In the stages of photosynthesis, what is the first major step that initiates the process in chloroplasts?

• A. Absorption of light energy by chlorophyll and other pigments in the thylakoid membranes (light-dependent reactions)
• B. Fixation of $C_{\mathrm{O}}{}_2$ by RuBisCO in the stroma (Calvin cycle)
• C. Conversion of glucose into $ATP$ through glycolysis in the cytosol
• D. Movement of electrons through the mitochondrial electron transport chain to produce $NADH$

Answer: (A)

Verified step by step guidance

1. Understand that photosynthesis occurs in two main stages: the light-dependent reactions and the Calvin cycle (light-independent reactions).
2. Identify that the process begins in the chloroplasts, specifically in the thylakoid membranes where pigments like chlorophyll are located.
3. Recognize that the first major step is the absorption of light energy by chlorophyll and other pigments, which excites electrons and initiates the light-dependent reactions.
4. Note that this absorption of light energy leads to the splitting of water molecules and the generation of ATP and NADPH, which are essential for the next stage, the Calvin cycle.
5. Distinguish this from other processes mentioned, such as CO2 fixation by RuBisCO in the stroma (which happens after light-dependent reactions), glycolysis (which occurs in the cytosol and is not part of photosynthesis), and mitochondrial electron transport (which is part of cellular respiration, not photosynthesis).