Explain 3 different processes by which substances pass the cell membrane.
The cell membrane is the organelle of the cell that serves as a protective barrier between the interior of the cell and the exterior of the cell. The cell membrane possesses a quality of being semi-permeable, meaning it will allow some substances to pass, while denying the same for other substances. It accomplishes this by several processes: diffusion, osmosis, mediated transport, endocytosis, and exocytosis. Diffusion is the transfer of molecules from a high to a low concentration. Osmosis is the diffusion of water from a high to a low concentration. Both those processes work naturally, with no energy required. Sometimes energy is required, as in the case of mediated transport, endo- and exocytosis. With mediated transport, the molecule must be brought in through a special "gateway" cell in the membrane. The prefix "endo-" means into, this is when a part of the membrane wraps around a molecule to package it and bring it into the cell. The reverse occurs with exocytosis, used to export waste materials from the interior of the cell to the exterior of the cell.