Question

During photosynthesis, the oxygen in glucose is from

• A. Water
• B. Carbon dioxide
• C. Both from CO${}_2$ and water
• D. Oxygen in air

Answer 1

Answer: (B)

Carbon dioxide

Photosynthesis is the process of carbon fixation which mainly takes place in two steps, light reaction, and dark reaction. ATP and NADPH${}_2$ are produced in the light reaction which is utilized for carbon fixation during the dark reaction. When the plant makes the glucose molecule, it gets the carbon and oxygen atoms it needs from carbon dioxide, which is taken by the air through the stomatal openings. Carbon dioxide doesn't have any hydrogen in it, though, so the plant must use another source for hydrogen. The source that it uses is water.

So, the correct answer is option B.