Part of every biological cell, the cell membrane is a semipermeable bilayer of phospholipids and protein molecules that envelops the cell, dividing the cytoplasm inside the cell from the environment outside it. The membrane is often pocked with receptor proteins, cell adhesion proteins, and other specialized structures (almost always proteins) that the cell uses to perform vital functions.
In animals, the cell membrane is the outermost layer of the cell; in yeast, bacteria, and plants, a secondary cell wall surrounds the cell membrane to provide additional support.
The cell membrane is often described as a fluid mosaic because it is not homogeneous; instead, it consists of diffusing lipids dotted with proteins functioning as channels into and out of the cell or as receptors. Cellular proteins and other molecules, like cholesterol, assist in regulating the fluidity of the membrane.
Beneath the cell membrane, the cytoskeleton, composed of microtubules and filaments, provide it with structure. The outside of the membrane may be dotted with microvilli, increasing surface area. Many cell membranes, like neurological synapses, are highly specialized.
Transport of molecules through the cell membrane may be active or passive in nature; active transport requires the expenditure of APT. Most passive transport comes about through diffusion reactions of hydrophobic, polar, and ionic molecules.
Every cell, from Archaea to the most specialized Eukarya, are enclosed by a cell membrane. The membrane serves as an individuating boundary between the cell and the outside world.
All cells in the modern world have a phospholipid cell membrane. Phospholipids are lipids (oils or fats) with a phosphate group attached. The end of the phospholipid with an attached phosphate group is attracted to water (hydrophillic) while the end with the lipid is repelled by water (hydrophobic). Each cell membrane is constructed with two layers of phospholipids. On the outer layer, the side attracted to water faces the outside world. On the inner layer, the side that repels water faces the internal cell structure. This two layer setup serves to keeps water and other materials from easily passing through the membrane and allows the cell to regulate the passage of materials through special channels.
The membrane can also serve different functions in various cells. For example, in single celled organisms the cell membrane serves an essential role as an environment sensor. In addition, the membrane can play a role in energy production, food collection, signal transduction, and adhesion.
Web Resources On Cell Membranes
Book Resources On Cell Membranes
Cell Membrane: The Red Blood Cell as a Model by Yoshihito Yawata
The Cell Membrane: Its Role in Interaction With the Outside World by Edgar Haber (Editor)