Which of the following best describes what happens during the Calvin cycle of photosynthesis?

• A. Water is split to release oxygen and electrons.
• B. Carbon dioxide is fixed into organic molecules using ATP and NADPH.
• C. Glucose is broken down to produce ATP.
• D. Light energy is absorbed by chlorophyll to excite electrons.

Answer: (B)

Verified step by step guidance

1. Step 1: Begin by understanding the Calvin cycle, which is the light-independent reaction of photosynthesis. It occurs in the stroma of chloroplasts and does not require light directly.
2. Step 2: Recognize that the Calvin cycle's primary function is carbon fixation, where carbon dioxide (CO₂) is incorporated into organic molecules. This process is catalyzed by the enzyme RuBisCO.
3. Step 3: Note that ATP and NADPH, produced during the light-dependent reactions, are used as energy sources in the Calvin cycle to drive the synthesis of these organic molecules.
4. Step 4: Understand that the Calvin cycle involves three main phases: carbon fixation, reduction, and regeneration of RuBP (ribulose-1,5-bisphosphate). These steps ensure the cycle continues and produces molecules like glyceraldehyde-3-phosphate (G3P), which can be used to form glucose and other carbohydrates.
5. Step 5: Compare the options provided in the problem. The correct description of the Calvin cycle is 'Carbon dioxide is fixed into organic molecules using ATP and NADPH,' as this accurately summarizes its role in photosynthesis.