Which gas is fixed into organic molecules during photosynthesis?

Which gas is fixed into organic molecules during photosynthesis?

• A. Oxygen
• B. Carbon dioxide
• C. Nitrogen
• D. Hydrogen

Answer: (B)

Carbon dioxide is fixed into organic molecules during photosynthesis. In the Calvin cycle, CO2 is captured by the enzyme RuBisCO and attached to a five-carbon sugar called RuBP, forming two molecules of 3-phosphoglycerate. These are then converted, using the energy-rich compounds ATP and NADPH produced by the light-dependent reactions, into glyceraldehyde-3-phosphate. This becomes the building block for glucose and other carbohydrates, providing the carbon skeletons plants need. Oxygen released during photosynthesis comes from water splitting in the light reactions, not from carbon dioxide. Nitrogen fixation is a separate process carried out by certain bacteria, and hydrogen is not fixed into organic molecules in the same way carbon is during photosynthesis.

Carbon dioxide is fixed into organic molecules during photosynthesis. In the Calvin cycle, CO2 is captured by the enzyme RuBisCO and attached to a five-carbon sugar called RuBP, forming two molecules of 3-phosphoglycerate. These are then converted, using the energy-rich compounds ATP and NADPH produced by the light-dependent reactions, into glyceraldehyde-3-phosphate. This becomes the building block for glucose and other carbohydrates, providing the carbon skeletons plants need.

Oxygen released during photosynthesis comes from water splitting in the light reactions, not from carbon dioxide. Nitrogen fixation is a separate process carried out by certain bacteria, and hydrogen is not fixed into organic molecules in the same way carbon is during photosynthesis.