Oxygen and glucose are carried in the bloodstream and enter individual cells by passing through the cell membrane via diffusion. Oxygen enters the cells through simple diffusion, while glucose, amino acids and other large insoluble compounds enter through facilitated diffusion.
Glucose first enters the body in certain foods, which are broken down into smaller particles through digestion. These small particles then pass through the walls of the small intestine to enter the bloodstream, where the glucose is dissolved into blood plasma. The glucose particles then travel through the body and are absorbed into individual cells in the capillaries.
Oxygen enters the lungs through the process of breathing. Inside the lungs, it fills tiny air sacs known as alveoli. The individual oxygen particles then pass through the alveoli into the bloodstream, where they bind with a substance in red blood cells known as hemoglobin. When the oxygenated blood reaches the capillaries, the red blood cells release the oxygen molecules, which then diffuse into the cells.
Glucose and oxygen are the two components necessary for cellular aerobic respiration. This type of respiration is not the same as breathing; instead, it is a reaction through which cells extract energy from glucose. During the process, oxygen and glucose combine to produce carbon dioxide (CO2) and water. The CO2 is then removed in the same way oxygen entered the body.