Cell membrane

The plasma membrane, which is also called the cell membrane, has many functions. The most basic of which is to define the borders of the cell and maintain cell function. The plasma membrane is selectively permeable. This means that the membrane allows some materials to freely enter or leave the cell, while other materials cannot move freely, but require the use of a specialized structure, and occasionally, even energy investment to pass.

Among the most sophisticated functions of the plasma membrane is the ability to transmit signals by means of complex, integral proteins known as receptors. These proteins act both as receivers of extracellular inputs and as activators of intracellular processes. These membrane receptors provide extracellular attachment sites for effectors like hormones and growth factors, and they activate signal transduction response cascades when their effectors are bound.

The fluid mosaic model (Figure 1) describes the structure of the plasma membrane as a mosaic of components—including phospholipids, cholesterol, proteins, and carbohydrates—that gives the membrane a fluid character. Plasma membranes range from 5 to 10 nm nanometers in thickness.

The membrane surfaces that face the interior and exterior of the cell are hydrophilic. In contrast, the interior of the cell membrane is hydrophobic and will not interact with water. Therefore, phospholipids form an excellent two-layer cell membrane that separates fluid within the cell from the fluid outside of the cell.

Fluid mosaic model of the cell membrane. The model shows the two phospholipid layers that form the membrane and various other components embedded in the membrane. Glycoproteins are embedded in the outer layer of the membrane. The glycoproteins consist of a round protein in the membrane and an attached branching carbohydrate chain that extends into the extracellular environment. Glycolipids are are shown as a lipid that forms part of the outer layer of the membrane, with an attached carbohydrate chain extending into the extracellular environment. Peripheral membrane proteins are embedded within one of the membrane layers. Integral membrane proteins form a rod like structure that reaches across both layers of the cell membrane. Protein channels consist of a tube like structure embedded in both layers of the membrane, with openings on both the extracellular and intracellular side to allow molecules to cross the membrane. Cholesterol is a lipid that is embedded within the bilayer membrane, with the hydrophobic tails of the phospholipids.

Figure 1. The fluid mosaic model of the plasma membrane describes the plasma membrane as a fluid combination of phospholipids, cholesterol, and proteins. Carbohydrates attached to lipids (glycolipids) and to proteins (glycoproteins) extend from the outward-facing surface of the membrane.

Acknowledgement The content of these theory pages has been developed based on the resources provided by: OpenStax College, Biology. (OpenStax CNX. Mar 13, 2015)

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