BioInteractive Photosystems I and II 2 — Questions and Answers
Question 1: Which protein immediately accepts electrons from Photosystem I before NADPH is produced?
- Plastocyanin
- Pheophytin
- Ferredoxin (Correct answer)
- Plastoquinone
Correct answer: Ferredoxin
Ferredoxin is the immediate electron acceptor from PSI; it then passes electrons to the enzyme NADP+ reductase (FNR), which reduces NADP+ to NADPH.
Question 2: What reduced molecule is produced when NADP+ accepts electrons in Photosystem I?
- ATP
- NADPH (Correct answer)
- Oxygen
- Water
Correct answer: NADPH
NADP+ reductase uses electrons from ferredoxin and protons from the stroma to reduce NADP+ to NADPH, the key electron carrier used in the Calvin cycle.
Question 3: Which mobile electron carrier transfers electrons from Photosystem II to the cytochrome b6f complex?
- Ferredoxin
- NADPH
- Plastocyanin
- Plastoquinone (Correct answer)
Correct answer: Plastoquinone
Plastoquinone (PQ) is a lipid-soluble molecule embedded in the thylakoid membrane that picks up electrons and protons from PSII and shuttles them to the cytochrome b6f complex.
Question 4: Which copper-containing protein transfers electrons from the cytochrome b6f complex to Photosystem I?
- Plastoquinone
- Ferredoxin
- Plastocyanin (Correct answer)
- NADPH
Correct answer: Plastocyanin
Plastocyanin is a small, water-soluble copper protein located in the thylakoid lumen that carries electrons from the cytochrome b6f complex to PSI.
Question 5: How many water molecules must be split by the oxygen-evolving complex to release one molecule of O2?
- One
- Four
- Two (Correct answer)
- Three
Correct answer: Two
Two water molecules (2H2O) are oxidized to release four electrons, four protons (H+), and one O2 molecule: 2H2O → 4H+ + 4e− + O2.
Question 6: What mechanism drives ATP synthesis by ATP synthase during the light reactions?
- Oxygen release from water splitting
- NADPH production by ferredoxin
- Proton gradient across the thylakoid membrane (chemiosmosis) (Correct answer)
- Direct electron transfer to ferredoxin
Correct answer: Proton gradient across the thylakoid membrane (chemiosmosis)
Proton pumping by plastoquinone and the cytochrome b6f complex creates a high proton concentration in the thylakoid lumen; these protons flow back through ATP synthase (chemiosmosis), driving ATP production.
Question 7: What are the three main products/byproducts of the complete non-cyclic electron flow pathway?
- CO2, glucose, and water
- O2, glucose, and ATP
- ATP, NADPH, and O2 (Correct answer)
- NADPH, glucose, and CO2
Correct answer: ATP, NADPH, and O2
Non-cyclic electron flow uses light energy to transfer electrons from water through PSII and PSI, producing ATP via chemiosmosis, NADPH via NADP+ reductase, and releasing O2 as a byproduct of water splitting.
BioInteractive Photosystems I and II 2
Which protein immediately accepts electrons from Photosystem I before NADPH is produced?