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The Plasma Membrane (formerly known as the cell membrane) forms the border of a neuron and acts to control the movement of substances into and out of the cell. This membrane is composed of two layers of lipid or fat molecules (phospholipids, in particular), with the water-repelling tails of these molecules directed toward the middle of the membrane and away from the fluid found outside and inside the cell. The water-attracting heads of the phospholipid molecules, therefore, are found in contact with extra- and intracellular fluid. Protein molecules are embedded within this bi-lipid membrane. Channel proteins serve as channels for controlling the rate of transport of specific molecules across the plasma membrane. Signal proteins respond to the binding of specific substances (often hormones) by sending messages into the cell to activate particular cellular mechanisms.
The phospholipid structure of the plasma membrane is noteworthy in gaining a better understanding of how this structure functions (Arms & Camp, 1995). The phospholipids are bipolar molecules. One end of the molecule contains phosphorous, is polar, and hydrodphilic (meaning "water loving"). This end of the molecule is referred to as the "head". The opposite or "tail" end of the phospholipid molecule is a two fatty acid chain that is nonpolar and hydrophobic (meaning "water dislike"). The plasma membrane of a living cell consists of two layers of phospholipids. The hydrophilic, phosphorous heads of each layer orient themselves in contact with the water found in the extracellular fluid ouside the cell and with the water found in the intracellular fluid inside the cell. The hydrophobic, fatty acid tails orient themselves to the inside of the plasma membrane, away from the extracellular and intracellular fluid.
Cholesterol, proteins, and sugars are also found in the plasma membrane. The cholesterol seems to provide structural stability to the membrane, wedging between the phospholipid molecules, which are in constant motion. Proteins take two forms; they either span the full width of the plasma membrane (integral proteins) or they attach to only one side of the membrane (called peripheral proteins). Integral proteins form the channels for movement of substances in and out of the cell. Some of the channels are controlled by the action of specific enzymes and are called "gated". The peripheral proteins act as carriers, helping molecules move through the membrane. Sugar molecules are found attached to the proteins and lipids of the plasma membrane. Indeed, the typing of blood into groups A, B, and O is based on the presence or absence of specific sugars attached to proteins (glycoproteins) on the plasma membrane of your red blood cells. The immune system is able to identify foreign proteins (such as those of bacteria or viruses) in the body based on the composition of glycoproteins in the plasma membrane. In addition, the layer of sugar that coats the plasma membrane appears to diminish with age. Evidence suggests that other cells detect when membrane sugar levels reach a low threshold, and this triggers a process that destroys the aging cell.
Arms, K. & Camp, P. (1995), Biology (4th ed.). New York: Harcourt Brace College Publishers.