Phospholipid molecules in the cell membrane are "fluid," in the sense that they are free to diffuse and exhibit rapid lateral diffusion. and are examples of cholesterol-enriched microdomains in the cell membrane.
Many proteins are not free to diffuse. The undergirds the cell membrane and provides anchoring points for integral membrane proteins. Anchoring restricts them to a particular cell face or surface – for example, the "apical" surface of that line the – and limits how far they may diffuse within the bilayer. Rather than presenting always a formless and fluid contour, the plasma membrane surface of cells may show structure. Returning to the example of epithelial cells in the gut, the apical surfaces of many such cells are dense with involutions, all similar in size. The finger-like projections, called microvilli, increase cell surface area and facilitate the absorption of molecules from the outside. are another example of highly-structured membrane.
New material is incorporated into the membrane, or deleted from it, by a variety of mechanisms.
Fusion of intracellular with the membrane not only excretes the contents of the vesicle, but also incorporates the vesicle membrane's components into the cell membrane. The membrane may form blebs that pinch off to become vesicles.
If a membrane is continuous with a tubular structure made of membrane material, then material from the tube can be drawn into the membrane continuously.
Although the concentration of membrane components in the aqueous phase is low (stable membrane components have low solubility in water), exchange of molecules with this small reservoir is possible.
In all cases, the mechanical tension in the membrane has an effect on the rate of exchange. In some cells, usually having a smooth shape, the membrane tension and area are interrelated by elastic and dynamical mechanical properties, and the time-dependent interrelation is sometimes called homeostasis, area regulation or tension regulation.
Structure
The outer cell membrane and the membranes surrounding inner cell organelles are . To perform the function of the organelle, the membrane is specialized in that it contains specific proteins and lipid components that enable it to perform its unique roles for that cell or organelle. In the cell membrane, phospholipid molecules create a spherical three dimensional lipid bilayer shell around the cell. A phospholipid molecule is composed of a head and two tails. The circle, or head, is the negatively charged phosphate group and the two tails are the two highly hydrophobic fatty acid chains of the phospholipid.
Functions
It attaches parts of the cytoskeleton to the cell membrane in order to provide shape.
It attaches cells to an extra-cellular matrix in grouping cells together to form tissues.
It transports molecules into and out of cells by such methods as ion pumps, channel proteins and carrier proteins.
It acts as receptor for the various chemical messages that pass between cells such as nerve impulses and hormone activity.
It takes part in enzyme activity which can be important in the metabolism or as part of the body's defense mechanism.
