Rucete ✏ AP Biology In a Nutshell
6. Photosynthesis — Practice Questions 2
This chapter explores how light energy is converted into chemical energy through light-dependent reactions and the Calvin cycle.
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(Multiple Choice — Click to Reveal Answer)
1. What molecule acts as the initial electron donor in the light-dependent reactions of photosynthesis?
(A) Oxygen
(B) Water
(C) Carbon dioxide
(D) Glucose
Answer
(B) — Water is split to supply electrons during the light-dependent reactions.
2. During the Calvin cycle, carbon dioxide is incorporated into an organic molecule through a process known as:
(A) Chemiosmosis
(B) Carbon fixation
(C) Oxidative phosphorylation
(D) Photolysis
Answer
(B) — Carbon fixation incorporates inorganic CO₂ into organic compounds.
3. What structure within the chloroplast contains the photosystems involved in the light reactions?
(A) Stroma
(B) Outer membrane
(C) Thylakoid membrane
(D) Inner envelope
Answer
(C) — Photosystems I and II are embedded in the thylakoid membranes.
4. What is the role of chlorophyll in photosynthesis?
(A) To capture carbon dioxide
(B) To synthesize glucose directly
(C) To absorb light energy
(D) To produce ATP directly
Answer
(C) — Chlorophyll captures photon energy and excites electrons.
5. Which of the following is directly produced by the Calvin cycle?
(A) ATP
(B) Oxygen gas
(C) G3P
(D) NADPH
Answer
(C) — G3P is the 3-carbon sugar produced in the Calvin cycle.
6. What product of the light reactions carries energy to the Calvin cycle?
(A) Oxygen
(B) Water
(C) NADPH
(D) Glucose
Answer
(C) — NADPH carries high-energy electrons to power reactions in the Calvin cycle.
7. The process that uses a proton gradient to produce ATP in chloroplasts is called:
(A) Photolysis
(B) Chemiosmosis
(C) Glycolysis
(D) Calvin cycling
Answer
(B) — Chemiosmosis uses a proton gradient to drive ATP synthesis.
8. Which gas is consumed during the Calvin cycle?
(A) Oxygen
(B) Water vapor
(C) Carbon dioxide
(D) Nitrogen
Answer
(C) — Carbon dioxide is fixed into organic molecules during the Calvin cycle.
9. What happens to the oxygen atoms from water molecules during photosynthesis?
(A) Incorporated into glucose
(B) Released as oxygen gas
(C) Stored as NADPH
(D) Attached to ATP molecules
Answer
(B) — Water is split and oxygen atoms are released as oxygen gas.
10. Which term best describes organisms that can perform photosynthesis?
(A) Heterotrophs
(B) Photoautotrophs
(C) Chemoautotrophs
(D) Decomposers
Answer
(B) — Photoautotrophs use light energy to synthesize organic compounds.
11. The main energy-capturing molecule in green plants is:
(A) Xanthophyll
(B) Chlorophyll a
(C) Anthocyanin
(D) Beta-carotene
Answer
(B) — Chlorophyll a is the primary pigment for light absorption in plants.
12. The Calvin cycle depends directly on which two products of the light-dependent reactions?
(A) CO₂ and O₂
(B) H₂O and NADP+
(C) ATP and NADPH
(D) Glucose and RuBP
Answer
(C) — ATP and NADPH supply energy and reducing power to the Calvin cycle.
13. In photosynthesis, the role of water is to:
(A) Serve as a reactant in the Calvin cycle
(B) Release oxygen and donate electrons
(C) Absorb light energy
(D) Form glucose directly
Answer
(B) — Water provides electrons and releases oxygen when split.
14. Photosystem I primarily functions to:
(A) Split water
(B) Regenerate RuBP
(C) Synthesize NADPH
(D) Create a proton gradient
Answer
(C) — Photosystem I re-energizes electrons and helps produce NADPH.
15. What is the main source of carbon atoms in the sugars produced during photosynthesis?
(A) Air
(B) Water
(C) Soil minerals
(D) Sunlight
Answer
(A) — Carbon dioxide from the air provides the carbon atoms for sugar molecules.
16. Which component of the chloroplast contains the Calvin cycle enzymes?
(A) Thylakoid membrane
(B) Stroma
(C) Grana
(D) Inner membrane
Answer
(B) — The stroma is the fluid where the Calvin cycle takes place.
17. What provides the energy needed for photophosphorylation in the light reactions?
(A) Oxygen
(B) Carbon dioxide
(C) Light energy
(D) Glucose
Answer
(C) — Light energy powers photophosphorylation to generate ATP.
18. In the chloroplast, what separates the thylakoid space from the stroma?
(A) Plasma membrane
(B) Thylakoid membrane
(C) Outer membrane
(D) Inner membrane
Answer
(B) — The thylakoid membrane separates the thylakoid interior from the stroma.
19. What is the first stable product of carbon fixation in the Calvin cycle?
(A) G3P
(B) 3-phosphoglycerate (3-PGA)
(C) Glucose
(D) RuBP
Answer
(B) — 3-PGA is the first stable 3-carbon product formed after carbon fixation.
20. Which event occurs during the reduction phase of the Calvin cycle?
(A) CO₂ binds with RuBP.
(B) ATP and NADPH are used to produce G3P.
(C) Oxygen is produced.
(D) Glucose is released directly.
Answer
(B) — ATP and NADPH reduce 3-PGA into G3P during the reduction phase.
21. The splitting of water during the light reactions provides which of the following?
(A) Glucose
(B) Electrons, protons, and oxygen
(C) ATP
(D) NADPH
Answer
(B) — Water splitting provides electrons, protons (H⁺), and oxygen gas.
22. Which factor most directly supplies energy to pump protons across the thylakoid membrane?
(A) Water splitting
(B) Electron transport chain
(C) Light absorption by chlorophyll b
(D) NADPH breakdown
Answer
(B) — The electron transport chain releases energy that pumps protons into the thylakoid space.
23. What is the final electron acceptor at the end of the light reactions’ electron transport chain?
(A) O₂
(B) H₂O
(C) NADP+
(D) ATP
Answer
(C) — NADP+ accepts electrons and forms NADPH at the end of the light reactions.
24. What process in photosynthesis most directly produces ATP?
(A) Carbon fixation
(B) Photolysis
(C) Chemiosmosis
(D) Oxygen evolution
Answer
(C) — ATP is produced by chemiosmosis across the thylakoid membrane.
25. If a plant’s thylakoid membranes were damaged, what process would be immediately affected?
(A) Calvin cycle carbon fixation
(B) ATP and NADPH production
(C) Glucose polymerization
(D) RuBP regeneration
Answer
(B) — Damage to thylakoid membranes would directly impair ATP and NADPH production in the light reactions.
26. If ATP synthase were inhibited in chloroplasts, what immediate effect would be observed?
(A) No NADPH formation
(B) No oxygen release
(C) Decreased ATP production
(D) Increased glucose output
Answer
(C) — ATP synthase inhibition would block ATP production during chemiosmosis.
27. How do the products of the light-dependent reactions interact with the Calvin cycle?
(A) They are broken down for energy.
(B) They provide energy and electrons for carbon fixation.
(C) They supply carbon dioxide.
(D) They replace water molecules.
Answer
(B) — ATP provides energy and NADPH provides electrons for the Calvin cycle.
28. Which best describes the flow of electrons in the light reactions of photosynthesis?
(A) Water → Photosystem I → Photosystem II → NADPH
(B) NADPH → ATP → Water
(C) Water → Photosystem II → Photosystem I → NADP+
(D) Oxygen → Chlorophyll → ATP
Answer
(C) — Electrons move from water to PSII, then PSI, and finally to NADP+.
29. In the Calvin cycle, ATP is mainly consumed during which phases?
(A) Carbon fixation only
(B) Reduction and regeneration
(C) Light absorption
(D) G3P export
Answer
(B) — ATP is used both in reducing 3-PGA and regenerating RuBP.
30. What would be the result if chlorophyll lost its ability to absorb light efficiently?
(A) Increased NADPH formation
(B) Decreased ATP production
(C) Enhanced carbon fixation
(D) Overproduction of oxygen gas
Answer
(B) — Without efficient light absorption, less energy would be available for ATP synthesis.
31. What process primarily drives the formation of the proton gradient during the light-dependent reactions?
(A) Active transport powered by ATP
(B) Energy release during electron transport
(C) Splitting of carbon dioxide
(D) NADPH oxidation
Answer
(B) — Energy from the electron transport chain pumps protons into the thylakoid lumen.
32. What would happen if NADP+ were absent during the light reactions?
(A) ATP would accumulate.
(B) Electrons would have no final acceptor.
(C) Carbon dioxide would be fixed faster.
(D) Water splitting would increase.
Answer
(B) — Without NADP+, electrons would have nowhere to go after PSI.
33. During the Calvin cycle, a deficiency of ATP would primarily affect which step?
(A) Carbon dioxide fixation
(B) Reduction of 3-PGA
(C) Regeneration of RuBP
(D) Oxygen release
Answer
(C) — ATP is crucial for regenerating RuBP from G3P molecules.
34. Which feature distinguishes cyclic electron flow from noncyclic electron flow in photosynthesis?
(A) Cyclic flow produces oxygen gas.
(B) Noncyclic flow generates ATP only.
(C) Cyclic flow recycles electrons back to PSI.
(D) Noncyclic flow occurs in mitochondria.
Answer
(C) — In cyclic flow, electrons return to PSI instead of moving to NADP+.
35. A scientist blocks the Calvin cycle enzymes in a plant. What immediate outcome would result?
(A) ATP production would stop.
(B) Oxygen would no longer be released.
(C) Carbon fixation would halt.
(D) NADPH synthesis would increase.
Answer
(C) — Blocking the Calvin cycle enzymes stops carbon dioxide fixation.
36. Explain why splitting water is essential for the light-dependent reactions.
Answer
Splitting water supplies electrons to replace those lost by chlorophyll in photosystem II, releases oxygen as a byproduct, and contributes protons to the proton gradient.
37. Describe the role of ATP synthase in the thylakoid membrane.
Answer
ATP synthase uses the energy of protons flowing down their gradient to catalyze the formation of ATP from ADP and inorganic phosphate.
38. How does the Calvin cycle maintain a continuous supply of RuBP?
Answer
Some G3P molecules produced during the Calvin cycle are used to regenerate RuBP using ATP, allowing the cycle to continue.
39. Predict the effect on photosynthesis if plants lacked chlorophyll b.
Answer
Photosynthesis would be less efficient because chlorophyll b broadens the spectrum of light wavelengths that can be absorbed.
40. Why is NADP+ reduction important during the light-dependent reactions?
Answer
Reducing NADP+ to NADPH stores high-energy electrons needed for the reduction phase of the Calvin cycle.
41. Explain why chemiosmosis is crucial for ATP production in chloroplasts.
Answer
Chemiosmosis uses the energy from the proton gradient across the thylakoid membrane to power ATP synthesis through ATP synthase.
42. What is the fate of oxygen molecules produced during photolysis?
Answer
Oxygen molecules diffuse out of the chloroplast and eventually out of the plant into the atmosphere as a waste product.
43. How does light energy initiate electron flow during the light reactions?
Answer
Light energy excites electrons in chlorophyll molecules, raising them to higher energy levels and enabling their transfer through the electron transport chain.
44. Why is the Calvin cycle referred to as the "dark reactions"?
Answer
The Calvin cycle does not directly require light to occur, although it depends on ATP and NADPH produced by the light reactions.
45. Explain the consequence if a plant could not regenerate RuBP.
Answer
Without RuBP regeneration, carbon dioxide could not be fixed into organic molecules, halting the Calvin cycle and glucose production.
46. What would happen if carbon dioxide concentrations dropped significantly inside a plant leaf?
Answer
The rate of the Calvin cycle would decrease because there would be less carbon available for fixation into organic molecules.
47. Compare the roles of photosystem I and photosystem II in the light reactions.
Answer
Photosystem II captures light to initiate electron flow and water splitting, while photosystem I re-energizes electrons for NADP+ reduction to NADPH.
48. Why is G3P considered a key intermediate in photosynthesis?
Answer
G3P can be used to synthesize glucose and other carbohydrates, serving as a crucial building block for plant metabolism.
49. How does the structure of a chloroplast enhance its ability to perform photosynthesis?
Answer
The arrangement of thylakoid membranes into stacks (grana) increases surface area for light absorption and proton gradient formation, boosting photosynthetic efficiency.
50. Predict the overall impact on the planet if photosynthesis ceased.
Answer
Without photosynthesis, oxygen production would stop, atmospheric CO₂ would rise, food webs would collapse, and most life forms would eventually die.