Mechanisms of movement across the cell membrane include both passive and active process. Passive transport mechanisms move substances down a concentration gradient and do not require cellular energy. These include simple diffusion, osmosis and facilitated diffusion. Active mechanisms move substances against a concentration gradient and require cellular energy in the form of ATP. Active transport is one example of active mechanisms of crossing a membrane. Facilitated diffusion and active transport both use protein carrier molecules. These two transport processes are limited by the number of carrier molecules that are present.
Passive transport moves oxygen, carbon dioxide, water, and glucose molecules across the membrane down its concentration gradient. Osmosis involves the movement of water molecules through a selectively permeable membrane. Facilitated diffusion moves large molecules that need some help in passing the membrane, for example, glucose molecules. Amino acids and many minerals are actively transported through the membrane by transport proteins.