When the light-independent reactions are occurring, gases diffuse between the chloroplast and the air that surrounds the plant. why is gas diffusion essential during this stage of photosynthesis? a. water vapor is used to assemble sugar molecules. b. carbon dioxide is used to assemble sugar molecules. c. molecular oxygen is used to assemble sugar molecules. d. water vapor is released as a waste product.

The correct answer is B. carbon dioxide is used to assemble sugar molecules.

The process of photosynthesis accomplished in two stages. First is the light- dependent stage which is called light reaction and second stage is the light-independent stage, called dark reaction or Calvin cycle. During light reactions green plants harness solar energy to produce assimilatory power (ATP and NADPH).

Using energy from assimilatory power produced in the light reactions, carbon dioxide is fixed into sugar molecule in the light–independent process or Calvin cycle which is carried out in the stroma of a chloroplast.  

thymine. in rna, thymine is replace by uracil