Photosynthesis Light Reactions 2 — Questions and Answers
Question 1: Which pigment is primarily responsible for absorbing light energy in the light reactions?
- Carotenoid
- Chlorophyll a (Correct answer)
- Chlorophyll b
- Phycoerythrin
Correct answer: Chlorophyll a
Chlorophyll a is the primary photosynthetic pigment that directly participates in the light reactions by absorbing red and blue-violet light.
Question 2: What is the role of the oxygen-evolving complex (OEC) in the light reactions?
- It synthesizes ATP from ADP
- It reduces NADP+ to NADPH
- It splits water molecules to release oxygen (Correct answer)
- It transfers electrons to plastoquinone
Correct answer: It splits water molecules to release oxygen
The oxygen-evolving complex (also called the water-splitting complex) oxidizes water molecules, releasing O2, protons, and electrons to replenish Photosystem II.
Question 3: How many photons of light are required to release one molecule of O2 during the light reactions?
- 2
- 4
- 6
- 8 (Correct answer)
Correct answer: 8
Splitting two water molecules to release one O2 requires 4 electrons, each needing 2 photons (one at PSII, one at PSI), totaling 8 photons.
Question 4: What happens to the electrons at the P700 reaction center when it absorbs light?
- They are transferred to plastoquinone
- They reduce oxygen to water
- They are excited to a higher energy level and passed to ferredoxin (Correct answer)
- They combine with protons to form hydrogen gas
Correct answer: They are excited to a higher energy level and passed to ferredoxin
When P700 absorbs light, its electrons are excited and transferred to the primary acceptor A0, ultimately reaching ferredoxin.
Question 5: Which statement correctly describes the Z-scheme of the light reactions?
- Electrons flow from PSI to PSII in a cyclic pathway
- Electrons flow from water through PSII, the electron transport chain, PSI, and finally to NADP+ (Correct answer)
- ATP is produced before NADPH in the Z-scheme
- The Z-scheme describes only cyclic photophosphorylation
Correct answer: Electrons flow from water through PSII, the electron transport chain, PSI, and finally to NADP+
The Z-scheme describes the noncyclic electron flow from water → PSII → ETC → PSI → NADP+, named for the Z-shape of the energy diagram.
Question 6: What is the primary function of plastocyanin in the light reactions?
- To split water at Photosystem II
- To transfer electrons from the cytochrome b6f complex to Photosystem I (Correct answer)
- To pump protons across the thylakoid membrane
- To reduce NADP+ to NADPH
Correct answer: To transfer electrons from the cytochrome b6f complex to Photosystem I
Plastocyanin is a copper-containing protein that shuttles electrons from the cytochrome b6f complex to the oxidized P700 of Photosystem I.
Question 7: During cyclic photophosphorylation, which of the following is produced?
- NADPH only
- ATP only (Correct answer)
- Both ATP and NADPH
- O2 and ATP
Correct answer: ATP only
Cyclic photophosphorylation involves electrons cycling from PSI back through the cytochrome b6f complex to generate a proton gradient for ATP synthesis, producing ATP but no NADPH.
Photosynthesis Light Reactions 2
Which pigment is primarily responsible for absorbing light energy in the light reactions?