USABO Plant Physiology and Photosynthesis 1 — Questions and Answers
Question 1: Which photosystem is responsible for the oxidation of water molecules during the light-dependent reactions of photosynthesis?
- Photosystem I (P700)
- The cytochrome b6f complex
- Photosystem II (P680) (Correct answer)
- Ferredoxin-NADP+ reductase
Correct answer: Photosystem II (P680)
Photosystem II (P680) catalyzes the light-driven oxidation of water, releasing O2, protons, and electrons that enter the electron transport chain.
Question 2: What is the primary CO2 acceptor molecule in the Calvin cycle that is carboxylated by Rubisco?
- 3-phosphoglycerate (3-PGA)
- Phosphoenolpyruvate (PEP)
- Glyceraldehyde-3-phosphate (G3P)
- Ribulose-1,5-bisphosphate (RuBP) (Correct answer)
Correct answer: Ribulose-1,5-bisphosphate (RuBP)
RuBP is the 5-carbon acceptor molecule that Rubisco carboxylates with CO2 to form two molecules of 3-PGA.
Question 3: In C4 plants, which cell type performs initial CO2 fixation, and which enzyme catalyzes this reaction?
- Bundle sheath cells; Rubisco
- Mesophyll cells; PEP carboxylase (Correct answer)
- Guard cells; carbonic anhydrase
- Mesophyll cells; Rubisco
Correct answer: Mesophyll cells; PEP carboxylase
C4 plants use PEP carboxylase in mesophyll cells to fix CO2 into oxaloacetate, concentrating CO2 in bundle sheath cells to suppress photorespiration.
Question 4: Which two products are generated by non-cyclic photophosphorylation (the Z-scheme) in photosynthesis?
- ATP and NADH
- NADPH and O2 only
- Glucose and ATP
- ATP and NADPH (Correct answer)
Correct answer: ATP and NADPH
Non-cyclic photophosphorylation generates both ATP (via chemiosmosis) and NADPH (via NADP+ reduction at PSI), with water oxidized to release O2.
Question 5: Photorespiration reduces photosynthetic efficiency primarily because it:
- Increases the rate of ATP hydrolysis in chloroplasts
- Enhances O2 production at the expense of NADPH
- Releases previously fixed CO2 via the glycolate pathway (Correct answer)
- Inhibits the regeneration of RuBP in the Calvin cycle
Correct answer: Releases previously fixed CO2 via the glycolate pathway
When Rubisco oxygenates RuBP, the products enter the glycolate pathway in peroxisomes and mitochondria, releasing CO2 and consuming ATP without net carbon gain.
Question 6: Chlorophyll a and chlorophyll b absorb light most efficiently in which wavelength ranges?
- Green (520-560 nm) and yellow (570-590 nm)
- Blue-violet (400-450 nm) and red (650-700 nm) (Correct answer)
- Ultraviolet (280-350 nm) and far-red (720-750 nm)
- Red (650-700 nm) and infrared (>750 nm)
Correct answer: Blue-violet (400-450 nm) and red (650-700 nm)
Chlorophylls absorb most strongly in the blue-violet (~430-450 nm) and red (~640-680 nm) regions and reflect green light, giving plants their color.
Question 7: What drives ATP synthesis by ATP synthase (CF0-CF1) in the thylakoid membrane during photosynthesis?
- Direct electron transfer from reduced plastoquinone to ATP synthase
- Substrate-level phosphorylation coupled to NADPH oxidation
- A proton electrochemical gradient across the thylakoid membrane (Correct answer)
- Conformational changes in Rubisco coupled to ADP phosphorylation
Correct answer: A proton electrochemical gradient across the thylakoid membrane
Electron transport pumps H+ into the thylakoid lumen, creating a proton gradient that drives H+ back through CF0-CF1 ATP synthase, powering ATP synthesis via chemiosmosis.
USABO Plant Physiology and Photosynthesis 1
Which photosystem is responsible for the oxidation of water molecules during the light-dependent reactions of photosynthesis?