Materials move into and out of cells through either passive transport or active transport. Passive transport includes diffusion and osmosis. Molecules tend to move from crowded to less crowded in order to achieve a balance or to reach homeostasis. The cell membrane is selectively permeable which allows the movement of substances, especially oxygen, water, food molecules, carbon dioxide, and waste products, into or out of the cell.
passive transport - movement of molecules from a more crowded to a less crowded area WITHOUT the use of energy. Movement occurs when there are unequal concentrations of a substance inside and outside of the cell.
active transport - movement of molecules from a less crowded to a more crowded area WITH the use of energy. Molecules are "carried" into or out of the cell using some of the cell's energy.