THE CELL MEMBRANE

The cell is enclosed in a thin membrane that separates the intracellular contents from the extracellular environment. To differentiate it from the other cell membranes, such as the mitochondrial or nuclear membranes, the cell mem­brane is often called the plasma membrane. In many re­spects, the plasma membrane is one of the most important parts of the cell. It acts as a semipermeable structure that separates the intracellular and extracellular environments. It provides receptors for hormones and other biologically active substances, participates in the electrical events that occur in nerve and muscle cells, and aids in the regulation

of cell growth and proliferation. The cell membrane may play an important role in the behavior of cancer cells (dis­cussed in Chapter 8).

The cell membrane consists of an organized arrange­ment of lipids, carbohydrates, and proteins (Fig. 4-9). A main structural component of the membrane is its lipid bilayer. It is a bimolecular layer that consists primarily of phospholipids, with glycolipids and cholesterol. Lipids form a bilayer structure that is essentially impermeable to all but lipid-soluble substances. Approximately 75% of the lipids are phospholipids, each with a hydrophilic (water-soluble) head and a hydrophobic (water-insoluble) tail. Phospholipid molecules, along with the glycolipids, are aligned such that their hydrophilic heads face outward on each side of the membrane and their hydrophobic tails pro­ject toward the middle of the membrane. The hydrophilic heads retain water and help cells stick to each other. At nor­mal body temperature, the viscosity of the lipid compo­nent of the membrane is equivalent to that of olive oil. The presence of cholesterol stiffens the membrane.

Although the lipid bilayer provides the basic structure of the cell membrane, proteins carry out most of the specific functions. Some proteins, called transmembraneproteins, pass directly through the membrane and communicate with the intracellular and extracellular environments. Integral proteins

 

 

IGURE 4-9 The structure of the cell mem­brane showing the hydrophilic (polar) heads and the hydrophobic (fatty acid) tails and the position of the integral and peripheral pro­teins in relation to the interior and exterior of the cell.

are transmembrane proteins tightly bound to lipids in the bilayer and are essentially part of the membrane. A second type of protein, the peripheral proteins, are bound to one or the other side of the membrane and do not pass into the lipid bilayer. Removal of peripheral proteins from the mem­brane surface usually causes damage to the membrane.

The manner in which proteins are associated with the cell membrane often determines their function. Thus, peri­pheral proteins are associated with functions involving the inner or outer side of the membrane where they are found. Peripheral proteins usually serve as receptors or are in­volved in intracellular signaling systems. By contrast, only the transmembrane proteins can function on both sides of the membrane or transport molecules across it.

Many integral transmembrane proteins form the ion channels found on the cell surface. These channel proteins have a complex morphology and are selective with respect to the substances they transmit. Mutations in these chan­nel proteins, often called channelopathies, are responsible for a host of genetic disorders. For example, in cystic fibrosis, the primary defect resides in an abnormal chloride chan­nel, which results in increased sodium and water reabsorp-tion that causes respiratory tract secretions to thicken and occlude the airways (see Chapter 31).

A fuzzy-looking layer surrounds the cell surface called the cell coat, or glycocalyx. The structure of the glycocalyx consists of long, complex carbohydrate chains attached to protein molecules that penetrate the outside portion of the membrane (i.e., glycoproteins); outward-facing membrane lipids (i.e., glycolipids); and carbohydrate-binding proteins called lectins. The cell coat participates in cell-to-cell recog­nition and adhesion. It contains tissue transplant antigens that label cells as self or nonself. The cell coat of a red blood cell contains the ABO blood group antigens. An in­timate relationship exists between the cell membrane and the cell coat. If the cell coat is enzymatically removed, the cell remains viable and can generate a new cell coat, but damage to the cell membrane usually results in cell death.

In summary, the cell is a remarkably autonomous structure that functions strikingly similarly to that of the total organism. In most cells, a single nucleus controls cell function and is the mastermind of the cell. It contains DNA, which provides the in­formation necessary for the synthesis of the various proteins that the cell must produce to stay alive and to transmit infor­mation from one generation to another.

The cytoplasm contains the cell's organelles and the en­zymes necessary for glycolysis. Ribosomes serve as sites for protein synthesis in the cell. The ER functions as a tubular com­munication system that transports substances from one part of the cell to another and as the site of protein (rough ER), car­bohydrate, and lipid (smooth ER) synthesis. Golgi bodies mod­ify materials synthesized in the ER and package them into secretory granules for transport within the cell or for export from the cell. Lysosomes, which are viewed as the digestive system of the cell, contain hydrolytic enzymes that digest worn-out cell parts and foreign materials. They are membra­nous structures formed in the Golgi complex from hydrolytic enzymes synthesized in the rough ER. Another organelle, the proteosome, digests misformed and misfolded proteins. The mitochondria serve as power plants for the cell because they transform food energy into ATP, to power cell activities. Mito­chondria contain their own extrachromosomal DNA, impor­tant in the synthesis of mitochondrial RNAs and proteins used in oxidative metabolism. Microtubules are slender, stiff, tubu­lar structures that influence cell shape, provide a means of moving organelles through the cytoplasm, and affect move­ment of the cilia and of chromosomes during cell division. Several types of threadlike filaments, including actin and myosin filaments, participate in muscle contraction.

The plasma membrane is a lipid bilayer that surrounds the cell and separates it from its surrounding external environment. A fuzzy-looking layer, the cell coat or glycocalyx, surrounds the cell surface; it contains tissue antigens and participates in cell-to-cell recognition and adhesion.