Multiple choice Which of the following is a reactant in aerobic respiration? CO2 O2 H2O C6H12O6 Reveal answer Fill a bubble to check yourself B Correct answer Explanation Oxygen is a reactant in aerobic respiration. Oxygen enters the cell by diffusion. It is first transported to the cells via the red corpuscles. These contain haemoglobin, a transporter protein that carries oxygen to the cells of the body.