Photosynthesis Carbon Fixation 2 — Questions and Answers
Question 1: Which enzyme catalyzes the carboxylation step in the Calvin cycle where CO₂ is added to RuBP?
- Phosphoglycerate kinase
- RuBisCO (Correct answer)
- Triose phosphate isomerase
- Aldolase
Correct answer: RuBisCO
RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyzes the addition of CO₂ to RuBP, producing two molecules of 3-phosphoglycerate.
Question 2: How many carbon atoms does ribulose-1,5-bisphosphate (RuBP) contain?
- 3
- 4
- 5 (Correct answer)
- 6
Correct answer: 5
RuBP is a 5-carbon compound that accepts one CO₂ during carbon fixation, briefly forming an unstable 6-carbon intermediate.
Question 3: In C4 photosynthesis, carbon is initially fixed by PEP carboxylase into which 4-carbon compound?
- Malate
- Oxaloacetate (Correct answer)
- Aspartate
- Fumarate
Correct answer: Oxaloacetate
PEP carboxylase combines CO₂ with phosphoenolpyruvate (PEP) to form oxaloacetate (OAA), the first stable 4-carbon product in C4 plants.
Question 4: What is the primary advantage of C4 carbon fixation over C3 fixation in hot, dry environments?
- It requires less ATP
- It concentrates CO₂ around RuBisCO, reducing photorespiration (Correct answer)
- It operates at night
- It uses a different electron acceptor
Correct answer: It concentrates CO₂ around RuBisCO, reducing photorespiration
C4 plants use a CO₂-concentrating mechanism that delivers high CO₂ to bundle sheath cells, suppressing RuBisCO's oxygenase activity and minimizing wasteful photorespiration.
Question 5: In CAM photosynthesis, when does carbon fixation by PEP carboxylase primarily occur?
- During the day when stomata are open
- At night when stomata are open (Correct answer)
- Continuously throughout the 24-hour cycle
- Only during the dawn transition period
Correct answer: At night when stomata are open
CAM plants open their stomata at night to fix CO₂ via PEP carboxylase into malic acid, which is stored in the vacuole until daytime use.
Question 6: The 3-phosphoglycerate (3-PGA) produced by carbon fixation has how many carbon atoms?
- 2
- 3 (Correct answer)
- 4
- 6
Correct answer: 3
3-PGA is a 3-carbon molecule; when CO₂ (1C) combines with RuBP (5C), the resulting 6-carbon intermediate splits into two molecules of 3-PGA.
Question 7: Which type of plant would most likely use the C4 photosynthetic pathway?
- Mosses in temperate forests
- Corn growing in a hot midwestern summer (Correct answer)
- Cacti in the Sonoran Desert
- Wheat in cool northern climates
Correct answer: Corn growing in a hot midwestern summer
Corn (maize) is a classic C4 plant, well-adapted to hot, high-light conditions where C4's CO₂-concentrating mechanism provides a significant photosynthetic advantage.
Photosynthesis Carbon Fixation 2
Which enzyme catalyzes the carboxylation step in the Calvin cycle where CO₂ is added to RuBP?