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

Step 1: Understanding the Question:
The question asks for the real reason C4 plants avoid photorespiration, unlike C3 plants.

Step 2: Key Formula or Approach:
In C4 plants, CO2 is first fixed in mesophyll cells by the enzyme PEP carboxylase into a four carbon acid. This acid moves into the bundle sheath cells and releases CO2 there, right next to RuBisCO, building up a high local CO2 concentration.

Step 3: Detailed Explanation:
Option 1 says C4 plants lack RuBisCO, but this is false, RuBisCO is present in the bundle sheath cells of C4 plants and still carries out the Calvin cycle there.
Option 2 says the cells are impermeable to oxygen, but oxygen is still produced and present in these cells, the cells are not sealed off from it.
Option 3 says C4 plants have a mechanism that raises CO2 concentration at the enzyme site, and this is exactly what the C4 pathway does. PEP carboxylase, which has no attraction to oxygen, first traps CO2 in the mesophyll, then bundle sheath cells receive concentrated CO2, so RuBisCO there mostly does its carboxylation job instead of its oxygenation job, and photorespiration is suppressed.
Option 4 says oxygen does not accumulate, but oxygen is a normal product of the light reactions in all these cells, this is not the actual reason photorespiration is avoided.

Step 4: Final Answer:
The correct reason is the CO2 concentrating mechanism around RuBisCO in the bundle sheath, so option 3 is correct.