The cell membrane is a thin layer of lipid, cholesterol, and proteins surrounding the cell. It isolates and protects the cell contents, such as the DNA material, from its extracellular environment, regulating what goes out and what goes into the cell cytosol. Also known as the plasma membrane, it constitutes the outermost limit of the cell; a boundary surrounding the cellular contents. It is an actively functioning part of the living material as many important reactions take place on its surface. Since a cell is the basic unit of a living organism, the cell membrane made life viable and possible on Earth.
Characteristics and structure
The cell membrane is so thin, that it is only visible with the aid of an electron microscope. It consists of two layers of lipid (in the form of phospholipid). The outer and an inner layer are united by molecules of cholesterol. Inserted in the membrane, there is a series of regulating gates, which are composed of proteins. The lipid molecules can move sideways within the plane of the membrane, thus forming a stable fluid film. The inner surface of the cell membrane is composed of fatty acid portion of the phospholipid molecules; therefore, it is oily. This is the reason the membrane is impermeable to various substances, such as sugars, protein fragments, and nucleic acids. Cholesterol molecules in the interior of the membrane also help to make it impermeable.
Most proteins in the cell membrane consists of tightly coiled, rod-like molecules, which are embedded in the phospholid bilayer. These proteins often function as receptors. However, there are group of cell membrane proteins that are compact and globular. Some of these proteins are integral proteins that are embedded in the interior of the phospholipd bilayer and completely span the membrane, providing mechanisms by which small molecules and ions can cross the phospholipid bilayer, getting in and out of the cell. Thus, these proteins function as receptors that are specialized to combine with specific kinds of molecules, such as hormones. Thus, these integral proteins form pore-like channels in the membrane which allow water molecule to go through, among other substances.
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| Above, schematic picture of the cell membrane. |