Rucete ✏ AP Biology In a Nutshell
6. Photosynthesis — Practice Questions
This chapter introduces the mechanisms of light-dependent reactions, the Calvin cycle, and the energy transformations in photosynthesis.
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(Multiple Choice — Click to Reveal Answer)
1. What is the main purpose of the light-dependent reactions in photosynthesis?
(A) Produce sugars
(B) Produce ATP and NADPH
(C) Absorb carbon dioxide
(D) Break down glucose
Answer
(B) — Light-dependent reactions produce ATP and NADPH needed for the Calvin cycle.
2. Where exactly do the light-dependent reactions occur in plants?
(A) Stroma
(B) Thylakoid membrane
(C) Cytosol
(D) Plasma membrane
Answer
(B) — Light-dependent reactions take place on the thylakoid membranes in chloroplasts.
3. What is released as a by-product of the light-dependent reactions?
(A) Glucose
(B) Carbon dioxide
(C) Oxygen gas
(D) Water
Answer
(C) — Splitting of water molecules during photolysis releases oxygen gas.
4. The Calvin cycle mainly occurs in which part of the chloroplast?
(A) Thylakoid membrane
(B) Stroma
(C) Matrix
(D) Cytoplasm
Answer
(B) — The Calvin cycle (light-independent reactions) occurs in the stroma.
5. Which molecule provides electrons to replace those lost by photosystem II during the light reactions?
(A) NADPH
(B) Carbon dioxide
(C) Water
(D) ATP
Answer
(C) — Water is split during photolysis to replace electrons in PSII.
6. In the light-dependent reactions, what is the final electron acceptor in the electron transport chain?
(A) NADP+
(B) Oxygen
(C) Water
(D) Carbon dioxide
Answer
(A) — NADP+ accepts electrons and forms NADPH during the light reactions.
7. What is the primary pigment involved in capturing light energy for photosynthesis?
(A) Carotenoid
(B) Chlorophyll
(C) Xanthophyll
(D) Anthocyanin
Answer
(B) — Chlorophyll is the main pigment that captures light energy for photosynthesis.
8. What is the main product of the Calvin cycle that plants use to build glucose?
(A) ATP
(B) NADPH
(C) G3P
(D) O2
Answer
(C) — G3P (glyceraldehyde-3-phosphate) is the key product that can be assembled into glucose and other sugars.
9. The ATP generated in photosynthesis is produced through which process?
(A) Glycolysis
(B) Chemiosmosis
(C) Fermentation
(D) Passive transport
Answer
(B) — ATP is produced via chemiosmosis during the light-dependent reactions.
10. Which structure in the chloroplast is essential for forming the proton gradient needed for ATP synthesis?
(A) Outer membrane
(B) Inner membrane
(C) Stroma
(D) Thylakoid membrane
Answer
(D) — The thylakoid membrane maintains the proton gradient essential for ATP production.
11. What does Rubisco catalyze in the Calvin cycle?
(A) Photolysis of water
(B) Formation of NADPH
(C) Fixation of carbon dioxide
(D) Generation of ATP
Answer
(C) — Rubisco is the enzyme that catalyzes carbon fixation in the Calvin cycle.
12. What two energy-carrying molecules drive the Calvin cycle?
(A) ATP and NADPH
(B) NADPH and NADH
(C) FADH2 and NADH
(D) ATP and oxygen
Answer
(A) — ATP and NADPH from the light-dependent reactions fuel the Calvin cycle.
13. During photosynthesis, which molecule is oxidized?
(A) Carbon dioxide
(B) Glucose
(C) Water
(D) Oxygen
Answer
(C) — Water is oxidized during photolysis, releasing oxygen gas and electrons.
14. Which product of photosynthesis is directly used for energy storage in plants?
(A) NADPH
(B) Glucose
(C) Oxygen
(D) Water
Answer
(B) — Glucose is used for energy storage and building materials in plants.
15. Where do prokaryotic organisms like cyanobacteria perform photosynthesis?
(A) On chloroplast membranes
(B) In the thylakoid lumen
(C) On infoldings of the plasma membrane
(D) In the cytosol
Answer
(C) — Prokaryotes lack organelles and perform photosynthesis on plasma membrane infoldings.
16. What supplies the electrons to photosystem II during the light-dependent reactions?
(A) NADPH
(B) Water molecules
(C) Oxygen gas
(D) ATP
Answer
(B) — Water molecules are split in photolysis to supply electrons to PSII.
17. What molecule captures light energy in the chloroplasts?
(A) Carbon dioxide
(B) Water
(C) Chlorophyll
(D) ATP synthase
Answer
(C) — Chlorophyll is the pigment that absorbs light energy for photosynthesis.
18. What term describes the splitting of water during the light reactions?
(A) Photophosphorylation
(B) Chemiosmosis
(C) Photolysis
(D) Carbon fixation
Answer
(C) — Photolysis refers to the light-driven splitting of water molecules.
19. In which photosystem is water split to replace lost electrons?
(A) Photosystem I
(B) Photosystem II
(C) Both Photosystem I and II
(D) Neither
Answer
(B) — Water is split to replace electrons lost from Photosystem II.
20. What is produced directly by the proton gradient in the thylakoid membrane?
(A) NADPH
(B) ATP
(C) Glucose
(D) Oxygen gas
Answer
(B) — The proton gradient drives ATP synthesis through chemiosmosis.
21. Which event occurs first during the light reactions of photosynthesis?
(A) Electron transfer to NADP+
(B) Water splitting
(C) ATP synthesis
(D) Carbon dioxide fixation
Answer
(B) — Water splitting (photolysis) supplies the electrons needed at the beginning of the light reactions.
22. In the Calvin cycle, which molecule initially reacts with carbon dioxide?
(A) G3P
(B) ATP
(C) RuBP
(D) NADP+
Answer
(C) — RuBP binds with CO₂ during carbon fixation in the Calvin cycle.
23. What happens to the NADPH produced during the light reactions?
(A) It powers the electron transport chain.
(B) It provides electrons for the Calvin cycle.
(C) It oxidizes oxygen gas.
(D) It breaks down water molecules.
Answer
(B) — NADPH supplies the reducing power for reactions in the Calvin cycle.
24. Why are light-independent reactions called "light-independent"?
(A) They use light energy stored in ATP.
(B) They occur only at night.
(C) They do not directly require light to proceed.
(D) They take place outside the chloroplast.
Answer
(C) — Light-independent reactions (Calvin cycle) use ATP and NADPH but do not require light directly.
25. What three-carbon molecule is produced during the Calvin cycle and can later form glucose?
(A) ATP
(B) G3P
(C) NADPH
(D) RuBP
Answer
(B) — G3P (glyceraldehyde-3-phosphate) is the key product of the Calvin cycle.
26. Which best explains why ATP is required during the Calvin cycle?
(A) To capture light energy
(B) To reduce oxygen to water
(C) To regenerate RuBP from G3P
(D) To split carbon dioxide molecules
Answer
(C) — ATP provides energy to regenerate RuBP, allowing the Calvin cycle to continue.
27. During the light-dependent reactions, the energy from excited electrons is used directly for which process?
(A) Formation of G3P
(B) Establishment of a proton gradient
(C) Fixation of carbon dioxide
(D) Production of glucose
Answer
(B) — Electron energy is used to actively transport protons, creating the proton gradient.
28. Which event would most likely occur if a plant’s thylakoid membranes were punctured?
(A) ATP production would decrease.
(B) More oxygen would be produced.
(C) Carbon fixation would speed up.
(D) Water splitting would stop.
Answer
(A) — A disrupted membrane would prevent proton gradient formation, reducing ATP synthesis.
29. Which product of the light-dependent reactions provides reducing power for the Calvin cycle?
(A) ATP
(B) Oxygen
(C) G3P
(D) NADPH
Answer
(D) — NADPH provides the high-energy electrons needed to reduce carbon compounds in the Calvin cycle.
30. What would happen if Rubisco activity were inhibited in a plant cell?
(A) No oxygen would be produced.
(B) No ATP would be generated.
(C) No carbon fixation would occur.
(D) No water would be split.
Answer
(C) — Rubisco catalyzes carbon fixation, so inhibition would halt carbon incorporation into organic molecules.
31. In photosynthesis, what role does chemiosmosis specifically play?
(A) It captures CO₂.
(B) It produces NADPH.
(C) It generates ATP.
(D) It reduces RuBP.
Answer
(C) — Chemiosmosis drives the production of ATP by utilizing the proton gradient across the thylakoid membrane.
32. In the absence of light, which photosynthetic process would cease first?
(A) Calvin cycle
(B) Carbon fixation
(C) ATP production in the thylakoid
(D) Glucose synthesis
Answer
(C) — Without light, electron excitation stops, halting the proton gradient and ATP production first.
33. Which photosynthetic pigment is most efficient at absorbing light in the blue-violet region of the spectrum?
(A) Chlorophyll a
(B) Chlorophyll b
(C) Carotenoids
(D) Anthocyanins
Answer
(A) — Chlorophyll a absorbs best around 425 nm (blue-violet region).
34. How is the oxygen gas produced during photosynthesis generated?
(A) By splitting water molecules in the light-dependent reactions
(B) By breaking down carbon dioxide during the Calvin cycle
(C) Through oxidative phosphorylation
(D) By the breakdown of ATP
Answer
(A) — Water is split during photolysis in the light-dependent reactions, releasing oxygen gas.
35. Which statement best describes the relationship between the Calvin cycle and light-dependent reactions?
(A) They are completely independent processes.
(B) Light-dependent reactions supply ATP and NADPH to the Calvin cycle.
(C) The Calvin cycle generates oxygen needed by light reactions.
(D) The Calvin cycle produces ATP used by light reactions.
Answer
(B) — Light-dependent reactions produce ATP and NADPH, which fuel the Calvin cycle.
36. Describe the role of water in the light-dependent reactions of photosynthesis.
Answer
Water is split during photolysis to supply electrons to photosystem II, release oxygen gas, and provide protons for the proton gradient.
37. Explain why the Calvin cycle is dependent on the light-dependent reactions.
Answer
The Calvin cycle requires ATP and NADPH, which are products of the light-dependent reactions, to power carbon fixation and reduction steps.
38. How does the structure of the thylakoid membrane contribute to ATP production?
Answer
The thylakoid membrane separates protons to create a proton gradient, which drives ATP synthesis via chemiosmosis through ATP synthase.
39. Predict what would happen to the rate of photosynthesis if a plant were exposed only to green light.
Answer
The rate of photosynthesis would decrease significantly because chlorophyll reflects green light rather than absorbing it for energy.
40. Why is the regeneration of RuBP critical for the continuation of the Calvin cycle?
Answer
Without RuBP regeneration, carbon dioxide could not continue to be fixed, halting the Calvin cycle and sugar production.
41. Explain why ATP is needed during the regeneration phase of the Calvin cycle.
Answer
ATP provides the energy required to rearrange G3P molecules into RuBP, allowing the cycle to repeat.
42. What role does NADPH play in the Calvin cycle?
Answer
NADPH provides high-energy electrons and hydrogen atoms necessary to reduce 3-carbon molecules into G3P during the Calvin cycle.
43. Describe what happens to electrons after they pass through the electron transport chain in the thylakoid membrane.
Answer
After the electron transport chain, electrons are re-energized in photosystem I and transferred to NADP+, forming NADPH.
44. Why is light necessary for the light-dependent reactions to occur?
Answer
Light energy excites electrons in chlorophyll molecules, providing the energy needed to drive electron transport and ATP/NADPH formation.
45. What would happen to the Calvin cycle if ATP production in the light-dependent reactions stopped?
Answer
The Calvin cycle would eventually stop because it depends on ATP to power carbon fixation and RuBP regeneration steps.
46. Explain why carbon fixation is an essential part of photosynthesis.
Answer
Carbon fixation incorporates inorganic carbon dioxide into organic molecules, enabling the production of sugars that fuel biological systems.
47. How does the proton gradient in the thylakoid membrane contribute to ATP synthesis?
Answer
The proton gradient drives protons through ATP synthase, providing the energy needed to phosphorylate ADP into ATP.
48. What would be the immediate effect if NADP+ were unavailable in the light-dependent reactions?
Answer
Electrons could not be accepted and stored as NADPH, halting electron flow and disrupting ATP and NADPH production.
49. Compare the roles of chlorophyll a and chlorophyll b in photosynthesis.
Answer
Both absorb light energy but at slightly different wavelengths; chlorophyll a is the primary pigment, while chlorophyll b broadens the absorption range.
50. Predict how a deficiency in Rubisco would affect a plant’s ability to grow.
Answer
A deficiency in Rubisco would severely impair carbon fixation, limiting sugar production and stunting the plant’s growth.