Question

During photosynthesis, oxygen in glucose comes from
A. Oxygen in air
B. Water
C. Carbon dioxide
D. Both (B) and (C)

Answer 1

Hint:- Photosynthesis is a multi step physico-chemical process in which light energy is used to convert inorganic compounds into organic compounds such as glucose. The hydrogen from the water reduces carbon dioxide into carbohydrates. The water is absorbed by the plant roots and carbon dioxide is taken from the atmosphere through stomata in the leaves.

Complete Answer:-
Photosynthesis does not involve the absorption of oxygen from the atmosphere. Therefore, the oxygen present in the glucose molecules after photosynthesis will not be derived from oxygen in air.
In the photochemical phase of photosynthesis, light is absorbed by photosystems I and II. In photosystem II, the electrons are continuously excited and are picked up by an electron acceptor. The electrons that are moved from photosystem II are replaced by the electrons achieved due to the splitting of water. Splitting of water forms hydrogen ions, electrons and oxygen. Therefore, the oxygen present in glucose is not obtained from water.
The products of the photochemical phase of photosynthesis are ATP, NADPH and oxygen. In the biochemical phase of folliculitis, ATP and NADPH2 are used along with carbon dioxide and water. Carbon dioxide entering the Calvin cycle undergoes carboxylation and reduction. The fixation of six carbon dioxide by six turns of the Calvin cycle facilitates the removal of one molecule of glucose from the pathway. Thus, the oxygen in glucose comes from carbon dioxide from the atmosphere.
Thus, the right option is C.

Note:- The oxygen formed by splitting of water is later released to the atmosphere. For the production of one molecule of glucose, six molecules of carbon dioxide, six molecules of ribulose-1,5- bisphosphate (RUBP), 18 ATP molecules and 12 NADPH molecules are used through six Calvin cycles. Each cycle requires 3 ATP and 2 NADPH molecules. The enzyme required in the Calvin cycle is RuBisCO or RUBP carboxylase-oxygenase. The enzyme utilises CO₂ for the carboxylation of RUBP.