Nutrients and waste both enter and leave cells through a process called

Diffusion. Diffusion is when particles in a highly concentrated area spread out to an area of lower concentration. The membranes of cells are very thin, which allows diffusion of different materials to happen easily. During respiration, oxygen and glucose concentration in the cell is lower than in the surrounding blood. This causes oxygen and glucose to move into the cell via diffusion. Carbon dioxide is produced inside the cell and this makes the concentration of CO2 higher in the cell than in the blood, so the excess diffuses out of the cell.