Oxygen and glucose react to form carbon dioxide and water along with ATP (a source of energy) in the process of aerobic respiration. Therefore, oxygen and glucose must be taken up by the cell, and typically the concentration of these molecules outside the cell is greater than inside. Therefore, the overall net movement of these molecules will be down the concentration gradient, and they will move into the cell via diffusion. Similarly, the carbon dioxide produced is a waste product and moves out of the cell, again via diffusion down its concentration gradient.
