Besides carbon dioxide, which small molecule is needed for photosynthesis?

Besides carbon dioxide, which small molecule is needed for photosynthesis?

• A. Water
• B. Oxygen
• C. Nitrogen
• D. Methane

Answer: (A)

Water is required in photosynthesis because the light-dependent reactions use it as the source of electrons and protons. When light energizes the chloroplasts, water is split in the oxygen-evolving complex of photosystem II, releasing electrons to replace those lost by chlorophyll, along with hydrogen ions to build a proton gradient and oxygen gas as a byproduct. The electrons flow through the electron transport chain, powering ATP formation, and NADP+ accepts electrons to make NADPH. Those ATP and NADPH feed the Calvin cycle to convert carbon dioxide into sugars. Without water, there are no electrons or protons to drive the light reactions, so carbon dioxide cannot be fixed even if it’s present. Oxygen is produced from water, not used as a reactant here; nitrogen and methane aren’t directly involved in the immediate steps of photosynthesis.

Water is required in photosynthesis because the light-dependent reactions use it as the source of electrons and protons. When light energizes the chloroplasts, water is split in the oxygen-evolving complex of photosystem II, releasing electrons to replace those lost by chlorophyll, along with hydrogen ions to build a proton gradient and oxygen gas as a byproduct. The electrons flow through the electron transport chain, powering ATP formation, and NADP+ accepts electrons to make NADPH. Those ATP and NADPH feed the Calvin cycle to convert carbon dioxide into sugars. Without water, there are no electrons or protons to drive the light reactions, so carbon dioxide cannot be fixed even if it’s present. Oxygen is produced from water, not used as a reactant here; nitrogen and methane aren’t directly involved in the immediate steps of photosynthesis.