During photosynthesis the oxygen in glucose comes from ______.

The correct answer is Carbon dioxide.

Key Points

• Photosynthesis is the physiochemical process, in which carbohydrates (organic compounds) are formed from inorganic substrates like H₂O & CO₂  in the presence of light and pigments.
• O₂ is evolved as a by-product during photosynthesis.
• The oxygen molecule present in glucose is derived from carbon dioxide during Calvin cycle of photosynthesis.
• The CO₂ molecule is first added to 3-phosphoglycerate during the carboxylation reaction of Calvin cycle, and is later used to form glucose.
• While molecular oxygen that is liberated during photosynthesis is produced from photolysis of water during the light reaction.

Additional Information

• Photosynthesis can be simply divided into 4 major processes

1. Absorption of light energy by Chlorophyll pigment.
2. Conversion of light energy into chemical energy.
3. Splitting of water or Photolysis during Non-cyclic photophosphorylation.

 H₂O → 2H⁺ +  2e^-- +  O₂ ↑

The oxygen that evolved during this process comes from water.

4. Reduction of CO₂ to carbohydrates (Glucose) during Dark reaction.

​The CO2 undergoes reduction and its O2 is utilized in the formation of Glucose.

Mistake Points

The O₂ released during photolysis is the evolved O₂, whereas the O₂ released during the reduction of CO₂ is the O₂ in glucose.