A lack of carbon dioxide would prevent which photosynthetic reactions from occurring?

• A. ATP synthesis in the thylakoid membrane
• B. The light-dependent reactions
• C. Photolysis of water
• D. The Calvin cycle (light-independent reactions)

Answer: (D)

Verified step by step guidance

1. Understand the role of carbon dioxide in photosynthesis: Carbon dioxide is a key reactant in the Calvin cycle, also known as the light-independent reactions, where it is fixed into organic molecules like glucose.
2. Recall the two main stages of photosynthesis: The light-dependent reactions occur in the thylakoid membrane and involve the production of ATP and NADPH, while the Calvin cycle occurs in the stroma and uses ATP, NADPH, and carbon dioxide to synthesize sugars.
3. Analyze the options provided: ATP synthesis in the thylakoid membrane, the light-dependent reactions, and photolysis of water are all part of the light-dependent reactions, which do not directly require carbon dioxide to proceed.
4. Focus on the Calvin cycle: The Calvin cycle specifically requires carbon dioxide as a substrate for the enzyme RuBisCO to catalyze the fixation of carbon into 3-phosphoglycerate (3-PGA). Without carbon dioxide, this process cannot occur.
5. Conclude that the absence of carbon dioxide would prevent the Calvin cycle (light-independent reactions) from occurring, as it is the only photosynthetic process directly dependent on carbon dioxide.