Introduction: Plasma Membranes
A cell’s plasma membrane defines the cell, outlines its borders, and determines the nature of its interaction with its environment (Figure 1). The plasma membrane allows cells to block, take in, and excrete substances, all in controlled quantities. To do this, it must be very flexible, which allows certain cells, such as red blood cells and white blood cells, to change shape as they pass through narrow capillaries. These are the more obvious functions of a plasma membrane. In addition, the surface of the plasma membrane carries markers that allow cells to recognize one another, which is vital for tissue and organ formation during early development and later plays a role in the “self” versus “non-self” distinction of the immune response.
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 as both 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 intracellular response cascades when their effectors are bound. Occasionally, receptors are hijacked by viruses (HIV, human immunodeficiency virus, is one example) that use them to gain entry into cells. At times, the genes that encode receptors become mutated, causing the process of signal transduction to malfunction with disastrous consequences.
The principal components of a plasma membrane are lipids (phospholipids and cholesterol), proteins, and carbohydrates attached to some of the lipids and proteins. The proportions of proteins, lipids, and carbohydrates in the plasma membrane vary with cell type and function.