Free-Response Question1. Cellular respiration and photosynthesis are basic cellular processes. Below are key

events in cellular respiration and/or photosynthesis.

(a) Explain how a photo system converts light energy to chem.ical energy.(b) Explain the specific role of glycolysis in cellular respiration.(c) Describe the function of water in cellular respiration and photosyn-

thesis.

Multiple-Choice QuestionsI 1. (C) is correct. The purpose of cellular respiration in eukaryotes is to pro-

duce energy for cellular work in the form of ATP. Respiration is an aerobiprocess, meaning that it requires oxygen. Answer choices A and Bare incorrebecause respiration involves the breakdown (not the synthesis) of carboh '-drates, fats, and proteins. Choice D is wrong because ADP is the product of thdephosphorylation of ATP-it is left over after the energy from ATP has beereleased. Choice E is wrong because oxygen is required for cellular respiration..

I 2. (B) is correct. In the course of the reaction shown, potassium (K) is oxidized. Oxidation involves the loss of an electron while reduction is the gainan electron by an atom or molecule. In this reaction, potassium is oxidized anbromine is reduced. Both cellular respiration and photosynthesis involxnumerous oxidation-reduction (redox) reactions.

20. CAM plants avoid photorespiration by(A) fixing CO

2into organic acids during the

night; these acids then release CO2during

the day.(B) Performing the Calvin cycle at night.(C) Fixing CO2 into four-carbon compounds

in the mesophyll, which release CO2in

the bundle-sheath cells.(D) Using PEP carboxylate to fix CO

2to ribu-

lose bisphosphate (RuBP).(E) Keeping their stomata open during the

day.

21. How many "turns" of the Calvin cycle arerequired to produce one molecule of glucose?(A) 1(B) 2(C) 3(D) 6(E) 12

ANSWERS AND EXPLANATIONS

22. What are the final electron acceptors for theelectron transport chains in the light reactionof photosynthesis and in cellular respiration?(A) 02 in both(B) CO2 in both(C) H20 in the light reactions and 02 in res-

piration(D) P700 and NADP+ in the light reactions

and NAD+ or FAD in repiration(E) NADP+ in the light reactions and 02 in

respiration

92 PART II: A REVIEW OF TOPICS WITH SAMPLE QUESTIONS, ANSWERS, AND EXPLANATIONS

I 3. (D) is correct. The net energy result of glycolysis is the production of twomolecules of ATP and two molecules of NADH. Glycolysis is the first of thethree stages of respiration-the second being the citric acid cycle and the thirdbeing oxidative phosphorylation. During glycolysis, glucose is broken downand oxidized to form two molecules of pyruvate. Glycolysis occurs in thecytosol, and the pyruvate it produces travels to the mitochondria, where it isused in the citric acid cycle.

I 4. (B) is correct. In the citric acid cycle, 2 ATPs are produced. The citric acidcycle takes in a molecule called acetyl CoA (pyruvate is converted into acetylCoA before it enters the citric acid cycle), and this is joined to a four-carbonmolecule of oxaloacetate to form a six-carbon compound citrate that is thenbroken down again to produce oxaloacetate; the oxaloacetate reenters thecycle. In the course of the citric acid cycle, the following are produced: 4 CO2, 2ATP,6 NADH, and 2 FADH2.

I s. (D) is correct. The process that produces the most ATP during cellular respi-ration is oxidative phosphorylation. FADH2 and NADH donate electrons tothe electron transport chain, which is coupled to ATP synthesis by chemios-mosis; the movement of electrons down the electron transport chain creates anH+ gradient across the mitochondrial membrane, which drives the synthesis ofATP from ADP.About 34 ATP are produced per glucose molecule.

I 6. (D) is correct. In glycolysis, glucose is oxidized to two molecules of pyruvate.This is the first step in cellular respiration, and it also produces 2 ATP and 2NADH.

I 7. (A) is correct. In chemiosmosis, the hydrogen ion gradient created by thetransfer of electrons in the electron transport chain provides the power to syn-thesize ATP from ADP.

I 8. (E) is correct. Fermentation is an anaerobic alternative to cellular respira-tion. It consists of glycolysis and several reactions that serve to regenerateNAD+. Electrons are transferred from NADH to pyruvate or its derivatives,and the NAD+ oxidizes sugar in glycolysis. There are two main types of fer-mentation: alcohol fermentation (which creates ethanol as a product) and lac-tic acid fermentation (which creates lactate).

I 9. (B) is correct. The electron transport chain is a series of inner mitochondrialmatrix membrane-embedded molecules that are capable of being oxidized andreduced as they pass along electrons. The energy produced from the passage ofthese electrons down the chain is used to create an H+ gradient across themembrane, and the flow of H+ down the gradient and back across the mem-brane powers the phosphorylation reaction of ADP to form ATP.

I 10. (C) is correct. The citric acid cycle includes the final reactions for thebreakdown of glucose that began in glycolysis. The pyruvate from glycolysis isconverted into acetyl CoA, which enters the cycle and is joined to oxaloacetateto create citrate, which is then converted to oxaloacetate again and reused. Thiscycle gives off CO2 and forms 1ATP, 3 NADH, and 1 FADH2. The cycle goesthrough one rotation to break down each of the molecules of pyruvate pro-duced in glycolysis (which of course is first converted to acetyl CoA), so the netresult of the breakdown of one glucose molecule is 2 ATP, 6 NADH, and2FADH2·

TOPIC 3: RESPIRATION AND PHOTOSYNTHESIS 93

I 11. (C) is correct. When muscle cells in the body are depleted of oxrgen, theyswitch from cellular respiration to lactic acid fermentation. In lactic acid fer-mentation, NADH reduces pyruvate directly, and lactate is formed as a wasteproduct. This lactic acid fermentation occurs when all of the cell's metabolicmachinery is using available oxygen to break down sugars in the cell.

I 12. (A) is correct. Groups of photosynthetic pigment molecules in the thy-lakoid membrane are called photosystems. The two photosystems involved inphotosynthesis are photosystem I and photosystem II. Both contain chloro-phyll molecules and many proteins and other organic molecules, and bothhave a light-harvesting complex that harnesses incoming light. Each of thesephotosystems contains a reaction center, where chlorophyll a and the primaryelectron acceptor are located.

I 13. (B) is correct. The main products of the light reactions of photosynthesisare NADPH andATP. NADPH andATP are used to convert CO2 to sugar in theCalvin cycle. The enzyme rubisco combines CO2 with ribulose bisphosphate(RuBP), and electrons from NADPH and energy from ATP to synthesize athree-carbon molecule called glyceraldehyde-3-phosphate.

I 14. (B) is correct. The process in photosynthesis that bears the closest resem-blance to chemiosmosis and oxidative phosphorylation in cellular respirationis linear photophosphorylation. In this process, energy from the transfer ofelectrons down the electron transport chain is used to create a hydrogen iongradient used in the making of ATP.Later, the energy stored in this ATP is usedduring the formation of carbohydrates in the Calvin cycle.

I 15. (E) is correct. The organic product of the Calvin cycle, which may be usedlater to build large carbohydrates in the cell, is glyceraldehyde-3-phosphate, orG3P. This molecule is created as a result of the fixation of three molecules ofCO2, which costs the cell ATP and NADPH that were created in the light reac-tions of photosynthesis.

III 16. (E) is correct. C4 and CAM plants both grow better than do C3 plants underconditions of increased median air temperature and decreased relative humid-ity. Both C4 and CAM plants use an alternative method of carbon fixation thatenables them to fix carbon into an acid intermediate for later deposit into theCalvin cycle.

I 17. (D) is correct. The electron transport chains pump protons across mem-branes from regions of low H+ concentrations to regions of high H+ concen-trations. This proton pumping occurs in both mitochondria and chloroplasts,and the protons then diffuse (with the concentration gradient) back across themembrane through ATP synthases. This drives the synthesis of ATP.

I 18. (D) is correct. ATP synthase is located in the thylakoid membrane. Noticethe direction of flow of protons through ATP synthase in photosynthesis: theprotons flow from the thylakoid space to the stroma. ATP is produced in thestroma, where it will be used by the Calvin cycle.

I 19. (C) is correct. When water is split, three products are formed: two protons,an oxygen atom that immediately bonds with another oxygen to form °2, andtwo electrons. The electrons immediately feed the P680 chlorophyll a in thereaction center of Photo system II. The ultimate electron donor in photosyn-thesis is water.

94 PART II: A REVIEW OF TOPICS WITH SAMPLE QUESTIONS, ANSWERS, AND EXPLANATIONS

I 20. (A) is correct. CAM plants separate the two stages of photosynthesis tem-porally to reduce photorespiration. This is accomplished by fixing CO2 at nightusing PEP carboxylase and storing the carbon in organic acids. During the daywhen CAM plants have their stomata closed to conserve water, the carbonfrom the organic acids is chemically released and used in the Calvin cycle.

I 21. (D) is correct. Each turn of the Calvin cycle involves the enzyme rubiscofixing one atom of carbon. It follows that it would take six turns to produce thesix carbon sugar glucose. Students sometimes miss this question by confusingthe Calvin cycle with the Krebs or citric acid cycle of cellular respiration. Readthese questions carefully, being disciplined enough to carefully identify whatthe question is asking.

I 22. (E) is correct. The light reactions of photosynthesis move electrons fromtheir low energy state in water to a higher energy level when the electrons aredonated to NADP+ to make NADPH. In cellular respiration electrons passdown the electron transport chain from high to low potential energy, ulti-mately combining with 02 and hydrogen ions to form water.

-Response Question(a) Although plants have two photosystems, they both work in the same way.Both

photo systems have two components: the light harvesting complexes and thereaction center. The light-harvesting complex is made up of many chlorophylland accessory pigment molecules. When one of the pigment moleculesabsorbs light energy in the form of photons, one of the molecule's electrons israised to an orbital of higher potential energy. The pigment molecule is thensaid to be in an "excited" state. The increase in potential energy is transferred tothe reaction center of the photosystem. The reaction center consists of twochlorophyll a molecules, which use the increased potential energy passed tothem by the photosynthetic pigments to donate electrons to the primary elec-tron acceptor. The solar-powered transfer of an electron from the reaction cen-ter chlorophyll a pair to the primary electron acceptor is the first step of thelight reactions. This is the conversion of light energy to chemical energy.

(b) Glycolysis is the first stage of cellular respiration and occurs in the cytoplasm.Glycolysis involves the breakdown of glucose to two pyruvate molecules. Toaccomplish this, 2 ATP molecules are invested, which helps to destabilize glu-cose, making it more reactive and allowing glucose to break into two three-carbon molecules. By the time the pathway has produced pyruvate, 4 ATPmolecules have been produced along with 2 NADH molecules. This gives a netenergy gain of 2 ATPs and 2 NADHs. Thus, one important role of glucose is toproduce energy molecules for the cell to use in its life processes. The secondrole is to produce pyruvate, which can feed into the citric acid cycle in themitochondria and ultimately into the electron transport chain where most ofthe ATP in cellular respiration is produced.

(c) In cellular respiration water is a product of the reaction, whereas in photosynthe-sis water is a reactant. In cellular respiration water is formed when the electronsat the end of the electron transport chain in the cristae membrane of the mito-chondria combine with hydrogen ions and an atom of oxygen to form water.Water is the ultimate electron acceptor in cellular respiration. In photosynthesis

TOPIC 3: RESPIRATION AND PHOTOSYNTHESIS 95

an enzyme splits a water molecule into two electrons, two hydrogen ions, and anoxygen atom. The electrons are supplied as needed directly to the chlorophyllmolecules in the reaction center of photo system II. In photosynthesis water is theultimate electron donor.

This response shows that the writer used the following key terms in context, show-ing the writer's knowledge of their meanings and relatedness:

photosystemslight harvesting complexesreaction centerchlorophyll

accessory pigmentsprimary electron acceptorATPNADPH

The response also contains an explanation of the following subjects and processes:

-role of photosystems in photosynthesis-how light energy becomes transformed to chemical energy-function of glycolysis in overall scheme of cellular respiration-ATP and NADPH as important energy molecules in the cell-role of water as an electron sink or an electron donor

96 ? fl. II: A REVIEW OF TOPICS WITH SAMPLE QUESTIONS, ANSWERS, AND EXPLANATIONS