Question

Photorespiration does not take place in C4 plants because such plants:

• A. Do not contain the fixation enzyme RUBISCO.
• B. Have cells that are impermeable to oxygen.
• C. Have a mechanism that increases the concentration of CO2 at the enzyme site.
• D. Cells do not allow oxygen to accumulate in them.

Hint:

Think about the role of PEP carboxylase and the bundle sheath cells in building up CO2 around RuBisCO.

Answer 1

The Correct Option is C

C4 plants like maize and sugarcane rarely lose carbon to photorespiration, and this question checks whether you know the actual reason for that advantage.

1. No RuBisCO enzyme: False, RuBisCO is present and active inside the bundle sheath cells of C4 plants, where the Calvin cycle still runs.
2. Cells impermeable to oxygen: False, oxygen is made during the light reactions in these cells just like in any green cell, it is not blocked out.
3. Mechanism that raises CO2 at the enzyme site: True. PEP carboxylase in mesophyll cells grabs CO2 first, ships it inward as a four carbon acid, and releases it in the bundle sheath, right where RuBisCO sits, so RuBisCO is surrounded by a high CO2 pool and rarely binds oxygen instead.
4. Cells do not allow oxygen to build up: False, oxygen is still generated normally, this is not the mechanism at play.

The concentrating step carried out by PEP carboxylase and the bundle sheath is what starves RuBisCO of the chance to react with oxygen, so photorespiration barely happens in C4 plants.

Let's summarize:

• C4 plants keep RuBisCO but surround it with a high CO2 environment through the bundle sheath pathway.
• This CO2 pumping mechanism, not a lack of RuBisCO or oxygen exclusion, is why photorespiration is suppressed.

Hence option 3, the CO2 concentrating mechanism at the enzyme site, is the correct answer.