Do C4 and CAM plants also use the C3 (Calvin cycle) pathway as part of photosynthesis?

• A. No; C4 and CAM plants completely bypass the Calvin cycle and make sugars only through PEP carboxylase reactions.
• B. Only CAM plants use the Calvin cycle; C4 plants rely exclusively on the light reactions to produce sugars.
• C. Only C4 plants use the Calvin cycle; CAM plants fix carbon only at night and cannot run the Calvin cycle.
• D. Yes; both C4 and CAM plants ultimately fix $C_1$ into carbohydrates via the Calvin cycle using Rubisco in bundle-sheath cells (C4) or during the day after decarboxylation (CAM).

Answer: (D)

Verified step by step guidance

1. Understand that the Calvin cycle, also known as the C3 pathway, is the fundamental process by which plants fix carbon dioxide (CO2) into carbohydrates using the enzyme Rubisco.
2. Recognize that C4 and CAM plants have evolved additional mechanisms to initially fix CO2 into four-carbon compounds using the enzyme PEP carboxylase, which helps them minimize photorespiration and conserve water under specific environmental conditions.
3. Note that in C4 plants, the initial CO2 fixation occurs in mesophyll cells, producing four-carbon compounds that are transported to bundle-sheath cells where CO2 is released and then fixed again by Rubisco through the Calvin cycle.
4. In CAM plants, CO2 fixation by PEP carboxylase happens at night when stomata are open, storing four-carbon acids in vacuoles; during the day, these acids release CO2 internally, which is then fixed by Rubisco via the Calvin cycle.
5. Conclude that both C4 and CAM plants ultimately use the Calvin cycle to synthesize carbohydrates, but they differ in how and when they initially fix CO2 before it enters the Calvin cycle.