When oxygen is released as a result of photosynthesis, it is a direct by-product of ________.

• A. the synthesis of ATP from ADP and inorganic phosphate
• B. the reduction of NADP\(^+\) to NADPH
• C. the splitting of water molecules during the light reactions
• D. the fixation of carbon dioxide in the Calvin cycle

Answer: (C)

Verified step by step guidance

1. Understand that photosynthesis consists of two main stages: the light reactions and the Calvin cycle. The light reactions occur in the thylakoid membranes of chloroplasts and involve the absorption of light energy to drive chemical processes.
2. Recall that during the light reactions, water molecules (H₂O) are split in a process called photolysis. This process is catalyzed by an enzyme complex known as the oxygen-evolving complex, which is part of Photosystem II.
3. Recognize that the splitting of water molecules produces electrons, protons (H⁺), and oxygen gas (O₂). The electrons are used to replenish the electron deficit in Photosystem II, while the oxygen gas is released as a by-product.
4. Understand that the synthesis of ATP from ADP and inorganic phosphate, as well as the reduction of NADP⁺ to NADPH, are also part of the light reactions but do not directly produce oxygen. These processes are driven by the energy captured from light and the electron transport chain.
5. Conclude that the oxygen released during photosynthesis is a direct by-product of the splitting of water molecules during the light reactions, not from ATP synthesis, NADP⁺ reduction, or carbon fixation in the Calvin cycle.