Cells and Energy

colored SEM; magnification 1000What makes these cellsso important to manyother organisms?These diatoms are single-celledalgae that use the process ofphotosynthesis to store chemicalenergy in sugars. Animals eat photosyntheticorganisms such as plantsand algae to get this chemical energy.Photosynthetic organisms also producethe oxygen that is required to releasemuch of the chemical energy in sugars.ConnectingCONCEPTSEcology Microscopicdiatoms are absolutelynecessary for the survivalof other marine organisms,including the humpbackwhale. Why? Diatoms areeaten by small, shrimplikeanimals called krill. In turn,humpbacks and severalother species of whales feedmainly on krill. In fact, a bluewhale’s stomach can holdmore than a ton of krill.Chapter 4: Cells and Energy 99

FIGURE 4.2 ATP and ADPAdding a phosphate group to ADP forms ATP. Infer Where are molecules from food involved in the cycle?When the phosphate is removed, energy is released and ATP becomesADP—adenosine diphosphate. ADP is a lower-energy molecule that can beconverted into ATP by the addition of a phosphate group. If ATP is a walletfilled with money, ADP is a nearly empty wallet. The breakdown of ATP toADP and the production of ATP from ADP can be represented by the cycleshown in FIGURE 4.2. However, adding a phosphate group to ADP to make ATPis not a simple process. A large, complex group of proteins is needed to do it.In fact, if just one of these proteins is faulty, ATP is not produced.Synthesize Describe the relationship between energy stored in food and ATP.TAKING NOTESUse a supporting main ideaschart to organize conceptsrelated to chemical energy.All cells need chemical energy.ATP carries energy.MAIN IDEAOrganisms break down carbon-based moleculesto produce ATP.Foods that you eat do not contain ATP that your cells can use. First, the foodmust be digested. One function of digestion is to break down food intosmaller molecules that can be used to make ATP. You probably know thatdifferent foods have different amounts of calories, which are measures ofenergy. Different foods also provide different amounts of ATP. The number ofATP molecules that are made from the breakdown of food is related to thenumber of calories in food, but not directly.The number of ATP molecules produced depends on the type of moleculethat is broken down—carbohydrate, lipid, or protein. Carbohydrates are notstored in large amounts in your body, but they are the molecules most commonlybroken down to make ATP. The breakdown of the simple sugar glucoseyields about 36 molecules of ATP.ConnectingCONCEPTSBiochemistry As you learned inChapter 2, carbon-based moleculesin living things—carbohydrates,lipids, proteins, andnucleic acids—have differentstructures and functions.Chapter 4: Cells and Energy 101

sunlightMAIN IDEAPhotosynthesis in plants occurs in chloroplasts.Chloroplasts are the membrane-bound organelles where photosynthesis takesplace in plants. Most of the chloroplasts are in leaf cells that are specialized forphotosynthesis, which has two main stages as shown in FIGURE 4.5. The twomain parts of chloroplasts needed for photosynthesis are the grana and thestroma. Grana (singular, granum) are stacks of coin-shaped, membraneenclosedcompartments called thylakoids (THY-luh-KOYDZ). The membranesof the thylakoids contain chlorophyll, other light-absorbing molecules, andproteins. The stroma is the fluid that surrounds the grana inside a chloroplast.FIGURE 4.5 Photosynthesis OverviewChloroplasts absorb energy from sunlight and produce sugarsthrough the process of photosynthesis.STAGE 1: Light-Dependent Reactions1Energy from sunlight isabsorbed. Water moleculesare broken downand oxygen is released.chloroplastgranum (stack of thylakoids)BIOLOGYView an animationof photosynthesisat ClassZone.com.6H 2 O 6O 2thylakoidSTAGE 2: Light-Independent Reactions2energyEnergy-carryingmolecules, includingATP, transfer energy.6CO 2stroma (fluid outside the thylakoids)1 six-carbon sugarC 6 H 12 O 63 Carbon dioxide4molecules are usedto build sugars.Identify What are the reactants and the productsin photosynthesis?Six-carbon simple sugarsare produced. The sugarsare often used to buildstarches and cellulose.104 Unit 2: Cells

The light-dependent reactions capture energy from sunlight. Thesereactions take place within and across the membrane of the thylakoids. Water(H 2 O) and sunlight are needed for this stage of photosynthesis.1 Chlorophyll absorbs energy from sunlight. The energy is transferredalong the thylakoid membrane. H 2 O molecules are broken down. Oxygenmolecules (O 2 ) are released.2 Energy carried along the thylakoid membrane is transferred to moleculesthat carry energy, such as ATP.The light-independent reactions use energy from the light-dependentreactions to make sugars. These reactions occur in the stroma of chloroplasts.Carbon dioxide molecules (CO 2 ) are needed during this stage of photosynthesis.3 CO 2 is added to a cycle of chemical reactions to build larger molecules.Energy from the light-dependent reactions is used in the reactions.4 A molecule of a simple sugar is formed. The sugar, usually glucose(C 6 H 12 O 6 ), stores some of the energy that was captured from sunlight.The equation for the whole photosynthesis process is shown below. As youcan see, there are many arrows between the reactants—CO 2 and H 2 O—andthe products—a six-carbon sugar and O 2 . Those arrows tell you that photosynthesishas many steps. For example, the light-independent reactions needonly one molecule of CO 2 at a time, and the six-carbon sugar comes from areaction that combines two three-carbon sugars. Also, enzymes and otherchemicals are needed, not just light, carbon dioxide, and water.Connecting CONCEPTSCalvin Cycle The lightindependentreactions includea series of chemical reactionscalled the Calvin cycle. You canread more about the Calvin cyclein Section 4.3.6CO 2 + 6H 2 O C 6 H 12 O 6 + 6O 2carbon dioxide water light, enzymesa sugar oxygenGlucose and other simple sugars, such as fructose, are not the only carbohydratesthat come from photosynthesis. Plants need the simple sugars tobuild starch and cellulose molecules. In effect, plants need photosynthesis fortheir growth and development. You will learn more about the importance ofanother product of photosynthesis—oxygen—in Sections 4.4 and 4.5.Summarize How is energy from sunlight used to make sugar molecules?4.2 ASSESSMENTONLINE QUIZClassZone.comREVIEWING MAIN IDEAS1. What are the roles of chloroplastsand chlorophyll inphotosynthesis?2. Describe the stages of photosynthesis.Use the terms thylakoid,light-dependent reactions, andlight-independent reactions inyour answer.CRITICAL THINKING3. Apply Suppose you wanted todevelop a light to help increaseplant growth. What characteristicsshould the light have? Why?4. Analyze Explain why photosynthesisis important for building thestructure of plant cells.Connecting CONCEPTS5. Chemical Reactions Overall,do you think photosynthesis isendothermic or exothermic?Explain your answer.Chapter 4: Cells and Energy 105

CHAPTER 4MATERIALS• young ivy leaves• hole punch• 2 100-mL beakers• 5 mL sodium bicarbonate/detergent solution• water• plastic 10-cc syringe• forceps• strong light source• other light sources• stopwatchPROCESS SKILLS• Designing Experiments• Analyzing• Calculating• Graphing• InferringDESIGN YOUR OWN INVESTIGATIONRates of PhotosynthesisPhotosynthesis converts some of the energy absorbed from sunlight into thechemical energy of sugars. The process is also the major source of oxygen inEarth’s atmosphere. In this lab you will design an experiment to determine theeffect of different light sources on the rate of photosynthesis in leaves.PROBLEM How does a light source affect the rate of photosynthesis?PROCEDURE1. Use the hole punch to make five disks from an ivy leaf.2. Fill one beaker halfway with the sodium bicarbonate/detergent solution. Fill asecond beaker with water.3. Remove the plunger from the syringe and place the ivy leaf disks into the syringe.Insert the plunger and draw 5 cc (5 mL) of the sodium bicarbonate/detergentsolution into the syringe as shown in the photograph below (left).4. Hold the syringe so that the tip is pointing upwards. Push on the plunger to squirtout any air in the syringe.5. Place your finger on the tip of the syringe, as shown in the photograph below(right). Withdraw the plunger to form a vacuum, but be careful to not pull theplunger all the way out of the syringe. When the vacuum is formed, the gases inthe air spaces in the leaf disks move into the syringe and the solution diffusesinto the air spaces. Shake the syringe several times while your finger is onthe tip.6. Take your finger off of the tip of the syringe. This causes the leaf disks to sink tothe bottom of the syringe because they become more dense from the diffusionof solution into the air spaces.7. Open the syringe by pulling the plunger almost all the way out. Place your fingerover the tip of the syringe and turn it so the tip is pointing down. Carefully removethe plunger and pour the contents of the syringe into the beaker of water. Use theforceps to remove the leaf disks if they stick to the walls of the syringe.8. Place the beaker with the leaf disks under the light source and immediately startthe stopwatch. Record the time it takes for each leaf disk to float to the top ofthe water. Use the rate at which the disks float as an indirect measurement of therate of photosynthesis.step 3 step 5106 Unit 2: Cells

DESIGN1. Decide how to test different light sources on the rate of photosynthesis.Identify your independent variable. Have your teacher approve your choice.2. Identify your control condition and the constants in the experiment.Examples of constants are the distance between the light and the beaker, andthe temperature of the water.3. Write the procedure for your experiment.4. Design a data table, such as the one shown below, to record and organizeyour results.TABLE 1. THE EFFECT OF LIGHT SOURCES ON THE RATE OF PHOTOSYNTHESISTime (sec) Type of light __________(# of disks)Type of light __________(# of disks)Type of light __________(# of disks)0 0 0 05. Gather the additional materials you need for your experiment.6. Test the rate of photosynthesis using the procedure on page 106. Be surenot to get any water on the light sources. Record your results. Continuecollecting data until all five disks float in each experimental condition. (Note: Ifyou test more than one condition at once, record a time and disk count for allof the beakers every time a disk floats in any of the beakers.)7. If time allows, conduct three trials of your experiment.ANALYZE AND CONCLUDE1. Identify What are the independent and dependent variables in yourexperiment?2. Calculate Determine the mean rate of photosynthesis in each condition ofyour experiment. To calculate the rate of photosynthesis, use the formulabelow:____________5 (# disks floating)= ____ disks/sectotal time (sec)3. Analyze Determine the best type of graph to use to represent your data.Explain your choice and construct the graph. Be sure to carefully label theaxes of the graph.4. Conclude Based on your data, what can you conclude about how yourindependent variable affects the rate of photosynthesis?5. Infer Why do you think sodium bicarbonate was used in this investigation?(Hint: Think about the equation for the overall process of photosynthesis.)6. Experimental Design What are possible sources of unavoidable error in yourdesign? Explain why they were present.7. Experimental Design Identify possible reasons for inconsistent results.Chapter 4: Cells and Energy 107

4.3Photosynthesis in DetailKEY CONCEPT Photosynthesis requires a series of chemical reactions.MAIN IDEAS• The first stage of photosynthesis capturesand transfers energy.• The second stage of photosynthesis usesenergy from the first stage to make sugars.VOCABULARYphotosystem, p. 108electron transportchain, p. 109ATP synthase, p. 110Calvin cycle, p. 111Reviewchlorophyll, thylakoid,light-dependentreactions, lightindependentreactionsConnect In a way, the sugar-producing cells in leaves are like tiny factories withassembly lines. In a factory, different workers with separate jobs have to worktogether to put together a finished product. Similarly, in photosynthesis manydifferent chemical reactions, enzymes, and ions work together in a precise orderto make the sugars that are the finished product.MAIN IDEAThe first stage of photosynthesis captures andtransfers energy.In Section 4.2 you read a summary of photosynthesis. However, the process ismuch more involved than that general description might suggest. For example,during the light-dependent reactions, energy is captured and transferredin the thylakoid membranes by two groups of molecules called photosystems.The two photosystems are called photosystem I and photosystem II.FIGURE 4.6 The light-dependentreactions capture energy from sunlightand transfer energy throughelectrons. The solar cells that powera solar car do the same thing.Overview of the Light-Dependent ReactionsThe light-dependent reactions are the photo- part of photosynthesis. Duringthe light-dependent reactions, chlorophyll and other light-absorbing moleculescapture energy from sunlight. Water molecules are broken down intohydrogen ions, electrons, and oxygen gas. The oxygen is given off as a wasteproduct. Sugars are not made during this part of photosynthesis.The main functions of the light-dependent reactions are to captureand transfer energy. In these reactions, as in the solar car in FIGURE 4.6,energy is transferred to electrons. The electrons are only used for energyin a few specific processes. Recall a time you have gone to an amusementpark. To go on rides, you needed special tickets that could only be usedthere. Similarly, the electrons are used for energy during photosynthesisbut not for the cell’s general energy needs.Energy from the electrons is used to make molecules that act asenergy carriers. These energy carriers are ATP and another moleculecalled NADPH. The ATP from the light-dependent reactions is usuallynot used for a cell’s general energy needs. In this case, ATP molecules,along with NADPH molecules, go on to later stages of photosynthesis.108 Unit 2: Cells

Photosystem II and Electron TransportIn photosystem II, chlorophyll and other light-absorbing molecules in thethylakoid membrane absorb energy from sunlight. The energy is transferredto electrons. Photosystem II, shown in FIGURE 4.7, needs water to function.1 Energy absorbed from sunlight Chlorophyll and other light-absorbingmolecules in the thylakoid membrane absorb energy from sunlight. Theenergy is transferred to electrons (e – ). High-energy electrons leave thechlorophyll and enter an electron transport chain, which is a series ofproteins in the membrane of the thylakoid.2 Water molecules split Enzymes break down water molecules. Oxygen,hydrogen ions (H + ), and electrons are separated from each other. Theoxygen is released as waste. The electrons from water replace thoseelectrons that left chlorophyll when energy from sunlight was absorbed.3 Hydrogen ions transported Electrons move from protein to protein inthe electron transport chain. Their energy is used to pump H + ions fromoutside to inside the thylakoid against a concentration gradient. The H +ions build up inside the thylakoid. Electrons move on to photosystem I.Photosystem I and Energy-Carrying MoleculesIn photosystem I, chlorophyll and other light-absorbing molecules in thethylakoid membrane also absorb energy from sunlight. The energy is added toelectrons, some of which enter photosystem I from photosystem II.FIGURE 4.7 Light-Dependent ReactionsPhotosystems II and I absorb energy from sunlightand transfer energy to the Calvin cycle.Light-dependent reactions takeplace in and across the thylakoidmembrane.Photosystem II and electron transport12Energy is absorbedfrom sunlight. 3 Hydrogen ions are 4 Energy is absorbed transported across the from sunlight. thylakoid membrane. 6 Hydrogen ions diffuse through a protein channel. Water molecules are broken down. Identify At what two points in the process areelectrons used in the transfer of energy?Photosystem I and energy-carrying molecules5NADPH is producedwhen electrons areadded to NADP + .7 ADP is changed intoATP when hydrogenions flow throughATP synthase.Chapter 4: Cells and Energy 109

4 Energy absorbed from sunlight As in photosystem II, chlorophyll andother light-absorbing molecules inside the thylakoid membrane absorbenergy from sunlight. Electrons are energized and leave the molecules.5 NADPH produced The energized electrons are added to a moleculecalled NADP + , which functions like ADP. A molecule called NADPH ismade. In photosynthesis, NADPH functions like ATP. The molecules ofNADPH go to the light-independent reactions.ATP ProductionThe final part of the light-dependent reactions makes ATP. The productionof ATP depends on the H + ions that build up inside the thylakoid fromphotosystem II, and on a complex enzyme in the thylakoid membrane.6 Hydrogen ion diffusion Hydrogen ions flow through a protein channelin the thylakoid membrane. Recall that the concentration of H + ions ishigher inside the thylakoid than it is outside. This difference in H + ionconcentration is called a chemiosmotic gradient, which stores potentialenergy. Therefore, the ions flow through the channel by diffusion.7 ATP produced The protein channel in step 6 is part of a complex enzymecalled ATP synthase, shown in FIGURE 4.8. As the ions flow through thechannel, ATP synthase makes ATP by adding phosphate groups to ADP.FIGURE 4.8 Scientists have madedetailed computer models of ATPsynthase (top). Scientists are stillworking on viewing the actualmolecule (bottom). (colored TEM;magnification 1,800,000)Summary of the Light-Dependent Reactions• Energy is captured from sunlight by light-absorbing molecules. Theenergy is transferred to electrons that enter an electron transport chain.• Water molecules are broken down into H + ions, electrons, and oxygenmolecules. The water molecules provide the H + ions and electrons thatare used in the light-dependent reactions.• Energized electrons have two functions. They provide energy forH + ion transport, and they are added to NADP + to form NADPH.• The flow of H + ions through ATP synthase makes ATP.• The products are oxygen, NADPH, and ATP. Oxygen is given off as a wasteproduct. Energy from ATP and NADPH is used later to make sugars.Summarize Describe how energy from sunlight is transferred to ATP and NADPH.MAIN IDEAThe second stage of photosynthesis uses energyfrom the first stage to make sugars.The light-independent reactions, like the light-dependent reactions, take placeinside chloroplasts. But as the name implies, the light-independent reactionsdo not need sunlight. These reactions can take place anytime that energy isavailable. The energy sources for the light-independent reactions are themolecules of ATP and NADPH formed during the light-dependent reactions.The energy is needed for a series of chemical reactions called the Calvin cycle,which is named for the scientist who discovered the process.110 Unit 2: Cells

The Calvin CycleThe Calvin cycle cannot take place without the ATP from the light-dependentreactions. The chemical reactions of the Calvin cycle use carbon dioxide(CO 2 ) gas from the atmosphere and the energy carried by ATP and NADPHto make simple sugars. Because the light-independent reactions build sugarmolecules, they are the synthesis part of photosynthesis. Only one molecule ofCO 2 is actually added to the Calvin cycle at a time. The simplified cycle inFIGURE 4.9 shows three CO 2 molecules added at once.VOCABULARYThe light-dependent reactionsare the photo- part of photosynthesis.The light-independentreactions are the synthesis partof photosynthesis.1 Carbon dioxide added CO 2 molecules are added to five-carbonmolecules already in the Calvin cycle. Six-carbon molecules are formed.2 Three-carbon molecules formed Energy—ATP and NADPH—from thelight-dependent reactions is used by enzymes to split the six-carbonmolecules. Three-carbon molecules are formed and rearranged.3 Three-carbon molecules exit Most of the three-carbon molecules stay inthe Calvin cycle, but one high-energy three-carbon molecule leaves thecycle. After two three-carbon molecules have left the cycle, they arebonded together to build a six-carbon sugar molecule such as glucose.4 Three-carbon molecules recycled Energy from ATP molecules is used tochange the three-carbon molecules back into five-carbon molecules. Thefive-carbon molecules stay in the Calvin cycle. These molecules areadded to new CO 2 molecules that enter the cycle.FIGURE 4.9 Light-Independent Reactions (Calvin Cycle)The Calvin cycle produces sugars.2 Energy is added. The sixcarbonmolecules split toform three-carbon molecules.More energy is added and themolecules are rearrangedinto higher-energy molecules.Light-independent reactionstake place in the stroma.1Carbon dioxide (CO 2 )molecules enter the cycleand are added to five-carbonmolecules. Six-carbonmolecules are formed. Infer Why must the Calvin cycle occurmore than once to build a sugar molecule? 4 3Three-carbon molecules arechanged back to five-carbonmolecules by energy from ATP. A high-energy threecarbonmolecule exits forevery 3 CO 2 moleculesthat enter. After 2 threecarbonmolecules haveexited, they bond to form1 six-carbon sugar.Chapter 4: Cells and Energy 111

Summary of the Light-Independent Reactions• Carbon dioxide enters the Calvin cycle.• ATP and NADPH from the light-dependent reactions transfer energy tothe Calvin cycle and keep the cycle going.• One high-energy three-carbon molecule is made for every three moleculesof carbon dioxide that enter the cycle.• Two high-energy three-carbon molecules are bonded together to make asugar. Therefore, six molecules of carbon dioxide must be added to theCalvin cycle to make one six-carbon sugar.• The products are a six-carbon sugar such as glucose, NADP + , and ADP. TheNADP + and ADP molecules return to the light-dependent reactions.ConnectingCONCEPTSEcology Photosynthesis is amajor part of the carbon cycle.You will learn more about thecarbon cycle in Chapter 13.Functions of PhotosynthesisPhotosynthesis is much more than just a biochemical process. Photosynthesisis important to most organisms on Earth, as well as to Earth’s environment.Recall that plants produce food for themselves and for other organismsthrough photosynthesis. Both plant cells and animal cells release the energystored in sugars through cellular respiration. Cellular respiration, which usesthe oxygen that is a waste product of photosynthesis, is the process that makesmost of the ATP used by plant and animal cells.Photosynthesis does more than make sugars. It also provides materials forplant growth and development. The simple sugars from photosynthesis arebonded together to form complex carbohydrates such as starch and cellulose.Starches store sugars until they are needed for energy. Cellulose is a major partof plant structure—it is the building block of plant cell walls. Photosynthesisalso helps to regulate Earth’s environment. The carbon atoms used to makesugar molecules come from carbon dioxide gas in the air, so photosynthesisremoves carbon dioxide from Earth’s atmosphere.Summarize How does the Calvin cycle build sugar molecules?4.3 ASSESSMENTONLINE QUIZClassZone.comREVIEWINGMAIN IDEAS1. How do the two photosystemswork together to capture energyfrom sunlight?2. Explain the relationship betweenthe light-dependent and the lightindependentreactions.CRITICAL THINKINGConnecting CONCEPTS5. Cell Functions Explain how3. Connect Explain how the Calvin cycle6CO 2 + 6H 2 O C 6 H 12 O 6 + 6O 2is a bridge between carbon in theatmosphere and carbon-basedmolecules in the food you eat.4. Evaluate Explain why the chemicalboth passive transportand active transport arenecessary for photosynthesisto occur.equation for photosynthesis (below)is a highly simplified representationof the process. How is the equationaccurate? How is it inaccurate?112 Unit 2: Cells

4.4Overview ofCellular RespirationKEY CONCEPT The overall process of cellular respiration converts sugar into ATP using oxygen.MAIN IDEAS• Cellular respiration makes ATP bybreaking down sugars.• Cellular respiration is like a mirrorimage of photosynthesis.VOCABULARYcellular respiration, p. 113aerobic, p. 113glycolysis, p. 113anaerobic, p. 113Krebs cycle, p. 115Reviewmitochondria, ATP,electron transport chainConnect The term cellular respiration may lead you to form a mental picture ofcells breathing. This image is not correct, but it is useful to remember. Your cellsneed the oxygen that you take in when you breathe. That oxygen helps your bodyrelease the energy in sugars and other carbon-based molecules. Indirectly, yourbreathing is connected to the ATP that your cells need for everything you do.mitochondrionMAIN IDEACellular respiration makes ATP by breakingdown sugars.Plants use photosynthesis to make their own food. Animals eat other organismsas food. But food is not a direct source of energy. Instead, plants, animals,and other eukaryotes break down molecules from food to produce ATP.Cellular respiration releases chemical energy from sugars and other carbonbasedmolecules to make ATP when oxygen is present. Cellular respiration is anaerobic (air-OH-bihk) process, which means that it needs oxygen to take place.Cellular respiration takes place in mitochondria, which are often called thecell’s “powerhouses” because they make most of a cell’s ATP.A mitochondrion, shown in FIGURE 4.10, cannot directly make ATP from food.First, foods are broken down into smaller molecules such as glucose. Thenglucose is broken down, as shown below. Glycolysis (gly-KAHL-uh-sihs) splitsglucose into two three-carbon molecules and makes two molecules of ATP.Glycolysis takes place in a cell’s cytoplasm and does not need oxygen. Glycolysisis an anaerobic process because it does not need oxygen to take place.However, glycolysis is necessary for cellular respiration. The products ofglycolysis are broken down in mitochondria to make many more ATP.animal cellFIGURE 4.10 Mitochondria, foundin both plant and animal cells, produceATP through cellular respiration.(colored TEM; magnification 4500) Explain What is the function of cellular respiration?Chapter 4: Cells and Energy 113

MAIN IDEACellular respiration is like a mirror image ofphotosynthesis.ConnectingCONCEPTSPhotosynthesis Review the overallprocess of photosynthesis inSection 4.2 and compare photosynthesisto cellular respiration.Photosynthesis and cellular respiration are not true opposites, but you canthink about them in that way. For example, chloroplasts absorb energy fromsunlight and build sugars. Mitochondria release chemical energy to make ATP.The chemical equation of cellular respiration is also basically the reverse ofphotosynthesis. But the structures of chloroplasts and mitochondria aresimilar. A mitochondrion is surrounded by a membrane. It has two parts thatare involved in cellular respiration: the matrix and the inner mitochondrialmembrane. In mitochondria, cellular respiration takes place in two mainstages, as shown in FIGURE 4.11.FIGURE 4.11 Cellular Respiration OverviewWhen oxygen is available, ATP is produced by cellular respiration in mitochondria.STAGE 1: Krebs Cycle1Three-carbon moleculesfrom glycolysis entercellular respiration inmitochondria.matrix (area enclosedby inner membrane)mitochondrionATPAND6CO 2BIOLOGYView an animationof cellular respirationat ClassZone.com.energy2Energy-carryingmolecules transferenergy to Stage 2.STAGE 2: Electron Transport3Energy-carrying molecules fromglycolysis and the Krebs cycle enterStage 2 of cellular respiration.inner membraneenergy from glycolysisATPAND6O 2AND6H 2 OIdentify What are the reactants andproducts in cellular respiration?4ATP molecules are produced.Heat and water are releasedas waste products.114 Unit 2: Cells

The Krebs cycle produces molecules that carry energy to the second part ofcellular respiration. The Krebs cycle, named for the scientist who discoveredthe process, takes place in the interior space, or matrix, of the mitochondrion.1 Three-carbon molecules from glycolysis are broken down in a cycle ofchemical reactions. A small number of ATP molecules are made. Othertypes of energy-carrying molecules are also made. Carbon dioxide isgiven off as a waste product.2 Energy is transferred to the second stage of cellular respiration.An electron transport chain made of proteins needs energy-carryingmolecules from the Krebs cycle and oxygen to make ATP. This part of theprocess takes place in and across the inner mitochondrial membrane.To learn more about ATP and howit is used, go to scilinks.org.Keycode: MLB0043 Energy is transferred to a chain of proteins in the innermembrane of the mitochondrion.4 A large number of ATP molecules are made. Oxygenenters the process and is used to make water molecules.Water and heat are given off as waste products.Up to 38 ATP molecules are made from the breakdown ofone glucose molecule—2 from glycolysis and 34 or 36 fromcellular respiration. The equation for cellular respiration isshown below, but it actually has many more steps. For example,the cellular respiration equation includes glycolysis. And manyenzymes are also part of the process.C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 Oa sugar oxygen carbon dioxide waterFIGURE 4.12 COMPARING PROCESSESPhotosynthesisREACTANTSCO 2H 2 OCellular RespirationPRODUCTSCO 2H 2 OPRODUCTSSugars (C 6 H 12 O 6 )O 2REACTANTSSugars (C 6 H 12 O 6 )O 2Use FIGURE 4.12 to compare cellular respiration with photosynthesis.As you can see, photosynthesis uses the products of cellularrespiration. It converts energy from sunlight into sugars. Cellularrespiration needs the products of photosynthesis. It releasesstored energy from sugars to make ATP that can be used by cells.Apply Does glucose actually react with oxygen during cellular respiration? Explain.The products of photosynthesis—sugars andO 2 —are the reactants in cellular respiration.4.4 ASSESSMENTONLINE QUIZClassZone.comREVIEWINGMAIN IDEAS1. How are cellular respiration andglycolysis related?2. Summarize the aerobic stagesof cellular respiration. Be sure todiscuss the Krebs cycle andthe electron transport chain inyour answer.CRITICAL THINKING3. Analyze Describe the relationshipbetween cellular respiration andphotosynthesis. Discuss the functions ofchloroplasts and mitochondria.4. Apply Is glucose a reactant in theaerobic stages of cellular respiration?Explain.Connecting CONCEPTS5. Chemical Reactions Is theprocess of cellular respirationexothermic or endothermic?Explain your answer.Chapter 4: Cells and Energy 115

DATA ANALYSISINTERPRETING GRAPHSPhotosynthesis and PlantsAfter scientists record their data in tables, they usually makegraphs to display it for analysis. Graphs show the relationshipbetween two variables. The pattern of curve or line that isdrawn will help you to form conclusions about your data.EXAMPLEScientists study the net amount of sugar production permeter squared per year in forest plants. Look at graph below.Notice that the amount of net sugar production increasesrapidly at first, then levels off after about 2500 mm ofprecipitation. Scientists can conclude that forests need about2500 mm of rain annually for maximum net sugar production.DATA ANALYSISClassZone.comGRAPH 1. NET SUGAR PRODUCTIONInfrared imaging techniquescan be used to study sugarproduction and plant growthconditions. Source: H. Leith, Human Ecology 1INTERPRET A GRAPHLook at the graph to the right. It showsthe amount of carbon dioxide in the airduring different times of the day inBiosphere 2, an enclosed research andeducation center in Arizona.1. Interpret What is the relationshipbetween the time of day and theamount of carbon dioxide in theresearch facility?2. Infer Using your knowledge of theprocess of photosynthesis, draw aconclusion about the pattern ofthe data.GRAPH 2. CO 2 LEVELS OVER TIMECarbon dioxide (parts per million)100090080070012am 4:00 8:00 12:00noon4:00 8:00 12amTime of day116 Unit 2: Cells

4.5Cellular Respirationin DetailKEY CONCEPT Cellular respiration is an aerobic process with two main stages.MAIN IDEAS• Glycolysis is needed for cellular respiration.• The Krebs cycle is the first main part of cellular respiration.• The electron transport chain is the second main part ofcellular respiration.Reviewglycolysis, Krebs cycle,electron transport chain,cellular respiration,aerobic respirationConnect If chloroplasts are like tiny factories that make products, mitochondriaare like power plants that burn fuel to produce electricity. In a power plant, aprocessed fuel is burned in the presence of oxygen and energy is released asuseful electricity. During cellular respiration, oxygen and digested moleculesfrom food are used to produce useful energy in the form of ATP.MAIN IDEAGlycolysis is needed for cellular respiration.In Section 4.4 you read a summary of how cellular respiration produces ATPmolecules. But cellular respiration, like photosynthesis, is a very complexprocess. For example, glucose and oxygen do not react directly with oneanother, and many chemical reactions, such as glycolysis, must take place.Glycolysis is an ongoing process in all cells, including yours. It takes placein the cytoplasm before cellular respiration, and it does not require oxygen.Glycolysis makes a small number of ATP molecules, but its other products aremuch more important. If oxygen is available, the products of glycolysis areused to produce many more ATP molecules through cellular respiration. Theprocess of glycolysis can be summarized as follows.1 Two ATP molecules are used to energize a glucose molecule. The glucosemolecule is split into two three-carbon molecules. A series of enzymesand chemical reactions rearranges the three-carbon molecules.2 Energized electrons from the three-carbon molecules are transferred tomolecules of NAD + . Molecules of NADH are formed. A series of reactionsconverts the three-carbon molecules to pyruvate (py-ROO-vayt),which enters cellular respiration. Four ATP molecules are made. 1 2 Chapter 4: Cells and Energy 117

ConnectingCONCEPTSFermentation When cells do nothave a supply of oxygen for theaerobic processes of cellular respiration,the anaerobic processesof fermentation take place. Youwill learn about fermentation inSection 4.6.Although glycolysis makes four ATP molecules, recall that two ATP moleculesare used to first split the glucose molecule. So the breakdown of oneglucose molecule by glycolysis gives a net gain of two ATP molecules. Thepyruvate and NADH produced by glycolysis are used for cellular respirationwhen oxygen is present. NADH is an electron carrier like NADPH, the electroncarrier in photosynthesis.Summarize How does glycolysis result in a net gain of two ATP molecules?MAIN IDEAThe Krebs cycle is the first main part of cellularrespiration.Cellular respiration makes many more ATP molecules than does glycolysis. Itbegins with the breakdown of pyruvate in steps 1 and 2 below. The processcontinues with the Krebs cycle, shown in FIGURE 4.14. Notice that steps 1, 4, and5 below are very similar. In those steps, a carbon-based molecule is split, amolecule of carbon dioxide is formed, and energy-carrying NADH moleculesare made. In fact, the main function of the Krebs cycle is to transfer highenergyelectrons to molecules that carry them to the electron transport chain.The Krebs cycle is also sometimes called the citric acid cycle because citricacid is the first molecule formed, as you can see in step 3 below.FIGURE 4.13 Gasoline engines burn carbonbasedmolecules in the presence of oxygen,and they release water, carbon dioxide, andenergy. The overall process of cellular respirationis similar.1 Pyruvate broken down A pyruvate molecule is split into a twocarbonmolecule and a molecule of carbon dioxide, which is givenoff as a waste product. High-energy electrons are transferred fromthe two-carbon molecule to NAD + , forming a molecule of NADH.The NADH moves to the electron transport chain.2 Coenzyme A A molecule called coenzyme A bonds to the twocarbonmolecule made from the breakdown of pyruvate. Thisintermediate molecule goes to the Krebs cycle.3 Citric acid formed The two-carbon part of the intermediatemolecule is added to a four-carbon molecule to form a six-carbonmolecule called citric acid. Coenzyme A goes back to step 2.4 Citric acid broken down The citric acid molecule is broken downby an enzyme and a five-carbon molecule is formed. A moleculeof NADH is made and moves out of the Krebs cycle. A moleculeof carbon dioxide is given off as a waste product.5 Five-carbon molecule broken down The five-carbon molecule isbroken down by an enzyme. A four-carbon molecule, a moleculeof NADH, and a molecule of ATP are formed. The NADH leavesthe Krebs cycle. Carbon dioxide is given off as a waste product.6 Four-carbon molecule rearranged Enzymes rearrange the fourcarbonmolecule. High-energy electrons are released. Molecules ofNADH and FADH 2 , which is another electron carrier, are made.They leave the Krebs cycle and the four-carbon molecule remains.118 Unit 2: Cells

FIGURE 4.14 The Krebs CycleThe Krebs cycle breaks down citric acid andtransfers energy to the electron transport chain.1Pyruvate (from glycolysis)is broken down. 2 Coenzyme A bonds to thetwo-carbon molecule. Thisintermediate molecule entersthe Krebs cycle.Infer Does glycolysis directly produce as much ATPas the Krebs cycle? Indirectly? Explain your answer. 3 Citric acid is formed.4Citric acid is brokendown and NADH ismade. 6 The Krebs cycle takes place inthe mitochondrion matrix. 5 The five-carbonmolecule is brokendown. NADH andATP are made.The four-carbon molecule isrearranged. NADH and FADH 2are formed.The products from the breakdown of one molecule of pyruvate are• three molecules of carbon dioxide that are given off as a waste product• one molecule of ATP• four molecules of NADH to the electron transport chain• one molecule of FADH 2 to the electron transport chainRemember, glycolysis produces two pyruvate molecules. Therefore, theproducts above are half of what comes from one glucose molecule. The totalsare six carbon dioxide, two ATP, eight NADH, and two FADH 2 molecules.Analyze In what two ways is the Krebs cycle important for making ATP?MAIN IDEAThe electron transport chain is the second mainpart of cellular respiration.The electron transport chain takes place in and across the inner membrane of amitochondrion. As with electron transport in photosynthesis, proteins make upthe electron transport chain in cellular respiration. The proteins use energy fromthe electrons supplied by NADH and FADH 2 to pump hydrogen ions against aconcentration gradient, and across the inner mitochondrial membrane.Chapter 4: Cells and Energy 119

The ions later flow back through the membrane to produce ATP. Oxygen isneeded at the end of the process to pick up electrons that have gone throughthe chain. The electron transport chain is shown in FIGURE 4.15.1 Electrons removed Proteins inside the inner membrane of the mitochondriontake high-energy electrons from NADH and FADH 2 . Twomolecules of NADH and one molecule of FADH 2 are used.2 Hydrogen ions transported High-energy electrons travel through theproteins in the electron transport chain. The proteins use energy fromthe electrons to pump hydrogen ions across the inner membrane toproduce a chemiosmotic gradient, just as in photosynthesis. The hydrogenions build up on the inside of the inner mitochondrial membrane.3 ATP produced Just as in photosynthesis, the flow of hydrogen ions isused to make ATP. Hydrogen ions diffuse through a protein channel inthe inner membrane of the mitochondrion. The channel is part of theATP synthase enzyme. ATP synthase adds phosphate groups to ADP tomake ATP molecules. For each pair of electrons that passes through theelectron transport chain, an average of three ATP molecules are made.4 Water formed Oxygen finally enters the cellular respiration process. Theoxygen picks up electrons and hydrogen ions to form water. The watermolecules are given off as a waste product.FIGURE 4.15 The Electron Transport ChainEnergy from the Krebs cycle is used to produce ATP. 2Hydrogen ions aretransported acrossthe membrane. The electron transport chain is in theinner mitochondrial membrane.1 Electrons areremoved fromNADH and FADH 2 . Explain How are hydrogen ions involved in the electrontransport chain? 4 3ADP is changed into ATPwhen hydrogen ions flowthrough ATP synthase.Water is formed whenoxygen picks up electronsand hydrogen ions.120 Unit 2: Cells

The products of cellular respiration—including glycolysis—are• Carbon dioxide from the Krebs cycle and from the breakdown ofpyruvate before the Krebs cycle• Water from the electron transport chain• A net gain of up to 38 ATP molecules for every glucose molecule—2 from glycolysis, 2 from the Krebs cycle, and up to 34 from theelectron transport chainComparing Cellular Respiration and PhotosynthesisAgain, think about how photosynthesis and cellular respiration areapproximately the reverse of each other. Photosynthesis stores energyfrom sunlight as chemical energy. In contrast, cellular respirationreleases stored energy as ATP and heat. Look at FIGURE 4.17, and thinkabout other similarities and differences between the processes.FIGURE 4.17 PHOTOSYNTHESIS AND CELLULAR RESPIRATIONPHOTOSYNTHESIS CELLULAR RESPIRATIONOrganelle for process chloroplast mitochondrionFIGURE 4.16 Like sandbags passeddown a line of people, high-energyelectrons are passed along a chainof proteins in the inner mitochondrialmembrane.Reactants CO 2 and H 2 O sugars (C 6 H 12 O 6 ) and O 2Electron transport chainCycle of chemical reactionsproteins within thylakoidmembraneCalvin cycle in stroma ofchloroplasts builds sugarmoleculesproteins within innermitochondrial membraneKrebs cycle in matrix ofmitochondria breaks downcarbon-based moleculesProducts sugars (C 6 H 12 O 6 ) and O 2 CO 2 and H 2 ORecall the roles of electrons, hydrogen ions, and ATP synthase. In bothprocesses, high-energy electrons are transported through proteins. Theirenergy is used to pump hydrogen ions across a membrane. And the flow ofhydrogen ions through ATP synthase produces ATP. As you can see, the partsof the processes are very similar, but their end points are very different.Analyze How does the electron transport chain depend on the Krebs cycle?4.5 ASSESSMENTONLINE QUIZClassZone.comREVIEWINGMAIN IDEAS1. What is the role of pyruvate incellular respiration?2. Describe in your own words thefunction of the Krebs cycle.3. Explain the functions of electrons,hydrogen ions, and oxygen in theelectron transport chain.CRITICAL THINKING4. Compare and Contrast Describethe similarities and differencesbetween the Krebs cycle and theCalvin cycle.5. Evaluate Is oxygen necessary forthe production of all ATP in yourcells? Why or why not?Connecting CONCEPTS6. Common Ancestry Proteinmolecules called cytochromesare part of the electrontransport chain. They are nearlyidentical in every knownaerobic organism. How dothese molecules show theunity of life on Earth?Chapter 4: Cells and Energy 121

4.6 FermentationKEY CONCEPT Fermentation allows the production of a small amount of ATP without oxygen.MAIN IDEAS• Fermentation allows glycolysis to continue.• Fermentation and its products are importantin several ways.VOCABULARYfermentation, p. 122lactic acid, p. 123ReviewATP, glycolysis, cellular respiration,aerobic, anaerobicConnect Think about a time that you worked or exercised hard. Maybe youmoved heavy boxes or furniture. Maybe, playing basketball, you found yourselfrepeatedly running up and down the court. Your arms and legs began to feelheavy, and they seemed to lose strength. Your muscles became sore, and evenwhen you rested you kept breathing hard. Your muscles were using fermentation.MAIN IDEAFermentation allows glycolysis to continue.FIGURE 4.18 Muscle cells use anaerobicprocesses during hard exercise.The cells in your body cannot store large amounts of oxygen for cellularrespiration. The amount of oxygen that is provided by breathing is enough foryour cells during normal activities. When you are reading or talking to friends,your body can maintain its oxygen levels. When you are doing high levels ofactivity, as the sprinter is in FIGURE 4.18, your body cannot bring in enoughoxygen for your cells, even though you breathe faster. How do your cellsfunction without enough oxygen to keep cellular respiration going?Recall that glycolysis yields two ATP molecules when it splits glucose intotwo molecules of pyruvate. Glycolysis is always occurring and does not requireoxygen. If oxygen is available, the products of glycolysis—pyruvate and theelectron carrier NADH—are used in cellular respiration. Then, oxygen picksup electrons at the end of the electron transport chain in cellular respiration.But what happens when oxygen is not there to pick up electrons? The productionof ATP without oxygen continues through the anaerobic processes ofglycolysis and fermentation.Fermentation does not make ATP,but it allows glycolysis to continue.Fermentation removes electrons fromNADH molecules and recycles NAD +molecules for glycolysis. Why is thisprocess important? Because glycolysis,just like cellular respiration, needs amolecule that picks up electrons. Itneeds molecules of NAD + .VISUAL VOCABFermentation is an anaerobic processthat allows glycolysis to continue.glycolysiswithO 2withoutO 2cellularrespirationfermentation122 Unit 2: Cells

Without NAD + to pick up high-energy electrons from the splitting of glucose,glycolysis would stop. When the high-energy electrons are picked up, though, aeukaryotic cell can continue breaking down glucose and other simple sugars tomake a small amount of ATP.Suppose that a molecule of glucose has just been split by glycolysis in oneof your muscle cells, but oxygen is unavailable. A process called lactic acidfermentation takes place. Lactic acid fermentation occurs in your muscle cells,the cells of other vertebrates, and in some microorganisms. Lactic acid,C 3 H 6 O 3 , is what causes your muscles to “burn” during hard exercise.1 Pyruvate and NADH from glycolysis enter the fermentation process. TwoNADH molecules provide energy to convert pyruvate into lactic acid. Asthe NADH is used, it is converted back into NAD + .2 Two molecules of NAD + are recycled back to glycolysis. The recycling ofNAD + allows glycolysis to continue.GLYCOLYSISLACTIC ACID FERMENTATIONTAKING NOTESUse a mind map to take noteson the processes involved infermentation.glycolysis2 ATPfermentation 12As you can see, the role of fermentation is simply to provide glycolysis witha steady supply of NAD + . By itself, fermentation does not produce ATP.Instead, it allows glycolysis to continue to produce ATP. However, fermentationdoes produce the lactic acid waste product that builds up in muscle cellsand causes a burning feeling. Once oxygen is available again, your cells returnto using cellular respiration. The lactic acid is quickly broken down andremoved from the cells. This is why you continue to breathe hard for severalminutes after you stop exercising. Your body is making up for the oxygendeficit in your cells, which allows the breakdown of lactic acid in your muscles.Sequence Which process must happen first, fermentation or glycolysis? Explain.ConnectingCONCEPTSHuman Biology Muscle cellsneed ATP to contract. You willlearn how muscles produce yourmovements in Chapter 33.MAIN IDEAFermentation and its products are important inseveral ways.How would your diet change without cheese, bread, and yogurt? How wouldpizza exist without cheese and bread? Without fermentation, a pizza crustwould not rise and there would be no mozzarella cheese as a pizza topping.Cheese, bread, and yogurt are just a few of the foods made by fermentation.Milk is changed into different cheeses by fermentation processes carried outby different types of bacteria and molds. Waste products of their fermentationprocesses give cheeses their different flavors and textures. Additionally, sometypes of bacteria that use lactic acid fermentation sour the milk in yogurt.Chapter 4: Cells and Energy 123

Lactic acid fermentation is not the only anaerobic process. Alcoholicfermentation is used by many yeasts and by some types of plants. Alcoholicfermentation begins at the same point as lactic acid fermentation. That is,glycolysis splits a molecule of glucose and produces two net ATP molecules,two pyruvate molecules, and two NADH molecules. Pyruvate and NADHenter alcoholic fermentation.1 Pyruvate and NADH from glycolysis enter alcoholic fermentation. TwoNADH molecules provide energy to break down pyruvate into an alcoholand carbon dioxide. As the NADH molecules are used, they are convertedback into molecules of NAD + .2 The molecules of NAD + are recycled back to glycolysis. The recycling ofNAD + allows glycolysis to continue.GLYCOLYSISALCOHOLIC FERMENTATION 1 2The products of this process are two molecules of an alcohol, often ethylalcohol, two molecules of carbon dioxide, and two molecules of NAD + . Justlike lactic acid fermentation, alcoholic fermentation recycles NAD + and soallows glycolysis to keep making ATP.QUICK LABDESIGN YOUR OWNFermentationOne waste product of alcoholic fermentation is carbon dioxide. In thislab you will determine which beverage causes yeast to undergo a higherrate of fermentation.PROBLEM What factors affect the rate of fermentation in yeast?PROCEDURE1. Write an operational definition for the dependent variable that you will use tomeasure the rate of fermentation.2. Develop a technique using a balloon to measure fermentation rate.3. Design your experiment. Have your teacher approve your experimental design. Writeyour experimental procedure and conduct your experiment.4. Construct a data table to record your data. Construct a graph to display your data.ANALYZE AND CONCLUDE1. Identify What are the independent variable, dependent variable, and constants?2. Analyze How did the independent variable affect the rate of fermentation? Why?3. Experimental Design Identify possible reasons for any inconsistent results you observed.MATERIALS• 2 empty plastic bottles• 1 package of yeast• 2 100-mL graduated cylinders• 2 250-mL beakers• 2 beverages• 2 round balloons• 30 cm string• metric ruler124 Unit 2: Cells

FIGURE 4.19 Fermentation bymolds and bacteria produces thedifferent flavors and textures ofvarious cheeses.Alcoholic fermentation in yeast is particularly useful. When bread or pizzacrust is made, yeast is used to cause the dough to rise. The yeast breaks downsugars in the dough through glycolysis and alcohol fermentation. The carbondioxide gas produced by alcoholic fermentation causes the dough to puff upand rise. When the dough is baked, the alcohol that is produced duringfermentation evaporates into the air. The yeast in dough is killed by the heatof baking.Bacteria that rely upon fermentation play a very important role in thedigestive systems of animals. Microorganisms in the digestive tracts of animals,including humans, must obtain their ATP from anaerobic processesbecause oxygen is not available. Without them, neither you nor other animalswould be able to fully digest food. Why? These bacteria continue the breakdownof molecules by taking in undigested material for their needs. Theadditional breakdown of materials by digestive bacteria allows the host animalto absorb more nutrients from food.Apply Explain the importance of alcoholic fermentation in the production ofbread’s light, fluffy texture.4.6 ASSESSMENTONLINE QUIZClassZone.comREVIEWINGMAIN IDEAS1. What is the relationship betweenglycolysis and fermentation?2. Summarize the process ofalcoholic fermentationin yeast.CRITICAL THINKING3. Compare and Contrast How arelactic acid fermentation andalcoholic fermentation similar?How are they different?4. Compare and Contrast Describethe similarities and differencesbetween cellular respiration andfermentation.Connecting CONCEPTS5. Aerobic Respiration How isthe role of oxygen in cellularrespiration similar to the roleof NAD + in fermentation ?Chapter 4: Cells and Energy 125

CHAPTER 4OPTIONS FOR INQUIRYUse these inquiry-based labs and online activities to deepen yourunderstanding of photosynthesis and cellular respiration.INVESTIGATIONCellular RespirationPlant cells use cellular respiration to make ATP fromthe sugars they produce during photosynthesis. Inthis experiment, you will compare cellularrespiration rates in dormant and germinated seeds.SKILLS Measuring, Collecting Data, Graphing,Analyzing DataPROBLEM How do respiration rates compare indormant and germinated seeds?PROCEDURE1. Label the three test tubes as follows:Tube 1: pre-soaked seeds; Tube 2: dry seeds;Tube 3: no seeds2. Layer into the test tubes the following materials.Be sure that the layers of materials are compact,but not too tight.MATERIALS• 3 test tubes• marking pen• 6 pre-soaked seeds• 6 dry seeds• 6 cotton plugs• 1.5 teaspoons potassiumhydroxide (KOH) powder• rubber band• 100-mL beaker• 25 mL colored water• metric rulerTube 1: pre-soaked seeds, cotton plug, 1/2 tsp. potassium hydroxide, cotton plugTube 2: dry seeds, cotton plug, 1/2 tsp. potassium hydroxide, cotton plugTube 3: cotton plug, 1/2 tsp. potassium hydroxide, cotton plugCaution: Potassium hydroxide is a strong base; avoid contact with eyes, skin,and clothing.3. Use the rubber band to hold the three test tubes together.4. Pour the 25 mL of colored water into the beaker.5. Invert the test tubes and place them in the beaker.6. Construct a data table and record the distance that the colored water travels up eachof the three test tubes.ANALYZE AND CONCLUDE1. Analyze Choose the type of graph that best represents the data.Construct your graph.2. Analyze Describe your observations and measurements of test tubes 1and 2. What do the data show? Explain why differences between thetwo tubes might exist.3. Evaluate What is the function of test tube 3 in the experiment? What dothe measurements from this test tube demonstrate?4. Infer Potassium hydroxide reacts with, and removes, carbon dioxidefrom the air. Why might cellular respiration be responsible for waterrising into some of the test tubes?5. Predict Suppose this experiment were allowed to run an additional day.What might you expect to encounter? What about after a week?126 Unit 2: Cells

INVESTIGATIONInvestigate Fermentation in FoodsCarbon dioxide gas is released as a waste productof fermentation in yeasts. In this experiment youwill change an independent variable to changethe rate of fermentation.SKILL Designing ExperimentsMATERIALS• plastic bottles• yeast• foods or beverages containing sugar• warm water• round balloons• balance• spatula• graduated cylinders• paper towels• string• metric rulerBIOLOGYCLASSZONE.COMVIRTUAL LABCarbon Dioxide Transfer Through Snailsand ElodeaHow does CO 2 cycle through a simplebiological system? In this interactive lab,you will place snails and elodea in varyingconditions and test for the presence of CO 2 .PROBLEM What conditions affect fermentation?DESIGN1. Choose a variable, such as temperature orconcentration, to test in the experiment. Useat least two experimental conditions.2. Design your procedure and get your teacher’sapproval. List the experimental constants.3. Design a data table that includes both theindependent variable and the dependent variable.4. Conduct your experiment. Measure carbondioxide production for 15 minutes during eachexperimental trial.ANALYZE AND CONCLUDE1. Identify What is your independent variable?What is the dependent variable? How did youoperationally define the dependent variable?2. Analyze Choose the best way to graph your data.Construct your graph.3. Analyze How did your manipulation of theindependent variable affect fermentation rate?4. Experimental Design What are possible sourcesof unavoidable error in your design? Explain whythey were present.ANIMATED BIOLOGYMirror ProcessesAre the light-dependent reactions ofphotosynthesis really a “mirror image” of theelectron transport chain in cellular respiration?Build a diagram of each to compare howelectrons move through the system.WEBQUESTAn athletic challenge, such as weightlifting,requires a lot of energy and can stressmuscles to their limits. Complete thisWebQuest to learn how athletes cope withthe burn. Review how cells process energyand explore how athletes train.Chapter 4: Cells and Energy 127

CHAPTER4@ CLASSZONE.COMKEY CONCEPTS Vocabulary Games Concept Maps Animated Biology Online Quiz4.1 Chemical Energy and ATPAll cells need chemical energy. Adenosine triphosphate(ATP) is the primary source of energyin all cells. ATP transfers energy for cell processessuch as building new molecules and transportingmaterials.4.2 Overview of PhotosynthesisThe overall process of photosynthesis producessugars that store chemical energy. Photosynthesisuses energy captured from sunlight to changecarbon dioxide and water into oxygen and sugars.Sunlight is absorbed during the light-dependentreactions, and sugars are made during the lightindependentreactions.4.3 Photosynthesis in DetailPhotosynthesis requires a series of chemicalreactions. Energy from sunlight is absorbed in thethylakoid membrane by photosystems II and I inthe light-dependent reactions. The energy istransferred to the Calvin cycle, which builds sugarmolecules from carbon dioxide.Synthesize Your Notes4.4 Overview of Cellular RespirationThe overall process of cellular respiration convertssugar into ATP using oxygen. Glycolysissplits glucose and when oxygen is present theproducts of glycolysis are used in cellular respiration.The Krebs cycle transfers energy to theelectron transport chain, which produces mostof the ATP in eukaryotic cells.mitochondrionThree-carbonmoleculeschloroplast6O 26H 2 O 6O 26CO 2ATP4.5 Cellular Respiration in DetailCellular respiration is an aerobic process withtwo main stages. The Krebs cycle breaks downcarbon-based molecules and transfers energy toelectron carriers. The electron carriers provideenergy to the electron transport chain. ATP is producedby the electron transport chain whenhydrogen ions flow through ATP synthase.4.6 FermentationFermentation allows the production of a smallamount of ATP without oxygen. Fermentationallows glycolysis to continue producing ATP whenoxygen is unavailable. Lactic acid fermentationoccurs in many cells, including human muscle cells.AND6H 2 OTwo-Column Chart Compare and contrast photosynthesisand cellular respiration. Use your notes to make detailedcharts that include details about both processes. Highlightimportant vocabulary and processes.PhotosynthesisCellular Respirationabsorbs sunlightproduces ATP6CO 2 +6H 2 O C 6 H 12 O 6 +6O 2 C 6 H 12 O 6 +6O 2 6CO 2 +6H 2 Ooccurs in chloroplasts occurs in mitochondriaConcept Map Use a concept map like the one below tosummarize and organize the processes of photosynthesis,cellular respiration, and fermentation.splitsglucoseglycolysisproduces2 ATP128 Unit 2: Cells

Chapter AssessmentChapter Vocabulary4.1 ATP, p. 100ADP, p. 101chemosynthesis, p. 1024.2 photosynthesis, p. 103chlorophyll, p. 103thylakoid, p. 104light-dependent reactions, p. 105light-independent reactions,p. 1054.3 photosystem, p. 108electron transport chain, p. 109ATP synthase, p. 110Calvin cycle, p. 1114.4 cellular respiration, p. 113aerobic, p. 113glycolysis, p. 113anaerobic, p. 113Krebs cycle, p. 1154.6 fermentation, p. 122lactic acid, p. 123Reviewing VocabularyKeep It ShortFor each vocabulary term below, write a short, precisephrase that describes its meaning. For example, a shortphrase to describe ATP could be “energy for cells.”1. photosynthesis2. light-dependent reactions3. cellular respiration4. aerobic5. Krebs cycle6. fermentationGreek and Latin Word PartsUse the definitions of the word parts to answer thenext three questions.Prefix or Rootphoto-syn-aero-spirareMeaninglighttogetherairto breathe7. Describe how the meaning of the term photosynthesisis a combination of the meanings of the prefixes photoandsyn-.8. Explain how the prefix aero- is related to the termsaerobic and anaerobic.9. Why is the root spirare the basis of the term cellularrespiration? Explain your answer.Reviewing MAIN IDEAS10. Describe the roles of ADP and ATP in the transfer anduse of energy in cells.11. What types of carbon-based molecules are most oftenbroken down to make ATP? Explain how ATP productiondiffers depending on the type of carbon-basedmolecule that is broken down.12. Describe how and where energy from light is absorbedduring photosynthesis. What happens to the energyafter it is absorbed?13. Write the chemical equation for photosynthesis andexplain what it represents.14. What roles do electrons and hydrogen ions play in thelight-dependent reactions of photosynthesis?15. Describe how the light-independent reactions are thesynthesis part of photosynthesis.16. How does glycolysis contribute to the overall process ofcellular respiration?17. Write the chemical equation for cellular respiration andexplain what it represents.18. What is the function of the Krebs cycle? In your answer,describe the products of the Krebs cycle and whathappens to them.19. Explain the function of the electron transport chain incellular respiration. Why is oxygen needed for theelectron transport chain?20. Fermentation does not produce ATP. Why isfermentation such an important process in cells?21. How is alcoholic fermentation similar to lactic acidfermentation? How is it different?Chapter 4: Cells and Energy 129

Critical Thinking22. Infer Human brain cells do not use fermentation.Explain why a lack of oxygen for even a short period oftime might result in the death of brain cells.23. Apply Energy is transferred in several different waysduring photosynthesis and cellular respiration. Give twoexamples of how energy is transferred in the processes.Explain both examples.24. Analyze How do photosynthesis and cellular respirationform a cycle of energy storage and use?25. Synthesize How do cellular respiration and fermentationdepend on glycolysis? How does glycolysis depend onaerobic and anaerobic processes?26. Analyze Consider the following two groups ofprocesses:• light-dependent reactions and electron transport chain• Calvin cycle and Krebs cycleWhich pair is nearly identical? Which is nearlyopposite? Explain.Interpreting VisualsUse the diagram to answer the next three questions.Analyzing DataUse information in the text and the graph below toanswer the next two questions.Plants have several different molecules that togetherabsorb all of the different wavelengths of visible light.Visible light ranges between the wavelengths of about400 and 700 nanometers (nm).ABSORPTION OF LIGHT BY CHLOROPHYLL 30. Analyze Data What range of wavelengths is absorbedby chlorophyll a? chlorophyll b?31. Synthesize Suppose a type of plant has onlychlorophylls a and b and is exposed to differentwavelengths of light. At which wavelengths would therebe the greatest amounts of carbon dioxide in the airaround the plants? The least? Explain your answers.27. Apply Which process stores energy? Which processreleases energy? How do you know?28. Infer Use information in the visual to explainwhy a plant would be able to survive in a sealedtransparent container.29. Infer Which of the processes in the diagram is necessaryfor most living things to survive? Explain.Connecting CONCEPTS32. Write an Analogy Suppose that photosynthesis orcellular respiration takes place in a factory. You are atour guide at the factory, explaining each step of theprocess to a group of visitors. Use analogies to describewhat happens at each step. For example, you coulddescribe the photosystems of photosynthesis as “thegreen machines next to windows to absorb light.” Besure to include important details of the processyou select.33. Analyze Look at the micrograph of diatoms and thephotograph of the whale on pages 98–99. Write aparagraph that explains the two ways in which the whaledepends on diatoms for the energy that its cells need.130 Unit 2: Cells

For more test practice,go to ClassZone.com.1. A group of students wants to find out how muchcarbon dioxide is used by plants during thedaytime. What control could be used in thisexperiment?A the amount of water used during the daytimeB the amount of oxygen released at nightC the amount of carbon dioxide used at nightD the amount of oxygen used during the daytime2. Which of the following groups of organismsuses cellular respiration in mitochondria toproduce ATP for their energy needs?A plants onlyB eukaryotesC animals onlyD prokaryotes3.carbohydrateslipidsproteins4. Photosynthesis is a part of various cycles thathelp to move oxygen and carbon through theenvironment. What form of abiotic carbon doplants remove from the environment?A glucoseB starchC carbon dioxideD ATPTHINK THROUGH THE QUESTIONSome terms in questions, such as biotic or abiotic,may be unfamiliar to you. Often, terms like theseare used to present an example but are notnecessary to answer the question. Find theimportant pieces of information in the questionand focus on those points.5. Aare types ofcarbon compoundsthat are brokendown to produce??This concept map shows some of the carbonbasedmolecules in cells. Some of thesemolecules can be broken down to produceusable chemical energy. Which of the followingterms best completes this concept map?A electronsB ATPC lactic acidD hydrogen ionsWhich of the following best represent thefinal products of the chemical reactions thattake place inside the organelle labeled A inthis diagram?A sugars, oxygenB ATP, electronsC ATP, sugarsD carbon dioxide, water6. Which process is represented by the followingchemical equation?6CO 2 + 6H 2 O C 6 H 12 O 6 + 6O 2A photosynthesisB fermentationC glycolysisD cellular respirationChapter 4: Cells and Energy 131