3.11: Endocytosis and exocytosis are used for bulk transport of particles.

Many substances are just too big to get into or out of a cell by passive or active transport. To absorb large particles, such as bacterial invaders, cells engulf them with their plasma membrane in a process called endocytosis. To export large particles, such as digestive enzymes manufactured for use elsewhere in the body, they often use the process of exocytosis.

There are three types of endocytosis: phagocytosis, pinocytosis, and receptor-mediated endocytosis. All three involve the basic process of the plasma membrane oozing around an object outside the cell, surrounding it, forming a little pocket called a vesicle, and then pinching off the vesicle so that it is inside the cell but separated from the rest of the cell contents.

Phagocytosis and Pinocytosis Relatively large particles are engulfed by cells in a process called phagocytosis (FIGURE 3-22). Amoebas and other unicellular protists, as well as white blood cells, use phagocytosis to consume entire organisms, either as food or as their way of defending against pathogens (disease-causing organisms or substances). Whereas “phagocytosis” comes from the Greek for “eat” and “container,” the term pinocytosis comes from the Greek for “drink” and “container” and describes the process of cells taking in dissolved particles and liquids. The two processes are largely the same, except that the vesicles formed during pinocytosis are generally much smaller than those formed during phagocytosis.

Receptor-Mediated Endocytosis The third type of endocytosis, receptor-mediated endocytosis, is much more specific than either phagocytosis or pinocytosis. Receptor molecules on the surface of a cell recognize and bind to one specific type of molecule. For one receptor it might be insulin, for another it might be cholesterol. Many receptors of the same type are often clustered together in a cell’s plasma membrane. When the appropriate molecule binds to each of the receptor proteins, the membrane begins to fold inward, first forming a little pit and then completely engulfing the molecules, which are still attached to their receptors.

One of the most important examples of receptor-mediated endocytosis involves cholesterol (FIGURE 3-23). Most cholesterol that circulates in the bloodstream is in the form of particles called low-density lipoproteins, or LDL. Each molecule of LDL is a cholesterol globule coated by phospholipids. Proteins embedded within the LDL’s phospholipid coat are recognized by receptor proteins built into the plasma membranes of liver cells. Once bound to the receptors, the LDL molecule is consumed by the cell by endocytosis. Inside the cell, the cholesterol is broken down and used to make a variety of other useful molecules, such as the hormones estrogen and testosterone.

Circulating cholesterol often builds up on the walls of arteries, reducing blood flow and causing the artery to harden. Too much circulating cholesterol in LDL molecules can lead to cardiovascular disease and death (FIGURE 3-24). Individuals lucky enough to have large numbers of LDL receptors on the plasma membranes of their liver cells have a significantly lower risk of cardiovascular disease.

Conversely, some individuals are at risk of early onset of cardiovascular disease because they consume food laden with too much cholesterol (such as egg yolks, cheese, and sausages) or have the misfortune of inheriting genes that code for faulty liver cell membranes that have few LDL receptors—a disorder called familial hypercholesterolemia. In the extreme case where an individual is born with no LDL receptors, circulating cholesterol accumulates in the arteries so rapidly that cardiovascular disease begins to develop even before puberty, and death from a heart attack can occur before the age of 30.

Exocytosis The movement of molecules out of cells takes place throughout the body. For example, cells in the pancreas produce a chemical called insulin that moves throughout the circulatory system. Insulin informs body cells that there is glucose in the bloodstream that ought to be taken in and used for energy. The insulin molecule is much too large to pass out through the plasma membranes of the cells where it is manufactured. As a result, after molecules of insulin are produced, they are coated with a phospholipid membrane to form a vesicle. The insulin-carrying vesicle then moves through the cytoplasm to the inner surface of the cell’s plasma membrane. Once there, the phospholipid membrane surrounding the insulin and the phospholipid membrane of the cell fuse together, dumping the contents of the vesicle out of the cell, where it can enter the bloodstream (FIGURE 3-25).

Exocytosis is not restricted to large molecules. In the brain and other parts of the nervous system, for example, communication between cells occurs as one cell releases large numbers of very small molecules, called neurotransmitters, by exocytosis.