Select all that apply
Select all of the following that occur in C4 ?plants.
ATP is produced.
Carbon is fixed twice.
The Calvin cycle does not take place.
Photorespiration is minimized.
CO2 ?is first fixed to form a four-carbon molecule.
CO2 ?is first fixed in mesophyll cells using rubisco to form a six-carbon molecule.
The correct answer and explanation is:
Correct Answers:
- ATP is produced.
- Carbon is fixed twice.
- Photorespiration is minimized.
- CO₂ is first fixed to form a four-carbon molecule.
Explanation:
C4 plants, such as maize and sugarcane, have a specialized mechanism to increase photosynthetic efficiency under high light, temperature, and low CO₂ conditions. This mechanism involves distinct steps that differentiate them from C3 plants. Below are detailed explanations for each correct answer:
- ATP is produced:
ATP is essential for the energy-demanding processes in photosynthesis, including carbon fixation in the Calvin cycle and the regeneration of key molecules. In C4 plants, additional ATP is required for the transport of carbon compounds between the mesophyll and bundle-sheath cells. - Carbon is fixed twice:
In C4 plants, carbon is fixed twice in separate processes. First, in mesophyll cells, CO₂ is captured and fixed to phosphoenolpyruvate (PEP) by the enzyme PEP carboxylase, forming a four-carbon compound (usually oxaloacetate or malate). This compound is then transported to bundle-sheath cells, where it releases CO₂ for fixation by Rubisco in the Calvin cycle. This two-step fixation ensures efficient carbon capture and minimizes photorespiration. - Photorespiration is minimized:
C4 plants concentrate CO₂ around Rubisco in bundle-sheath cells, significantly reducing the oxygenase activity of Rubisco that leads to photorespiration. By segregating the initial CO₂ fixation and the Calvin cycle in different cells, C4 plants enhance photosynthetic efficiency under stress conditions. - CO₂ is first fixed to form a four-carbon molecule:
The hallmark of C4 photosynthesis is the initial fixation of CO₂ into a four-carbon molecule. This process occurs in mesophyll cells, where PEP carboxylase fixes CO₂ to form oxaloacetate. This mechanism gives C4 plants their name and distinguishes them from C3 plants.
Incorrect Statements:
- The Calvin cycle does not take place: The Calvin cycle occurs in C4 plants but is localized to bundle-sheath cells rather than mesophyll cells.
- CO₂ is first fixed in mesophyll cells using Rubisco to form a six-carbon molecule: In C4 plants, the first fixation uses PEP carboxylase, not Rubisco, and forms a four-carbon compound, not a six-carbon molecule.