Cell (Plasma) Membrane: The living membrane that surrounds the cytoplasm of all cells. It is composed of a phospholipid bilayer with embedded glycoproteins. In the "sandwich model" the two phospholipid layers are sandwiched between two layers of protein. The membranes of organelles are also composed of a phospholipid bilayer, including vacuoles, nuclei, mitochondria and chloroplasts. [Riubosomes are not membrane-bound.] Embedded glycoproteins in plasma membranes include membrane transport "carrier molecules" and cell recognition antigens. The plasma membrane is permeable to water molecules by osmosis, but not to other molecules and ions by simple diffusion. Ions pass through the plasma membrane via carrier molecules by active transport and facilitated diffusion. Active transport requires ATP.