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Photosynthesis provides food for living organisms andreplenishes oxygen in the atmosphere. Photosynthesisutilizes the portion of the electromagnetic spectrumknown as visible light. Chlorophyll—both chlorophylla and chlorophyll b—absorbs violet, blue, and red lightbetter than light of other colors. These pigments arepresent in the thylakoid membrane within the grana orchloroplasts. They participate in the so-called light-dependent reactions.

The light-dependent reactions involve a cyclic elec-tron pathway and a noncyclic electron pathway. Onlyphotosystem I is required for the cyclic electron path-way, in which electrons energized by the sun leave thereaction-center chlorophyll a and then pass down an elec-tron transport system with the concomitant buildup ofATP before returning to chlorophyll a. In the noncyclicelectron pathway, energized electrons leave chlorophylla of photosystem II, pass down an electron transportsystem, and enter photosystem I, where they are ener-gized once more before being accepted by NADPH. Theoverall result from the noncyclic pathway is the produc-tion of NADPH and ATP. Oxygen is also liberated whenwater is split and electrons enter chlorophyll a (photosystemII) to replace those lost. The NADPH and ATP from thelight-dependent reactions of photosynthesis are used tobuild a carbohydrate in the light-independent reactions,which occur in the stroma of chloroplasts.

ATP production during the light-dependent reac-tions requires chemiosmosis. Hydrogen ions are con-centrated in the thylakoid space; when water splits, itreleases hydrogen ions, and carriers within the cytochromecomplex of the electron transport system pump the hy-drogen ions to the thylakoid space. The hydrogen ions flowdown their concentration gradient through a channel ina protein having an ATP synthase, which forms ATPfrom ADP and P .

The light-independent reactions occur during theCalvin cycle: carbon dioxide is fixed by RuBP, it is re-duced to PGAL (this requires the ATP and NADPH fromthe light-dependent reactions), and RuBP is regenerated.One PGAL out of every six joins with another PGAL toform glucose-6-phosphate.

C3 photosynthesis (the first molecule after fixationis a C3 molecule) occurs when a plant uses the Calvincycle directly. Plants have also evolved two other typesof photosynthesis: C4 photosynthesis and CAM photo-synthesis. C4 plants fix carbon dioxide in mesophyll cells(which results in a C4 molecule) and then transport it tobundle sheath cells, where it enters the Calvin cycle.CAM plants fix carbon dioxide at night and then releaseit during the day to the Calvin cycle. C4 and CAM pho-tosynthesis are adaptations to hot, dry environments,since these processes allow the stomates to close to con-serve water.

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PART II PLANT BIOLOGY

8PHOTOSYNTHESIS

CHAPTER REVIEW

STUDY QUESTIONS

Study the text section by section as you answer the questions that follow.

8.1 RADIANT ENERGY (P. 136)

• Plants make use of solar energy in the visible light range when they carry on photosynthesis.

1. Indicate whether the following statements about the importance of photosynthesis are true (T) or false (F):a. It makes food for animals.b. It promotes the breakdown of biodegradable wastes.c. It returns carbon dioxide to the atmosphere.d. It returns oxygen to the atmosphere.

2. Indicate whether the following statements about solar energy are true (T) or false (F):a. Chlorophylls a and b absorb violet, blue, and red light best.b. Photons of visible light energize electrons without harming cells.c. Photosynthesis uses infrared light efficiently in the daytime.d. Photosynthesis uses ultraviolet light efficiently at night.

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8.2 STRUCTURE AND FUNCTION OF CHLOROPLASTS (P. 138)

• Photosynthesis takes place in chloroplasts, organelles that contain membranous thylakoids surrounded by afluid-filled stroma.

• Photosynthesis has two sets of reactions: solar energy is captured by the pigments in thylakoids, and carbondioxide is reduced by enzymes in the stroma.

3. Photosynthesis refers to the ability of plants, algae, and a few kinds of bacteria to make their

own a.______________ in the presence of b.______________. In plants, photosynthesis is carried on

in c.______________.4. The green pigment a.______________ is found within the membrane of the b.______________, and it is here

that c.______________ energy is captured.

5. A chloroplast contains flattened, membranous sacs called a.______________ that are stacked like poker chips

into b.______________. The fluid-filled space surrounding the grana is called the c.______________.

6. Label this diagram of a chloroplast using the following terms:granumstromathylakoid

7. From the diagram in question 6, the light-

dependent reactions would be associated with

the a.______________, and the light-independent

reactions would be associated with

the b.______________.

8. The light-dependent reactions drive the light-

independent reactions. The light-independent

reactions use the NADPH and a.______________

from the light-dependent reactions to

reduce b.______________ to a c.______________.

9. Label each of the following as either light-dependent (LD) or light-independent (LI) reactions:a. energy-capturing reactionsb. synthesis reactionsc. carbon dioxide becomes carbohydrated. water gives off oxygene. NADPH and ATP are madef. NADPH and ATP are used

8.3 SOLAR ENERGY CAPTURE (P. 140)

• Solar energy energizes electrons and permits a buildup of ATP.

10. The thylakoid membrane has two light-gathering units called a.______________ and b.______________. Within

each unit are green pigments called c.______________ and yellow-orange pigments called d.______________.

11. Label this diagram of the cyclic electron pathwaywith the following terms:

ADPATPelectron acceptorlight-dependent reactionsphotosystem Ipigment complex

12. Label this diagram of the noncyclic electronpathway with the following terms (some are usedmore than once):

electron acceptor (used twice)electron transport systemlight-dependent reactionslight-independent reactionsphotosystem Iphotosystem II

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63

2 PGA

1.

2.

3.

4.

PGAL

PGAL

other organicmolecules

glucose phosphate

RuBP

C6

8.4 CARBOHYDRATE SYNTHESIS (P. 143)

• Carbon dioxide reduction requires energized electrons and ATP.

16. Match the numbers in the diagram to thefollowing descriptions of events in the Calvincycle (some numbers are used more than once):_____ a. ATP only required_____ b. CO2 reduction reaction_____ c. NADPH and ATP required_____ d. CO2 taken up, CO2 fixation_____ e. glucose formed after six turns of cycle_____ f. five PGAL required to form three

molecules of product

13. What is the role of each of these in the noncyclic electron pathway?

reaction-center chlorophyll a a. _____________________________________________________________________

electron acceptors b. _______________________________________________________________________________

NADP+ c. ________________________________________________________________________________________

electron transport system d. _________________________________________________________________________

water e. __________________________________________________________________________________________

ATP Production (p. 142)

• Chemiosmosis depends on an electrochemical gradient that the electron transport system establishes.

14. The concentration of H+ in the a.______________ space is b.______________, compared to the lower H+

concentration in the c.______________. The flow of H+ down its concentration gradient provides the energy

for an enzyme called d.______________ to produce e.______________ from ADP + P .

The Thylakoid Membrane (p. 142)

15. Match the complexes in the thylakoid membrane with the following functions:1 produces ATP from ADP + P2 transports electrons and stores H+ in the thylakoid space3 captures solar energy; water is split, releasing electrons and oxygen4 captures solar energy; NADP+ is reduced to NADPH

a. photosystem IIb. cytochrome complexc. photosystem Id. ATP synthase complex

17. Rearrange the letters to indicate the order that the

molecules appear during or as a result of the

Calvin cycle. _______________________________

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a. CO2

b. glucose phosphatec. PGAd. PGALe. starch

8.5 OTHER ASPECTS OF PHOTOSYNTHESIS (P. 145)

• Photosynthesis provides most of the food for the biosphere.• Plants use either C3, C4, or CAM photosynthesis, named for the manner in which CO2 is fixed.

18. Label the following as describing C3, C4, and/or CAM plants:a. predominate in spring and cooler summer weatherb. PEPCasec. succulent plants, cacti; live in hot, arid regionsd. wheat, rice, oatse. predominate in hot, dry summer weatherf. product of CO2 fixation is PGAg. CO2 fixation occurs at night and C4 molecules are stored until daylighth. photorespiration occursi. stomates are closed during the day to conserve waterj. end product of CO2 fixation is oxaloacetatek. sugarcane, corn, Bermuda grassl. chloroplasts only in mesophyll cells

m. chloroplasts in bundle sheath cells and mesophyll

8.6 PHOTOSYNTHESIS VERSUS AEROBIC CELLULAR

RESPIRATION (P. 147)

• In photosynthesis, carbon dioxide is reduced to glucose and water is oxidized, releasing oxygen gas. Inaerobic cellular respiration, carbohydrate is oxidized to carbon dioxide, and oxygen is reduced to water.

Are these statements True (T) or False (F)? If false, rewrite the statement as a true one.

19. Carbon dioxide is oxidized during photosynthesis and oxygen is oxidized during aerobic cellular respiration.

Answer: __________ Restatement: ____________________________________________________________

________________________________________________________________________________________

20. The coenzyme active during photosynthesis is NADP+, and the coenzyme active during cellular respiration is

NAD+.


________________________________________________________________________________________

21. PGAL becomes PGA during photosynthesis and PGA becomes PGAL during aerobic cellular respiration.


________________________________________________________________________________________

22. Oxygen is given off by photosynthesis and carbon dioxide is given off by aerobic cellular respiration.


________________________________________________________________________________________

Photosynthesis: A Play in Two Acts

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Photosynthesis, which is now playing in chloroplasts,has two acts. The first act is the light-dependent reac-tions, and the second act is the light-independent reac-tions. The theater has a revolving stage of two parts,shown in the diagram.

1. Place these “props” on the correct “stage”: grana(thylakoids) and stoma.

2. Place these “props” on the correct “stage”:Calvin cycle, energy-capturing reactions, andsynthesis reactions.

3. Place these “props” on the correct “stage”:chlorophyll and RuBP.

4. Certain actors/actresses walk on during either thefirst act or the second act (not both acts), andcertain others walk off during either the first actor the second act. Entrances and exits arerepresented by the arrows. Show where thefollowing “actresses/actors” either walk on orwalk off: CO2, H2O, light, O2, and PGAL.

5. Certain actors/actresses are in the first act, butthen they change their costumes and move to thesecond act. Naturally, they have to get readyagain for the first act. Movement of theseactors/actresses is indicated by arrows betweenthe acts. Show where the following“actresses/actors” belong: ADP, ATP, NADP+,and NADPH.

CHAPTER TEST

OBJECTIVE QUESTIONS

Do not refer to the text when taking this test. In ques-tions 1–6, match the terms with the following definitions:

a. organic product of photosynthesisb. released by photosynthesisc. reactant of photosynthesisd. site of light-dependent reactionse. site of light-independent reactionsf. molecule-absorbing solar energy

1. chlorophyll2. oxygen3. stroma4. sugar5. thylakoid membrane6. water

7. Each of the following is a product of photosyn-thesis EXCEPTa. carbon dioxide.b. organic food.c. oxygen.d. carbohydrate.

8. Photosynthesis occurs best at wavelengths thatarea. blue.b. gamma.c. infrared.d. ultraviolet.

9. Each is a product of light-dependent reactions EXCEPTa. ATP.b. NADPH.c. oxygen.d. sugar.

10. The cyclic pathways of photosynthesis producea. ATP only.b. NADPH only.c. ATP and NADPH.d. organic sugars only.

11. Carbon dioxide fixation occurs when CO2 com-bines witha. ATP.b. NADPH.c. PGAL.d. RuBP.

12. Which of the following pathways uses the en-zyme PEPCase?a. C2

b. C3

c. CAMd. CAP

13. The enzyme that produces ATP from ADP + Pin the thylakoid isa. RuBP carboxylase.b. rubisco.c. ATPase.d. ATP synthase.e. coenzyme A.

14. Which statement is NOT true regarding chemiosmosis?a. H+ concentration is higher in the stroma than

in the thylakoid space.b. The electron transport system pumps H+ from

the stroma into the thylakoid space.c. The ATP synthase complex is present in the

thylakoid membrane.d. All of these are true.

15. Which of the following is NOT a stage in theCalvin cycle?a. carbon dioxide fixationb. carbon dioxide oxidationc. carbon dioxide reductiond. ribulose bisphosphate regeneration

16. Which of these descriptions is NOT true of photosynthesis?a. not affected by temperatureb. not affected by solar energyc. requires a supply of oxygend. involves a reduction reactione. more likely to occur during the day

17. Which of these descriptions is NOT true ofchlorophyll?a. absorbs solar energyb. located in the granac. located in thylakoid membranesd. passes electrons directly to NADP+

e. passes electrons to an acceptor molecule18. The two major sets of reactions involved in pho-

tosynthesis area. the cyclic and noncyclic electron pathways.b. glycolysis and the Krebs cycle.c. the Calvin and Krebs cycles.d. the Calvin cycle and the electron transport

system.e. the light-dependent and light-independent

reactions.19. Which of the following statements is NOT true

of the Calvin cycle?a. RuBP is regenerated with the use of ATP.b. Glucose phosphate is synthesized from PGAL.c. NADPH is used to reduce PGAL to PGA.d. Five molecules of PGAL are used to reform

three molecules of RuBP.20. Photosystem II gets replacement electrons from

a. the sun.b. water molecules.c. ATP.d. photosystem I.e. NADPH.

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THOUGHT QUESTIONS

Answer in complete sentences.21. How is life dependent on photosynthesis?

22. Why is photosynthesis dependent on the high degree of compartmentalization in chloroplasts?

Test Results: ______ Number right ÷ 22 = ______ × 100 = ______ %

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1. a. T b. F c. F d. T 2. a. T b. T c. F d. F3. a. food b. sunlight c. chloroplasts 4. a. chloro-phyll b. thylakoids c. solar 5. a. thylakoids b. granac. stroma 6. a. thylakoid b. stroma c. granum7. a. thylakoid and granum b. stroma 8. a. ATP b. car-bon dioxide c. carbohydrate 9. a. LD b. LI c. LId. LD e. LD f. LI 10. a. photosystem I b. photosys-tem II c. chlorophyll d. carotenoids 11. a. light-de-pendent reactions b. photosystem I c. ADP d. ATPe. electron acceptor f. pigment complex 12. a. light-dependent reactions b. photosystem II c. electron ac-ceptor d. electron transport system e. photosystem If. electron acceptor g. light-independent reactions.13. a. releases electrons that become excited from solarenergy b. accepts energized electrons from the reaction-center chlorophyll a and sends them to the electron trans-port system c. accepts electrons and hydrogen and becomesNADPH d. stores energy as the electrons fall to a lowerenergy level e. splits, releasing oxygen and hydrogenions 14. a. thylakoid b. higher c. stroma d. ATP syn-thase e. ATP 15. a. 3 b. 2 c. 4 d. 1 16. a. 2, 3 b. 2c. 2 d. 1 e. 4 f. 3 17. a, c, d, b, e 18. a. C3 b. C4,CAM c. CAM d. C3 e. C4 f. C3 g. CAM h. C3

i. CAM j. C4 k. C4 l. C3 m. C4 19. F Carbon dioxideis reduced during photosynthesis and oxygen is reduceduring aerobic cellular respiration. 20. T 21. F. PGAbecomes PGAL during photosynthesis and PGAL be-comes PGA during aerobic cellular respiration. 22. T

STUDY EXERCISES PHOTOSYNTHESIS:A PLAY

IN TWO ACTS

lightgrana (thylakoids)energy-capturing reactionschlorophyll

stromaCalvin cyclesynthesis reactionsRuBP

NADP+

ADPNADPH

ATP

H2O O2

CO2 PGAL

ANSWER KEY

CHAPTER TEST

1. f 2. b 3. e 4. a 5. d 6. c 7. a 8. a 9. d10. a 11. d 12. c 13. d 14. a 15. b 16. c17. d 18. e 19. c 20. b 21. Through photosynthe-sis, plants (and algae) produce food for themselves andall other living things. These organisms are the produc-ers at the start of food chains of all types. Animals feeddirectly on photosynthesizers or on other animals thathave fed on photosynthesizers. 22. Chemiosmosis can-not occur without a membrane that maintains a differ-ence in hydrogen ion concentration. In plant cells, hydrogenions build up within the thylakoid space and then they flowdown their concentration gradient through an ATP syn-thase complex located in the thylakoid membrane.

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