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. But the cell membrane is not an impenetrable wall -- quite the contrary. If it were, the cell would quickly use up any nutrients and oxygen it may have had to start with and, having no way to acquire more, become depleted and die.

The cell membrane, also called the plasma membrane, functions more like a bag of tightly woven fabric than like a wall. 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. Other substances, however, can move into or out of the cell only through special openings called ion channels, GluT transporters, and protein pumps.

Like the drawstring opening of a cloth bag, 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. 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. In other cases it may provide protein "escorts" for the molecules in a process called facilitated transport, or it may simply provide an open channel that allows some large molecules and ions to move in and out.

To move extremely large molecules and particles across the cell membrane, the cell must temporarily alter the structure of its membrane. For example, enzymes, the large protein molecules produced by cells in the digestive tract, are far too large to pass through the protein pumps and ion channels of the cells that produce them. Instead, the cells package these huge molecules in pouches, called vacuoles, made of a lipid bilayer identical to the one that makes up the cell membrane. When the vacuole makes contact with the cell membrane, the cell membrane and the membrane of the vacuole split open at the point of contact and release the enzymes. This process is called exocytosis.

Endocytosis is similar to but in many ways the reverse of exocytosis. Cells "ingest" large molecules and other materials by folding their cell membrane around them to create vacuoles. When an object is completely enclosed, the vacuole is pinched off and released into the cytoplasm inside the cell. One form of endocytosis, called phagocytosis, allows certain types of cells (phagocytes), including white blood cells, to consume other types of cells. The process of phagocytosis is the same as endocytosis, except that the objects being ingested during phagocytosis are sometimes larger than the phagocyte itself.

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