Question

which statement about the light-dependent reactions of photosynthesis is correct?
○ carbon dioxide is a reactant, and glucose is a product.
○ water is a reactant, and glucose is a product.
○ carbon dioxide is a reactant, and oxygen is a product.
○ water is a reactant, and oxygen is a product.

Explanation:

1. Recall the process of light - dependent reactions in photosynthesis: The light - dependent reactions of photosynthesis use light energy to split water molecules ($\ce{H_2O}$) into hydrogen ions, electrons, and oxygen gas ($\ce{O_2}$). The reactants of light - dependent reactions are light, water, and ADP + Pi (for ATP synthesis), and the products are ATP, NADPH, and oxygen.
2. Analyze each option:

• Option 1: Carbon dioxide is a reactant in the light - independent (Calvin cycle) reactions, and glucose is a product of the light - independent reactions. So this option is incorrect.
• Option 2: Glucose is a product of the light - independent reactions, not the light - dependent reactions. Water is a reactant in light - dependent reactions, but glucose is not a product here. So this option is incorrect.
• Option 3: Carbon dioxide is a reactant in the light - independent reactions, not the light - dependent reactions. So this option is incorrect.
• Option 4: Water is split in the light - dependent reactions, so water is a reactant, and oxygen is released as a product. This option is correct.

Answer:

Water is a reactant, and oxygen is a product.