C4 pathway and Kranz anatomy — practice questions

Question 1 · difficulty L1 · recall

What is the primary carbon dioxide acceptor in the mesophyll cells of C4 plants?

• A. RuBP
• B. PGA
• C. PEP
• D. Malate

Question 2 · difficulty L2 · understanding

Why does PEP carboxylase favour carbon fixation in C4 mesophyll cells under warm conditions?

• A. It functions only when stomata are fully open
• B. It releases oxygen while fixing carbon dioxide
• C. It directly synthesises glucose
• D. High CO2 affinity and no oxygenase activity

Question 3 · difficulty L2 · understanding

What does Kranz anatomy refer to in a typical C4 leaf?

• A. Bundle-sheath rings around vascular bundles
• B. The absence of chloroplasts from mesophyll cells
• C. A single epidermal layer without stomata
• D. Parallel venation alone

Question 4 · difficulty L3 · application

In a C4 plant, a four-carbon acid formed in the mesophyll is transported to bundle-sheath cells and decarboxylated. What is the main advantage of this step?

• A. It supplies oxygen directly to RuBisCO
• B. Raises CO2 around RuBisCO
• C. It replaces the Calvin cycle
• D. It eliminates the need for ATP

Question 5 · difficulty L4 · analysis

Plant X fixes carbon dioxide first with PEP carboxylase in mesophyll cells and runs the Calvin cycle mainly in bundle-sheath cells. Plant Y performs primary fixation and the Calvin cycle in mesophyll chloroplasts. Which interpretation is correct?

• A. Both are C4 plants
• B. X is a C3 plant and Y is a C4 plant
• C. X is C4; Y is C3
• D. Both are C3 plants

Question 6 · difficulty L5 · analysis

A C4 leaf loses the ability to regenerate PEP from pyruvate, while its Calvin-cycle enzymes remain functional. What is the most likely consequence?

• A. Water splitting in Photosystem II will stop immediately
• B. The Calvin cycle will permanently switch to using PEP instead of RuBP
• C. Photorespiration will become impossible under all conditions
• D. The C4 carbon-concentrating cycle will progressively stop