Rucete ✏ AP Biology In a Nutshell
6. Photosynthesis — Practice Questions 3
This chapter explains how photoautotrophs capture light energy and transform it into chemical energy through light-dependent reactions and the Calvin cycle.
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(Multiple Choice — Click to Reveal Answer)
1. Which of the following best describes the role of NADP+ in photosynthesis?
(A) It captures light energy.
(B) It acts as the final electron acceptor in the light reactions.
(C) It splits water molecules.
(D) It fixes carbon dioxide into sugars.
Answer
(B) — NADP+ accepts electrons and protons to become NADPH at the end of the light reactions.
2. Which pigment primarily drives the light-dependent reactions by absorbing blue and red light?
(A) Chlorophyll a
(B) Chlorophyll b
(C) Carotenoids
(D) Xanthophyll
Answer
(A) — Chlorophyll a is the primary pigment that absorbs mainly blue and red wavelengths for photosynthesis.
3. In which compartment of the chloroplast does the proton gradient build up during photosynthesis?
(A) Stroma
(B) Thylakoid lumen
(C) Outer membrane
(D) Inner envelope
Answer
(B) — The proton gradient builds up inside the thylakoid lumen to power ATP synthesis.
4. What molecule directly provides the energy used in the Calvin cycle to form sugars?
(A) Light energy
(B) ATP
(C) NADP+
(D) Oxygen gas
Answer
(B) — ATP provides the chemical energy needed for carbon fixation and sugar formation.
5. What is the ultimate source of energy that powers the light-dependent reactions?
(A) Water
(B) Glucose
(C) Sunlight
(D) ATP
Answer
(C) — Sunlight excites electrons in chlorophyll to initiate the light-dependent reactions.
6. What is the main function of the photosystems in the thylakoid membrane?
(A) Pump protons into the stroma
(B) Capture light energy and excite electrons
(C) Break down glucose
(D) Synthesize oxygen directly
Answer
(B) — Photosystems absorb light and excite electrons to higher energy levels.
7. In photosynthesis, which event happens immediately after light is absorbed by chlorophyll?
(A) NADPH formation
(B) ATP breakdown
(C) Electron excitation
(D) Glucose synthesis
Answer
(C) — Absorption of light energy excites electrons in chlorophyll molecules.
8. What is the primary purpose of the Calvin cycle?
(A) Split water molecules
(B) Produce ATP and NADPH
(C) Fix carbon dioxide into organic molecules
(D) Release oxygen gas
Answer
(C) — The Calvin cycle converts inorganic CO₂ into organic compounds like G3P.
9. During the light reactions, oxygen is produced by:
(A) The fixation of carbon dioxide
(B) The splitting of water
(C) The breakdown of glucose
(D) The activation of Rubisco
Answer
(B) — Oxygen is released when water molecules are split during photolysis.
10. Where in the chloroplast does the Calvin cycle take place?
(A) Thylakoid space
(B) Stroma
(C) Thylakoid membrane
(D) Cytoplasm
Answer
(B) — The Calvin cycle occurs in the stroma of the chloroplast.
11. Which two molecules from the light-dependent reactions provide energy and electrons for the Calvin cycle?
(A) Oxygen and glucose
(B) ATP and NADPH
(C) RuBP and carbon dioxide
(D) Water and oxygen
Answer
(B) — ATP supplies energy, and NADPH provides high-energy electrons for the Calvin cycle.
12. What is the product formed when three carbon dioxide molecules complete one turn of the Calvin cycle?
(A) One glucose molecule
(B) One G3P molecule
(C) Two oxygen molecules
(D) Two ATP molecules
Answer
(B) — One G3P molecule is produced for every three CO₂ molecules fixed.
13. Which of the following best describes photophosphorylation in chloroplasts?
(A) Synthesis of glucose using carbon dioxide
(B) Formation of ATP using light energy
(C) Breakdown of water molecules
(D) Transfer of electrons to carbon dioxide
Answer
(B) — Photophosphorylation is the process of generating ATP using light-driven chemiosmosis.
14. What is the main product of photosystem I?
(A) ATP
(B) Oxygen
(C) NADPH
(D) Water
Answer
(C) — Photosystem I primarily functions to produce NADPH.
15. What provides the carbon atoms used to build organic molecules in photosynthesis?
(A) Sunlight
(B) Oxygen
(C) Water
(D) Carbon dioxide
Answer
(D) — Carbon dioxide provides the carbon source for organic molecules during photosynthesis.
16. Which process establishes a proton gradient during the light-dependent reactions?
(A) Electron transport chain
(B) Calvin cycle
(C) Carbon fixation
(D) Light absorption by chlorophyll b
Answer
(A) — The electron transport chain pumps protons into the thylakoid lumen, creating a proton gradient.
17. Which molecule is regenerated during the Calvin cycle to allow the cycle to continue?
(A) Glucose
(B) NADPH
(C) ATP
(D) RuBP
Answer
(D) — RuBP is regenerated so that carbon fixation can continue in the Calvin cycle.
18. What type of energy conversion occurs during photosynthesis?
(A) Light energy to chemical energy
(B) Mechanical energy to heat energy
(C) Chemical energy to light energy
(D) Heat energy to ATP energy
Answer
(A) — Photosynthesis transforms light energy into chemical energy stored in sugars.
19. In which photosystem is water split to replace lost electrons?
(A) Photosystem I
(B) Photosystem II
(C) Both photosystems
(D) Neither photosystem
Answer
(B) — Water is split at Photosystem II to replace electrons lost during light excitation.
20. Which event occurs directly as a result of light absorption by chlorophyll?
(A) Water molecule breakdown
(B) Excitation of electrons
(C) Production of glucose
(D) Synthesis of RuBP
Answer
(B) — Absorbed light energy excites electrons in chlorophyll.
21. What would happen if the proton gradient across the thylakoid membrane were eliminated?
(A) NADPH production would increase.
(B) ATP synthesis would decrease.
(C) Carbon dioxide fixation would immediately stop.
(D) Oxygen release would double.
Answer
(B) — Without a proton gradient, ATP synthase cannot produce ATP efficiently.
22. What is the fate of most of the G3P molecules produced during the Calvin cycle?
(A) Used to regenerate RuBP
(B) Immediately converted to oxygen
(C) Broken down for ATP production
(D) Stored directly as NADPH
Answer
(A) — Most G3P is used to regenerate RuBP, allowing the Calvin cycle to continue.
23. Which molecule directly donates electrons to the electron transport chain in photosystem II?
(A) Water
(B) NADPH
(C) Glucose
(D) Oxygen
Answer
(A) — Water donates electrons after photolysis at photosystem II.
24. Which gas is produced as a byproduct of the light reactions of photosynthesis?
(A) Nitrogen
(B) Oxygen
(C) Carbon dioxide
(D) Hydrogen
Answer
(B) — Oxygen gas is released when water molecules are split during the light reactions.
25. What describes the ultimate fate of the high-energy electrons captured by NADPH during the light reactions?
(A) Used to pump more protons
(B) Passed to carbon dioxide in the Calvin cycle
(C) Used to split additional water molecules
(D) Converted into ATP directly
Answer
(B) — High-energy electrons carried by NADPH reduce carbon dioxide during the Calvin cycle.
26. What would happen to the Calvin cycle if the supply of NADPH were suddenly cut off?
(A) Carbon fixation would accelerate.
(B) Reduction of 3-PGA would halt.
(C) RuBP regeneration would speed up.
(D) Water splitting would increase.
Answer
(B) — Without NADPH, the Calvin cycle cannot reduce 3-PGA into G3P.
27. Which event directly causes the proton concentration inside the thylakoid lumen to increase during the light reactions?
(A) Water splitting
(B) Glucose synthesis
(C) Carbon dioxide fixation
(D) ATP hydrolysis
Answer
(A) — Water splitting releases protons into the thylakoid lumen, raising the proton concentration.
28. If a mutation caused Rubisco to lose its ability to bind carbon dioxide, what would be the most immediate consequence for the plant?
(A) Oxygen production would stop.
(B) The Calvin cycle would halt at the fixation step.
(C) ATP would accumulate uncontrollably.
(D) Water splitting would cease.
Answer
(B) — Without Rubisco binding CO₂, carbon fixation cannot occur, stalling the Calvin cycle.
29. What is the fate of electrons that are excited in photosystem I during the light reactions?
(A) They return to photosystem II.
(B) They are transferred to NADP+ to form NADPH.
(C) They drive the formation of oxygen gas.
(D) They are used directly to produce glucose.
Answer
(B) — Electrons from PSI are transferred to NADP+ to produce NADPH.
30. Why is ATP needed in the regeneration phase of the Calvin cycle?
(A) To provide electrons for reduction
(B) To power the rearrangement of G3P into RuBP
(C) To split water molecules
(D) To generate oxygen gas
Answer
(B) — ATP supplies the energy to rearrange G3P molecules into RuBP during regeneration.
31. Which process is most directly responsible for creating the oxygen released during photosynthesis?
(A) Electron transfer through photosystems
(B) Oxidation of water during photolysis
(C) Splitting of carbon dioxide molecules
(D) Breakdown of glucose during light reactions
Answer
(B) — Oxygen gas is produced when water molecules are split during photolysis.
32. How would photosynthesis be affected if a plant's thylakoid membranes were damaged?
(A) Carbon fixation would become faster.
(B) Proton gradient formation would be impaired.
(C) NADP+ would be reduced more quickly.
(D) Rubisco would stop functioning.
Answer
(B) — Without intact thylakoid membranes, the proton gradient could not form, affecting ATP synthesis.
33. During photosynthesis, which molecule acts as the terminal electron acceptor at the end of the light reactions?
(A) Oxygen
(B) ATP
(C) NADP+
(D) RuBP
Answer
(C) — NADP+ accepts electrons at the end of the light reactions, forming NADPH.
34. Why is photosystem II essential for the overall process of photosynthesis?
(A) It produces ATP directly.
(B) It initiates electron flow and generates oxygen by splitting water.
(C) It creates glucose immediately.
(D) It regenerates RuBP for the Calvin cycle.
Answer
(B) — Photosystem II starts electron flow and splits water, releasing oxygen.
35. Which molecule accumulates if the Calvin cycle slows due to low ATP availability?
(A) NADPH
(B) RuBP
(C) G3P
(D) Carbon dioxide
Answer
(A) — If ATP is low, NADPH cannot be used efficiently, leading to its accumulation.
36. Explain why the splitting of water is crucial for the functioning of photosystem II.
Answer
Water splitting provides electrons to replace those lost by chlorophyll in PSII, allowing electron transport to continue and producing oxygen as a byproduct.
37. How does the proton gradient generated during the light reactions lead to ATP formation?
Answer
The proton gradient drives protons through ATP synthase, which uses the energy of the flow to synthesize ATP from ADP and inorganic phosphate.
38. Describe the role of Rubisco in the Calvin cycle.
Answer
Rubisco catalyzes the first step of carbon fixation by attaching CO₂ to RuBP, forming two molecules of 3-PGA.
39. Predict what would happen if photosystem I failed to function properly.
Answer
Without PSI, NADP+ would not be reduced to NADPH, limiting the supply of electrons for the Calvin cycle and hindering sugar synthesis.
40. Explain why ATP is necessary for regenerating RuBP during the Calvin cycle.
Answer
ATP provides the energy needed to rearrange G3P molecules into RuBP, enabling continuous carbon fixation in the Calvin cycle.
41. How does light intensity affect the rate of photosynthesis?
Answer
Increased light intensity generally raises the rate of photosynthesis up to a saturation point where other factors, like CO₂ concentration or temperature, become limiting.
42. What would happen to oxygen production if water were not available for photosynthesis?
Answer
Oxygen production would stop because water is the source of electrons and oxygen in the light-dependent reactions.
43. How do the light-dependent reactions indirectly affect the Calvin cycle?
Answer
The light reactions supply ATP and NADPH, which are necessary for powering the endergonic reactions of the Calvin cycle.
44. Compare the role of ATP and NADPH in the Calvin cycle.
Answer
ATP provides the energy for reactions like RuBP regeneration, while NADPH donates high-energy electrons for reducing 3-PGA to G3P.
45. Why is the Calvin cycle sometimes referred to as the "dark reactions"?
Answer
It is called the "dark reactions" because it does not require light directly, although it depends on products of the light reactions.
46. Describe the flow of electrons from water to NADP+ during photosynthesis.
Answer
Electrons from water move to PSII, then to PSI through an electron transport chain, and finally are transferred to NADP+ to form NADPH.
47. What would happen if the Calvin cycle enzymes became denatured?
Answer
The Calvin cycle would stop, preventing carbon fixation and sugar production even if ATP and NADPH were available.
48. Why does photosynthesis require two different photosystems?
Answer
Two photosystems allow a stepwise energy boost: PSII captures light to start electron flow and generate ATP, while PSI re-energizes electrons to reduce NADP+.
49. What environmental conditions can slow down photosynthesis even if light is abundant?
Answer
Low carbon dioxide concentration, extreme temperatures, and water scarcity can all limit the rate of photosynthesis despite abundant light.
50. Explain why the Calvin cycle is essential for life on Earth.
Answer
The Calvin cycle fixes atmospheric carbon into organic molecules, providing the foundation for food webs and sustaining nearly all life forms.