perform work. Work is performed whenever an object is moved against an opposing force. In other words, work moves things in directions they cannot move if left alone. For example, your leg muscles do work when you climb the steps to the top of a water slide– your legs move your body against the opposing force of gravity. You expend food energy to do the work of climbing.

    While you climb the stairs, food energy is converted to kinetic energy, the energy of motion, and potential energy, the energy of position. Anything that is moving has kinetic energy. In fact, the word kinetic is derived from a Greek word meaning "motion." Potential energy, on the other hand, is energy that is stored and held in readiness to do work. It is stored energy based on an object's position. As you climb to a higher position against the force of gravity, your body gains potential energy.

    What happens to the energy once you reach the top of the stairs? Because you are now standing still, your kinetic energy is zero. Is the energy gone? The answer is no. It is not possible to destroy or create energy. However, energy can be converted from one form to another. By climbing the stairs, you converted kinetic energy to potential energy. You now possess the potential energy of a higher position. Your potential energy is converted back to kinetic energy as you slide down (Figure 7.3).

    After you come to a stop at the bottom of the slide, where has the energy gone? It has all been converted to heat energy. Because heat is a type of kinetic energy, the energy hasn't disappeared. Heat is the motion of atoms and molecules. Sliding generated this heat energy in the material of the slide, in your swimsuit, in the water, and even in the surrounding air. Most of that heat was generated by friction between you and the slide. In fact, all energy conversions generate some heat. Although generating heat does not destroy energy, it does make the energy less useful. Of all energy forms, heat is the most difficult to harness to do useful work. The trip up and down the water slide was fun while it lasted, but all of the energy you expended has now been converted to heat energy. To climb up the water slide again, you must use more food energy.

    A Closer Look at Chemical Energy

    How do the chemicals of food provide energy for a climb up a water slide? The molecules of food, gasoline, and other fuels have a form of potential energy called chemical energy. Remember, potential energy is energy stored because of position. In the case of chemical energy, the potential to perform work is due to the positions of the atoms within the fuel molecules. Put another way, chemical energy is based on the structure of molecules. Organic molecules, such as the carbohydrates, fats, and proteins you learned about in Chapter 5, have structures that make them especially rich in chemical energy (Figure 7.4).

    Figure 7.5 compares potential energy based on heightened position and chemical structure (chemical energy). In the case of chemical energy, it is the breakdown of complex fuel molecules into simpler molecules that releases the potential energy. This energy is then available for work in your cells.

    Online Lab 7.1

    Investigating Chemical Energy Stored in Food

    How much energy does a peanut contain (in calories)? What kind of experiment would you design to find out? Go online to learn about burning a peanut in your lab.

    Putting Chemical Energy to Work

    Both cells and automobile engines use the same basic process to make the chemical energy stored in fuel available for work. In both cases, an organic fuel is broken into smaller-sized molecules that possess far less chemical energy than the fuel molecules did (Figure 7.6).

    An automobile engine (called an internal combustion engine) mixes oxygen with gasoline in an explosive chemical reaction that breaks down the fuel molecules. The force of the explosions powers the car. The waste products emitted from the exhaust pipe of the car are mostly carbon dioxide and water. Only about 25 percent of the energy extracted from the gasoline is converted into the car's kinetic energy (motion). The rest is lost as heat energy, which explains why it is very hot under the hood of a car.

    Within your cells, fuel (food) also reacts with oxygen in the process of cellular respiration. And similarly to an automobile engine, working cells emit carbon dioxide and water as their "exhaust." Fortunately, the process is a slow, gradual "burn" rather than an explosive one. This is one reason your cells are more efficient than automobiles. You convert about 40 percent of food energy into useful work, such as the contraction of muscles. Nevertheless, cellular respiration does generate considerable heat (the other 60 percent of the energy released by the breakdown of the fuel molecules). This explains why you feel so warm after running, inline skating, or dancing vigorously. Sweating and other cooling mechanisms enable your body to lose the excess heat, much as a car's radiator keeps the engine from overheating.

    The heat energy generated by cellular respiration is not completely useless. By retaining some of this heat, your body is able to maintain a constant, warm temperature, even when the surrounding air is cold. But even when sitting still in a room, you radiate about as much heat as a 100-watt lightbulb. You've probably experienced the discomfort of this heat while sitting in a closed room crowded with other "human lightbulbs."

    Calories: Units of Energy

    You have probably heard the term calorie used to refer to food or exercise. Scientifically, a calorie is the amount of energy that can raise the temperature of one gram (g) of water by one degree Celsius (ƒC). A calorie is such a tiny unit of energy that it is not very practical for measuring the energy content of food. Instead, the kilocalorie (kCal), which equals 1,000 calories, is often used. The "calories" shown on a food package are actually kilocalories.

    You can measure the calorie content of a food, such as a peanut, in the lab. All you need to do is dry the food and burn it under an insulated flask containing water, as shown in Figure 7.7. This converts the stored chemical energy to heat energy. By measuring the increase in water temperature and using the definition of a calorie, you can calculate the number of calories in the peanut. A peanut has about 5,000 calories, or 5 kilocalories. That is enough chemical energy to increase the temperature of 100 g of water by about 50ƒC.

    Of course, your cells don't burn fuel molecules from peanuts or other foods in the manner of Figure 7.7. Cells break down their fuel through the more controlled process of cellular respiration. The released energy is easier to manage for work as a result. As shown in Table 7.1, just a handful of peanuts provides enough fuel to power an hour-long walk.

    Table 7.1: Energy Consumed by Various Activities (kCals)
    Kilocalories Consumed per Hour by a 67.5-kg (150-lb) Person*
    *Not including energy necessary for body maintenance
    Bicycling (racing)
    Bicycling (slowly)
    Dancing (slow)
    Dancing (fast)
    Running (7 min/mi)
    Sitting (writing)
    Sleeping or lying still
    Swimming (2 mph)
    Walking (3 mph)

    Online Concept 7.2
    Burn a marshmallow.
    What is the difference between burning a marshmallow and eating it? Go online and compare the results.

    Concept Check 7.2

    1. Describe the energy conversions that occur when you lift a book from a table and place it on a high shelf.
    2. Explain why vigorous exercise generates so much body heat.
    3. Which form of energy is the most difficult to put to work? Why?

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    Concept 7.3 A molecule called ATP powers work in your cells.


  • Give examples of work that cells perform.
  • Explain how ATP drives work in cells.


  • ATP


    Concept 7.3
    See how ATP powers work.

    It's a good thing that food doesn't fuel your cells by burning like the peanut torched in Figure 7.7. In fact, the carbohydrates, fats, and other fuel molecules obtained from food do not drive work in your cells in any direct way. The food energy must first be converted to ATP energy. Energy is required for all forms of work. ATP provides that energy for the work that cells do.

    How ATP Packs Energy

    The initials ATP stand for adenosine triphosphate. This molecule consists of a complex organic molecule called adenosine attached to a tail of three phosphate groups. The tail is the "business" end of ATP; it is the direct source of energy used for most cellular work. The phosphate group at the very tip of the tail can break away. Loss of this phosphate group from the triphosphate tail makes energy available for working cells (Figure 7.8).

    Phosphate groups have a negative electrical charge. Because like charges repel, the crowding of negative charge in the triphosphate tail contributes to the potential energy stored in ATP. It is like storing energy by compressing a spring. When a compressed spring relaxes, its potential energy is released and the spring can perform work. The springy bond in Figure 7.8 symbolizes this analogy. In the case of ATP, the loss of a phosphate group releases energy for work. After losing a phosphate group, the tail of the molecule has only two phosphates left. It is now called adenosine diphosphate, or ADP.

    ATP and Cellular Work

    When ATP breaks up, phosphate groups don't remain unattached. Specialized enzymes transfer the phosphate groups to other molecules. Imagine again a climb up the stairs of a water slide. In your muscle cells, ATP lends phosphate groups to motor proteins. These proteins change shape, causing the muscle cells to contract.

    Figure 7.9 shows the three main kinds of work that your cells perform: chemical, mechanical, and transport. An example of chemical work is building giant molecules such as the protein hemoglobin– the protein that allows red blood cells to carry oxygen. An example of mechanical work is the movement of a muscle. In an example of transport work, ATP prepares your brain cells to transmit signals by enabling the brain cells to pump salts across their membranes.

    The ATP Cycle

    You spend ATP continuously. It is converted to ADP as your cells do work. Fortunately, ATP is a renewable resource. It can be restored from ADP by adding a phosphate group. This is where food energy reenters the story. The chemical energy from food drives ATP regeneration. Thus ATP operates in a cycle within your cells. Work consumes ATP, which is then regenerated from ADP and phosphate. Food energy keeps the cycle running.

    The ATP cycle churns at an astonishing pace. A working muscle cell recycles all of its ATP about once each minute. That's 10 million ATP molecules spent and regenerated per second per cell! If ATP could not be recycled, your body would have to consume its own weight in ATP each day. The next concept focuses on how your harvest of food energy manages to keep pace with this incredible demand for ATP.

    Online Concept 7.3

    See how ATP powers work.
    Go online and explore the many life processes that use ATP. How many can you list?

    Concept Check 7.3

    1. Explain how ATP powers muscle contraction.
    2. What is the source of energy for regenerating ATP from ADP?
    3. In what way is ATP like a loaded spring?

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    Concept 7.4 Electrons "fall" from food to oxygen during cellular respiration.


  • Explain the relationship between breathing and cellular respiration.
  • Describe the role of oxygen in cellular respiration.


  • aerobic


    Concept 7.4
    Make electrons fall.

    You have already learned that cells, like automobile engines, consume oxygen in the process of breaking down their fuel. The cell's living version of the internal combustion process is called cellular respiration. You use cellular respiration to convert food energy to ATP energy. But how is this related to breathing?

    Relationship of Cellular Respiration to Breathing

    Cellular respiration is an aerobic process, which is just another way of saying that it requires oxygen. Thus, cellular respiration can be defined as the aerobic harvesting of food energy in cells. You have probably heard the word respiration used in reference to breathing. Although breathing should not be confused with cellular respiration, the two processes are closely related.

    Review the cellular engine in Figure 7.6 and you'll see that a cell exchanges two gases with its surroundings during cellular respiration. The cell takes in oxygen and gets rid of carbon dioxide. Your bloodstream keeps cells supplied with oxygen and carries away carbon dioxide. Breathing exchanges these gases between your blood and the outside air. Oxygen in the air you inhale diffuses across the lining of the lungs and into the bloodstream. The carbon dioxide is disposed of when it diffuses from your blood across the lungs' lining and is then exhaled.

    Overall Equation for Cellular Respiration

    The sugar glucose is a major fuel for cellular respiration. The overall equation of what happens to glucose during cellular respiration is shown in Figure 7.12. The series of arrows indicates that cellular respiration consists of many chemical steps, not just a single chemical reaction.

    The main function of cellular respiration is to generate ATP for cellular work. In fact, the process can produce up to 38 ATPs for each glucose molecule consumed. This ATP energy is made available as cells break glucose down to carbon dioxide. Notice that cellular respiration also transfers hydrogen atoms from glucose to oxygen, thus forming water. Next, we'll take a closer look at how this hydrogen transfer to oxygen releases energy.

    "Falling" Electrons as an Energy Source

    In tracking hydrogen atoms from sugar to oxygen, you are also following the transfer of electrons. The atoms making up sugar and other molecules are bonded together because they share electrons (see Chapter 4). When hydrogen atoms and their bonding electrons change partners – from sugar to oxygen – energy is released.

    Why does the transfer of electrons from sugar to oxygen release energy? Oxygen is an electron grabber in chemical reactions. Oxygen atoms have a stronger pull on electrons than almost any other type of atom. As electrons move with hydrogen atoms from glucose to oxygen, the pull of the oxygen makes it seem as though the electrons were "falling." The electrons are not really falling in the sense of an apple dropping from a tree. However, falling objects and electron transfer to oxygen both represent an unlocking of potential energy. Instead of gravity, the pull of oxygen atoms on electrons causes the fall.

    A rapid electron fall generates an explosive release of energy in the form of heat and light. A spark triggers an explosive reaction between hydrogen gas and oxygen gas. The reaction produces water (Figure 7.13). The reaction also releases a large amount of energy. This energy is released as the electrons of the hydrogen atoms fall into their new bonds with oxygen. Although this all-at-once reaction releases a burst of energy, it is difficult to harness such an explosion to do useful work.

    NADH and Electron Transport Chains

    Cellular respiration also combines oxygen and hydrogen to produce water. However, compared with burning, cellular respiration is a more controlled fall of electrons– more like a step-by-step walk of electrons down an energy staircase. Instead of releasing food energy in a burst of flame, cellular respiration unlocks food energy in small amounts that cells can put to productive use– the conversion of food energy to ATP energy.

    Using Figure 7.14, track the path electrons take on their way down from food to oxygen. First stop is an electron carrier called NAD+ (an abbreviation for a very long chemical name). The transfer of electrons from food changes the NAD+ to NADH – this transfer is the electrons' first step down the energy staircase. The rest of the staircase consists of a series of molecules called an electron transport chain.

    Each molecule, or step, in an electron transport chain is usually a protein. Each member of the chain can first accept and then donate electrons. The electrons release a small amount of energy with each transfer. At the uphill end of the transport chain, the first molecule accepts electrons from NADH. Thus, the function of NADH is to accept electrons from food and deposit them at the top of electron transport chains. The electrons then cascade down the chain, from molecule to molecule. Oxygen pulls the electrons down the chain. The molecule at the bottom end of the chain finally drops the electrons to oxygen. The oxygen also combines with hydrogen to form water. Your cells use this step-by-step freeing of chemical energy during electron transport to make most of their ATP.

    Online Concept 7.4

    Make electrons fall.
    Go online to witness a historic explosion caused by oxygen reacting with hydrogen. How does this explosive "fall" of electrons compare with an electron transport chain?

    Concept Check 7.4

    1. How is breathing related to cellular respiration?
    2. What is the main function of cellular respiration?
    3. Explain the function of NADH.

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    Concept 7.5 Cellular respiration converts food energy to ATP energy.


  • Describe the functions of glycolysis, the Krebs cycle, and electron transport during cellular respiration.
  • Summarize the transfer of energy from food to ATP during cellular respiration.


  • metabolism
  • glycolysis
  • Krebs cycle
  • ATP synthase


    Concept 7.5
    Play respiration pinball.

    Lab 7.2
    Guided Research: Cellular Respiration in FastPlantsÆ

    Aerobic Exercise and Dietary Supplements

    Closer Look
    Cellular Respiration

    You have read that cellular respiration is an overall process that releases the energy stored in food. Cellular respiration actually occurs in a series of chemical reaction steps. More than two dozen reactions make up these steps. These reactions can be grouped into three main stages: glycolysis, the Krebs cycle, and electron transport. Let's see how these steps combine to harvest food energy.

    A Road Map for Cellular Respiration

    The total of all chemical processes that occur in cells is called metabolism. Cellular respiration is one part of metabolism. Because cellular respiration is made up of a series of reactions, it is called a metabolic pathway. A specific enzyme catalyzes (speeds up) each reaction in a metabolic pathway.

    Use the "road map" in Figure 7.15 to follow glucose through the metabolic pathway of cellular respiration. The three main stages are color-coded: glycolysis (green), the Krebs cycle (purple), and electron transport (gold). The road map also shows where in your cells each stage occurs. Notice that much of the action takes place in mitochondria. In Chapter 6, you learned that mitochondria are the organelles that power cells. Now you can see how– mitochondria convert food energy to ATP energy through cellular respiration.

    STAGE 1: Glycolysis

    Glycolysis is the first step in breaking down the glucose molecule. The enzymes for glycolysis are located outside mitochondria, in the cytosol. The word glycolysis means "splitting of sugar." As you can see in Figure 7.16, this is precisely what happens. Glycolysis breaks the six-carbon glucose in half, forming two three-carbon molecules. (Each ball in Figure 7.16 represents a carbon atom.) These three-carbon molecules then donate electrons to NAD+, the electron carrier. Some of the food energy is now stored in NADH. The fuel molecules also generate some ATP directly.

    The fractured glucose has been converted to two molecules of a substance called pyruvic acid. This molecule still holds most of the energy of the original glucose molecule.

    STAGE 2: The Krebs Cycle

    This stage is named for Hans Krebs, who worked out the steps of the cycle in the 1930s. The Krebs cycle finishes extracting the energy of the fuel molecules by breaking them down to carbon dioxide. The enzymes for the Krebs cycle are dissolved in the fluid within your mitochondria.

    The fuel for the Krebs cycle comes from pyruvic acid, the final product of glycolysis (Figure 7.17). First, the pyruvic acid loses a carbon as a carbon dioxide molecule is given off. This leaves a two-carbon fuel molecule that enters the Krebs cycle. The fuel joins a four-carbon acceptor molecule. This acceptor is later recycled by the loss of two carbon dioxide molecules, which is why this process is called a cycle.

    Thus, carbon enters the Krebs cycle in the form of organic fuel and leaves as exhaust carbon dioxide. The Krebs cycle uses a small portion of the energy to produce ATP directly. However, NADH traps most of the energy. The FADH2 shown in the diagram is another electron carrier that works similarly to NADH.

    STAGE 3: Electron Transport

    The electron transport stage converts NADH energy to ATP energy. The molecules of electron transport chains are built into the inner membranes of mitochondria. The entire chain functions as a pump – pumping hydrogen ions across the inner membranes of mitochondria (Figure 7.18). The pumping stores energy by making hydrogen ions more concentrated on one side of the membrane than on the other.

    The membrane that stores energy from electron transport behaves something like a dam. A dam stores water behind its walls, preventing the water from flowing out. The stored energy is then harnessed to do work (such as generating electricity) when the water is allowed to rush downhill, turning giant wheels called turbines as it goes. Similarly, the membranes of mitochondria retain hydrogen ions, keeping them from moving back to where they are less concentrated.

    Your mitochondria also have structures that act like miniature turbines. These protein "turbines," called ATP synthases, are built into the inner membranes of mitochondria. Electron transport powers ATP synthase indirectly. Hydrogen ions pumped by electron transport rush back "downhill" through the ATP synthase. This spins the ATP synthase – like water spins turbines in a dam. The rotation activates sites in the synthase that regenerate ATP from ADP. The turbines of your mitochondria are marvelous molecular dynamos that people are only now beginning to understand.


    Aerobic Exercise and Dietary Supplements
    How does high-level aerobic training help an athlete prepare for grueling sports competitions? What are the risks of using dietary supplements to gain a competitive edge? Explore these questions online.

    Review of Cellular Respiration: Adding Up the ATP

    When taking cellular respiration apart to see how all its metabolic machinery works, it's easy to forget the overall function. The result of cellular respiration is to generate ATP for cellular work. As you can see in Figure 7.19, a cell can convert the energy of each glucose molecule to as many as 38 ATPs. Glycolysis and the Krebs cycle each produce 2 ATPs. The ATP synthase machines produce the other 34 ATPs, powered by the fall of electrons from food to oxygen.

    Notice that most ATP production depends on the presence of oxygen. This is why the oxygen you breathe is so important. Without oxygen, most of your cells would be unable to produce much ATP. And without oxygen, you would only survive for a few minutes.

    Online Concept 7.5

    Play respiration pinball.
    Go online to find the ultimate ATP score. Which respiration stage makes the most ATP molecules? Study the simulations to help you understand the "road map" for cellular respiration.

    Concept Check 7.5

    1. Which of the three stages of cellular respiration uses oxygen directly?
    2. In what two chemical forms does glycolysis store energy extracted from food?
    3. What becomes of the carbon atoms that enter the Krebs cycle in the form of fuel?
    4. Which of the three stages of cellular respiration take(s) place within mitochondria?

    Online Lab 7.2

    Guided Research: Cellular Respiration in FastPlantsÆ

    In this laboratory exploration, you gain first-hand experience with cellular respiration. You build a sensitive device, called a microrespirometer, and use it to determine rate of respiration of germinating seeds. Before you build your microrespirometer, you observe how this device, shown at right, functions. After learning how a microrespirometer can help you to answer questions about respiration, you apply what you have learned to design and carry out your own investigation on respiration.

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    Concept 7.6 Some cells can harvest food energy without oxygen.


  • Explain the differences between fermentation and cellular respiration.
  • Give examples of commercial products that depend on fermentation in microorganisms.


  • anaerobic
  • fermentation


    Concept 7.6
    Play fermentation pinball.

    Closer Look

    When you are walking down the street, your lungs supply your cells with oxygen at a rate that keeps pace with ATP demand. But what happens when you sprint to catch a bus? Your leg muscles are forced to work without oxygen, because you are spending ATP more quickly than your lungs and bloodstream can deliver oxygen to your muscles. Fortunately, some of your cells can actually produce ATP and work for short periods without oxygen.

    Fermentation in Human Muscle Cells

    If you sprint for a certain amount of time, your muscles must regenerate ATP. Normally, the cells can produce ATP through cellular respiration. But in the sprinting example, the cells are working under anaerobic conditions – that is, without oxygen. The process of harvesting food energy in anaerobic conditions is called fermentation.

    The energy to make ATP in fermentation comes from glycolysis, the same process that is the first stage of cellular respiration. Glycolysis directly produces two ATPs. This may not seem very efficient compared to the 38 ATPs generated during respiration. However, by burning enough glucose, fermentation can energize your leg muscles long enough for you to catch that bus.

    Fermentation in muscle cells produces a waste product called lactic acid. The temporary buildup of lactic acid in muscle cells contributes to the soreness you feel during and after a long run or a set of push-ups. Your body consumes oxygen as it converts the lactic acid back to pyruvic acid. You pay this "oxygen debt" by breathing heavily for several minutes after you stop exercising.

    Fermentation in Microorganisms

    Like your muscle cells, yeast (a microscopic fungus) is capable of both cellular respiration and fermentation. When yeast cells are kept in an anaerobic environment, they are forced to ferment their sugar and other foods. In contrast to fermentation in your muscle cells, fermentation in yeast produces alcohol, instead of lactic acid, as a waste product (Figure 7.21). This fermentation reaction, called alcoholic fermentation, also releases carbon dioxide. For thousands of years, humans have put yeast to work producing alcoholic beverages such as beer and wine. The carbon dioxide is what makes champagne and beer bubbly. In another example of "taming" microbes, the carbon dioxide bubbles from baker's yeast make bread rise.

    There are also fungi and bacteria that produce lactic acid during fermentation, just as our muscle cells do. Humans use these microbes to transform milk into cheese and yogurt. The sharpness or sour flavor of yogurt and some cheeses is mainly due to lactic acid. The same kind of microbial fermentation turns soybeans into soy sauce, cucumbers into pickles, and cabbage into sauerkraut.

    Yeast cells and our muscle cells are versatile in their ability to harvest food energy by either respiration or fermentation. In contrast, some bacteria found in still ponds or deep in the soil are actually poisoned if they come into contact with oxygen. These bacteria generate all of their ATP by fermentation. If you had to do that – though you don't and you can't – you would have to consume almost 20 times more food than normal. Oxygen enables us to get the most from our food energy. In the next chapter, you'll learn about the original source of this food energy – photosynthesis.

    Online Concept 7.6

    Play fermentation pinball.
    Now go online and play fermentation pinball. What is the ultimate ATP score without oxygen? How are the fermentation pinball results different from the respiration pinball results?

    Concept Check 7.6

    1. Which metabolic stage is part of both fermentation and cellular respiration?
    2. When muscle cells make the switch from respiration to fermentation, they begin consuming glucose at a much faster rate. Explain this change in the rate of fuel consumption.
    3. What is the waste product of fermentation in your cells?

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    Chapter 7 Review


    Multiple Choice
    Choose the letter of the best answer.

    1. What are the waste products of cellular respiration?
    a. carbon dioxide and water
    b. ATP and ADP
    c. carbon dioxide and oxygen
    d. energy and glucose
    e. ADP and water

    2. What metabolic stage is part of both cellular respiration and fermentation?
    a. electron transport
    b. glycolysis
    c. Krebs cycle
    d. aerobic process
    e. ATP cycle

    3. The process shown in the diagram takes place in a cell's
    a. cytosol
    b. nucleus
    c. mitochondria
    d. chloroplasts
    e. cell wall

    4. Sports physiologists wanted to monitor athletes to determine at what point their muscles began to work anaerobically rather than aerobically. One way to test this would be to check for a buildup of
    a. ATP
    b. lactic acid
    c. carbon dioxide
    d. ADP
    e. oxygen

    If the statement is true, write, 'True.' If it is false, correct the underlined word or phrase to make the statement true.

    5. In the electron transport chain in cellular respiration, electrons "fall" from food to carbon dioxide.
    6. The stage of cellular respiration that generates the most ATP is electron transport.
    7. The energy stored in an ATP molecule is an example of kinetic energy.
    8. A kilocalorie is the amount of energy that can raise the temperature of one gram of water by one degree Celsius.

    Word Relationships
    Choose the letter of the second pair of words that are related in the same way as the first pair of words.

    9. Photosynthesis : oxygen as _____
    a. electron transport : electron
    b. autotroph : sunlight
    c. fermentation : lactic acid
    d. cellular respiration : carbon dioxide

    10. Calorie : energy as _____
    a. glycolysis : fermentation
    b. gram : mass
    c. temperature : molecule
    d. photosynthesis : cellular respiration

    11. ADP : ATP as _____
    a. glucose : pyruvic acid
    b. NAD+ : NADH
    c. water : carbon dioxide
    d. sugar : lactic acid

    12. Fermentation : alcohol as _____
    a. cellular respiration : mitochondria
    b. electron transport : ATP
    c. photosynthesis : oxygen
    d. glycolysis : pyruvic acid


    Analyzing Information

    13. Analyzing Diagrams Use the diagrams to answer the questions that follow.
    a. What are the reactants in photosynthesis? What are the products?
    b. How are photosynthesis and cellular respiration related? How are they different?

    14. Analyzing Data Refer to the table to answer the questions that follow.

    Energy Consumed by Various Activities
    kCals Consumed per Hour by a 67.5-kg (150-lb) Person
    Bicycling (racing)
    Bicycling (slowly)
    Running (7 min/mi)
    Swimming (2 mph)
    Dancing (fast)
    Walking (3 mph)

    a. How many hours would a 67.5-kg person have to walk to use up the energy contained in a piece of pizza containing 430 kCals?
    b. How far would this person have walked?
    c. What form of exercise would use up the pizza's calories in the shortest amount of time? Explain your answer.

    Critical Thinking

    15. Relating Cause and Effect The red blood cells in your body contain no mitochondria. Which stage of cellular respiration could take place in your red blood cells? Explain your answer.
    16. Comparing and Contrasting How is the process by which your body extracts energy from food similar to the process by which a car engine burns fuel? How is it different?
    17. What's Wrong With These Statements? Briefly explain why each statement is inaccurate or misleading.
    a. Photosynthesis creates energy.
    b. Animals carry out cellular respiration, while plants carry out photosynthesis.
    c. Eating a peanut releases energy in the same way as burning a peanut in the laboratory.
    d. Exercise releases the energy stored in calories.
    18. Biology Themes: Energy Conversions Write a few sentences explaining how cellular respiration relates to the biology theme of energy conversions discussed in Unit I.
    19. The Scientific Process You are a scientist investigating the factors that affect alcoholic fermentation in yeast. One of the products of fermentation is carbon dioxide gas. How could you design a technique to capture and measure the amount of carbon dioxide gas produced by fermentation in baker's yeast?
    Extra Challenge Choose a variable that might affect fermentation in yeast or bacteria, such as the concentration of sugar available to the fermenting cells. Develop a hypothesis about the effect of this variable. Design an experiment to test your hypothesis. Check with your teacher before conducting any experiments.
    20. Biology Research Project Identify some foods that are made with yeast, and then find out how yeasts are used in making one of those foods. With a partner, create a presentation of your findings. You can present your information in a cookbook style, on a poster, or as a computer presentation.


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    Chapter 7 Quiz

    Go online to check your understanding of the concepts in this chapter by taking the quiz.

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