Which two main stages comprise photosynthesis?

• A. Glycolysis and fermentation.
• B. Glycolysis and Krebs cycle.
• C. Electron transport chain and fermentation.
• D. Light-dependent reactions and the Calvin cycle.

Answer: (D)

Photosynthesis relies on two linked sets of reactions. First, the light-dependent reactions use light energy to drive the splitting of water and the transfer of electrons, producing ATP and NADPH and releasing oxygen gas as a byproduct. These take place in the chloroplasts’ thylakoid membranes and require light to occur. Second, the Calvin cycle uses the ATP and NADPH generated by the light reactions to fix carbon dioxide into sugars, assembling carbon skeletons into glucose precursors in the chloroplast stroma. This division allows light energy to be captured and converted into chemical energy, which is then used to synthesize sugars.

The other options mix processes from cellular respiration or anaerobic metabolism, which occur in different contexts and do not represent the two stages of photosynthesis.