In the stages of photosynthesis, which event occurs first?

• A. Synthesis of glucose from glyceraldehyde-3-phosphate (Calvin cycle output processing)
• B. Absorption of light energy by chlorophyll in photosystems (light-dependent reactions)
• C. Reduction of $N_{A_D}P+$ to $NADPH$ using electrons from the electron transport chain (Calvin cycle)
• D. Fixation of $C_{\mathrm{O}}2$ by Rubisco to form 3-phosphoglycerate (Calvin cycle)

Answer: (B)

Verified step by step guidance

1. Understand that photosynthesis consists of two main stages: the light-dependent reactions and the light-independent reactions (Calvin cycle).
2. Identify that the light-dependent reactions begin with the absorption of light energy by chlorophyll molecules in the photosystems located in the thylakoid membranes.
3. Recognize that this absorbed light energy excites electrons, which then move through the electron transport chain, leading to the reduction of NADP\+ to NADPH and the generation of ATP.
4. Know that the Calvin cycle (light-independent reactions) uses the ATP and NADPH produced in the light-dependent reactions to fix CO\_2 and synthesize glucose through a series of enzymatic steps, starting with the fixation of CO\_2 by Rubisco.
5. Conclude that since the Calvin cycle depends on the products of the light-dependent reactions, the very first event in photosynthesis is the absorption of light energy by chlorophyll in the photosystems.