The plasmalemma is another name for the plasma membrane of a cell. Generally, each cell has one of these membranes, which functions as a containing wall to keep the contents separate from the surrounding environment. The plasmalemma is constructed from lipids, more commonly known as fats, and proteins. Some of the proteins are fixed and some can move about within the cell membrane. A number of proteins are involved in transporting molecules into and out of the cell, while others help maintain cell structure, act as receptors or take part in chemical reactions as enzymes.
Proteins and lipids make up the plasmalemma, with the proteins effectively determining the shape and function of the cell. Most of the lipids are present in the form of phospholipids, which have a phosphate head and a fatty acid tail. Phosphate heads are water soluble while the tails are not. The phospholipids are arranged in the membrane so that the water soluble heads point toward the watery extracellular environment and the aqueous cytoplasm make up the inside of the cell. The tail ends, which are not water soluble, are oriented together in the middle of the membrane.
Around half of the plasmalemma consists of protein. These proteins are of two sorts: integral proteins inside the double layer of lipids and peripheral proteins loosely bound to the surface of the membrane. Of the integral proteins, some, called transmembrane proteins, pass right through the lipid bilayer and stick out at each side, while others are contained just within the inner or outer part of the membrane and protrude only from one of its surfaces.
A number of membrane proteins are involved in moving substances in and out of cells. For example, in bacterial cell structure, membrane proteins pump in ammonia from which the nutrient nitrogen may be extracted. Some proteins in the plasmalemma are cell adhesion molecules which join cells to one another and to basement membranes while others take part in chemical reactions and are known as enzymes, or act as receptors for substances like hormones. When hormones bind to such receptors, this causes changes within the cell. In muscle or nerve cells, membrane proteins are involved in creating and sending electrical impulses.
Although the plasmalemma is fluid and proteins can move around in it, some of them are restricted to certain areas where they enable specific functions. In epithelial cells lining the intestines some proteins are concentrated within the apical membrane, the upper surface of the cell that projects into the gut opening. Others are restricted to the basolateral membrane at the sides and lower surfaces of the cell. This ensures that uptake of nutrients from the gut occurs at one cell surface, before substances pass through the cell and are transported out to blood vessels on the other side of the cell.
