Cell Membrane Reference Sheet There may be a dozen different types of materials passing through the membrane of a cell at any one time. The job of the cell membrane is to regulate this movement in order to maintain the proper balance of ions, water, oxygen, carbon dioxide, nutrients, and other molecules. This interactive feature illustrates the movement of some of these materials and describes the structures that make it possible. The cell membrane is critical to the survival of a cell. It acts as a boundary between the cell and its environment, keeping the cytoplasm and organelles inside and harmful organisms and particles out. It is not an impenetrable wall. If it were, the cell would quickly use up any nutrients & oxygen it had to start with and, having no way to acquire more, become depleted & die. It is semi-permeable or selectively permeable. The cell membrane, also called the plasma membrane, functions more like a bag of tightly woven fabric than like a wall. The cell membrane is called a phospholipid bilayer because it is mostly made up of a double layer of phospholipids with some proteins mixed in. The membrane allows some molecules, including gases like oxygen and carbon dioxide, to pass readily through its surface. Water and other small molecules also move into and out of the cell with relative ease. Transport through the cell membrane that does not require energy is called passive transport. Simple diffusion is a form of passive transport where small molecules move across the phospholipids in the membrane from areas of high concentration to low concentration. Simple diffusion does not require energy. The openings in the cell membrane allow the cell to take in and release fairly large molecules like glucose, as well as ions (atoms with a positive or negative electrical charge) that are unable to pass through the main, lipid-bilayer portion of the membrane. This second form of passive transport is called facilitated diffusion. Molecules are still diffusing from areas of high concentration to low concentration without energy. However, these molecules are too large to move through the phospholipids. Large molecules like glucose need a protein “escort” to help them through the cell membrane. Depending on the type of molecule or particle involved and the environment inside and outside the membrane, the cell may have to actively pump molecules or ions along the channel in a process called active transport. Active transport moves molecules across the membrane from low concentration to high concentration and requires ATP energy.