Photosynthesis Photorespiration and C4/CAM Pathways 1 — Questions and Answers
Question 1: Which enzyme is responsible for both carbon fixation in the Calvin cycle and the oxygenase reaction that initiates photorespiration?
- PEP carboxylase
- RuBisCO (Correct answer)
- ATP synthase
- NADP+ reductase
Correct answer: RuBisCO
RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) can bind either CO2 or O2, and when it binds O2 it triggers the photorespiration pathway.
Question 2: During photorespiration, RuBisCO catalyzes the addition of which molecule to RuBP instead of CO2?
- Water (H2O)
- Glucose
- Oxygen (O2) (Correct answer)
- NADPH
Correct answer: Oxygen (O2)
Photorespiration occurs when RuBisCO fixes O2 to RuBP instead of CO2, an energetically wasteful reaction that releases previously fixed carbon.
Question 3: When RuBisCO catalyzes the oxygenase reaction with RuBP, what two products are formed?
- Two molecules of 3-phosphoglycerate
- One molecule of 3-phosphoglycerate and one molecule of 2-phosphoglycolate (Correct answer)
- One molecule of pyruvate and one molecule of PEP
- One molecule of oxaloacetate and one molecule of malate
Correct answer: One molecule of 3-phosphoglycerate and one molecule of 2-phosphoglycolate
The oxygenase reaction splits the 5-carbon RuBP into one 3-carbon molecule (3-PGA) and one 2-carbon molecule (2-phosphoglycolate), the entry point of photorespiration.
Question 4: In C4 plants, what is the first stable product of carbon fixation in mesophyll cells?
- 3-phosphoglycerate (3-PGA)
- Glyceraldehyde-3-phosphate (G3P)
- Oxaloacetate (OAA) (Correct answer)
- Ribulose-1,5-bisphosphate (RuBP)
Correct answer: Oxaloacetate (OAA)
In C4 plants, PEP carboxylase in mesophyll cells combines CO2 with PEP to form oxaloacetate, a 4-carbon compound — which is why these plants are called C4.
Question 5: Which enzyme fixes CO2 in the mesophyll cells of C4 plants?
- RuBisCO
- PEP carboxylase (PEPcase) (Correct answer)
- ATP synthase
- Malate dehydrogenase
Correct answer: PEP carboxylase (PEPcase)
PEP carboxylase fixes CO2 onto phosphoenolpyruvate (PEP) in mesophyll cells; unlike RuBisCO, it has no oxygenase activity and a very high affinity for CO2.
Question 6: In C4 plants, where does the Calvin cycle (carbon reduction) predominantly take place?
- Mesophyll cells
- Guard cells
- Bundle sheath cells (Correct answer)
- Epidermal cells
Correct answer: Bundle sheath cells
C4 plants spatially separate initial CO2 fixation (mesophyll cells) from the Calvin cycle (bundle sheath cells), concentrating CO2 around RuBisCO to suppress photorespiration.
Question 7: What is the primary advantage of C4 photosynthesis over C3 photosynthesis in hot, sunny conditions?
- C4 plants produce more ATP per glucose molecule synthesized
- C4 plants concentrate CO2 around RuBisCO, minimizing photorespiration (Correct answer)
- C4 plants do not require sunlight for the Calvin cycle
- C4 plants have significantly larger chloroplasts than C3 plants
Correct answer: C4 plants concentrate CO2 around RuBisCO, minimizing photorespiration
By using PEP carboxylase to concentrate CO2 in bundle sheath cells, C4 plants keep the CO2-to-O2 ratio high around RuBisCO, drastically reducing the wasteful photorespiration reaction.
Photosynthesis Photorespiration and C4/CAM Pathways 1
Which enzyme is responsible for both carbon fixation in the Calvin cycle and the oxygenase reaction that initiates photorespiration?