Which of the following represents a reactant in the light reactions of photosynthesis?

• A. C\(_6\)H\(_{12}\)O\(_6\) (glucose)
• B. NADP\(^+\) (nicotinamide adenine dinucleotide phosphate)
• C. O\(_2\) (oxygen)
• D. H\(_2\)O (water)

Answer: (D)

Verified step by step guidance

1. Understand the light reactions of photosynthesis: These reactions occur in the thylakoid membranes of chloroplasts and involve the conversion of light energy into chemical energy, producing ATP and NADPH while releasing oxygen as a byproduct.
2. Identify the key reactants in the light reactions: The primary reactants are water (H\(_2\)O), light energy, and NADP\(^+\), which is reduced to NADPH during the process.
3. Analyze the role of water (H\(_2\)O): Water is split during a process called photolysis, which occurs in Photosystem II. This splitting produces electrons (to replace those lost by chlorophyll), protons (H\(^+\) ions), and oxygen (O\(_2\)) as a byproduct.
4. Eliminate incorrect options: C\(_6\)H\(_{12}\)O\(_6\) (glucose) is a product of the Calvin cycle, not a reactant in the light reactions. O\(_2\) is a byproduct of the light reactions, not a reactant. NADP\(^+\) is a reactant, but the question specifies H\(_2\)O as the correct answer.
5. Conclude that H\(_2\)O (water) is the correct answer because it is directly involved in the photolysis process, providing the electrons needed for the light reactions to proceed.