In the context of photosynthesis, after carbon dioxide (CO\(_2\)) is fixed during the Calvin cycle (carbon source labeled 'c'), what is the next phase for this carbon in the cycle?

• A. Reduction phase, where 3-phosphoglycerate is converted to glyceraldehyde-3-phosphate (G3P)
• B. Photolysis, where water is split to release oxygen
• C. Photorespiration, where oxygen is consumed and CO\(_2\) is released
• D. Electron transport chain, where ATP is synthesized

Answer: (A)

Verified step by step guidance

1. Step 1: Begin by understanding the Calvin cycle, which is the process in photosynthesis where carbon dioxide (CO\(_2\)) is fixed into organic molecules. This cycle occurs in the stroma of chloroplasts and consists of three main phases: carbon fixation, reduction, and regeneration.
2. Step 2: Recall that during the carbon fixation phase, CO\(_2\) is incorporated into a 5-carbon sugar called ribulose-1,5-bisphosphate (RuBP) by the enzyme RuBisCO, forming 3-phosphoglycerate (3-PGA). This sets the stage for the next phase.
3. Step 3: In the reduction phase, ATP and NADPH (produced during the light-dependent reactions) are used to convert 3-phosphoglycerate (3-PGA) into glyceraldehyde-3-phosphate (G3P). This is a key step where energy is invested to produce a sugar molecule.
4. Step 4: Understand that photolysis, photorespiration, and the electron transport chain are processes that occur outside the Calvin cycle. Photolysis refers to the splitting of water during the light-dependent reactions, photorespiration occurs when oxygen is consumed instead of CO\(_2\), and the electron transport chain synthesizes ATP during the light-dependent reactions.
5. Step 5: Conclude that after carbon fixation, the next phase in the Calvin cycle is the reduction phase, where 3-phosphoglycerate is converted into glyceraldehyde-3-phosphate (G3P) using ATP and NADPH.