Plasma Membrane

The plasma membrane is a thin, continuous sheet that separates all living cells from their environment. This membrane maintains conditions within the cell and allows certain things to pass in and out of the cell. It is strong and can repair itself if damaged.

Every living cell in every living thing, plant or animal, is surrounded by a plasma membrane. If cells did not have some sort of barrier to separate them from their fluid environment, they could not perform any functions. Their organelles (tiny structures that each have a job to do) would be floating around all over the place. More important, they would not be cells, since cells are separate units of living matter. It is the plasma membrane that makes cells a separate entity, or a self-contained unit of living matter.

The membrane is so thin that it was not proven to exist until the late twentieth-century invention of the super powerful electron microscope. Although incredibly thin, this delicate structure is very strong and even stretchable. It is made up of a double layer of molecules of a lipid (fats, oils, and waxes) that each have a head and a tail. Their tails point in toward the center of the cell and repel water. Their larger heads face out-ward and attract water.

Plasma membranes are selectively permeable. This means that they allow certain molecules to enter the cell through its pores while blocking the path of others. The plasma membrane is sometimes called the living gatekeeper. Although strong and flexible, it can be damaged. However, the plasma membrane has the ability to repair any breaks it may suffer. Although the plasma membrane is one of the few structures common to every living cell, the plasma membrane in a plant cell is not identical to what is called its cell wall. Animal cells do not have cell walls. They are found only in plant cells and are located outside the plasma membrane. Unlike the plasma membrane, the cell wall is made of tough cellulose (the main component of plant tissue). Fungi and bacteria also have cell walls, but they are not made of cellulose. Cells walls give plant cells a rigid shape, while the plasma membrane of animal cells give them the appearance of a tiny, jelly-filled bag.

[See alsoCell ]