Which of the following plants handles O₂ buildup by physically separating RUBISCO from O₂?

• A. C3 Plants
• B. C4 Plants
• C. CAM Plants

Answer: (B)

Verified step by step guidance

1. Understand the role of RUBISCO: RUBISCO is an enzyme involved in the first major step of carbon fixation in the Calvin cycle. It catalyzes the reaction of carbon dioxide with ribulose bisphosphate (RuBP). However, RUBISCO can also react with oxygen, leading to a process called photorespiration, which is less efficient for the plant.
2. Identify the problem with O2 buildup: In environments where oxygen levels are high or carbon dioxide levels are low, RUBISCO may bind to oxygen instead of carbon dioxide, leading to photorespiration. This is a problem for plants because it reduces the efficiency of photosynthesis.
3. Explore how C4 plants handle O2 buildup: C4 plants have evolved a mechanism to minimize photorespiration by physically separating the initial CO2 fixation and the Calvin cycle. They do this by using two types of cells: mesophyll cells and bundle-sheath cells.
4. Describe the C4 pathway: In C4 plants, CO2 is initially fixed in the mesophyll cells into a four-carbon compound (hence the name C4) by an enzyme called PEP carboxylase, which does not react with oxygen. This compound is then transported to the bundle-sheath cells, where CO2 is released for use in the Calvin cycle, effectively keeping RUBISCO away from high oxygen concentrations.
5. Contrast with CAM plants: CAM plants also have a mechanism to reduce photorespiration, but they do so by temporally separating the processes of CO2 uptake and the Calvin cycle, rather than physically separating them as in C4 plants. CAM plants open their stomata at night to take in CO2, which is stored as an acid and then used during the day for photosynthesis.