Materials like oxygen, carbon dioxide and glucose enter and leave cells by diffusion. Diffusion occurs when particles spread. They move from a region where they are in high concentration to a region where they are in low concentration.
Diffusion happens when the particles are free to move (gases and particles dissolved in solutions). Particles diffuse down a concentration gradient, from an area of high concentration to an area of low concentration. No energy is required.
Examples of diffusion in living systems: