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CELLS, TISSUES, AND SKIN
normally, and also for body cells to maintain their normal fluid volumes. Because there is a continual “leak” of sodium into cells by diffusion, as well as a leak of potassium in the other direction, this pump operates more or less continuously. The pump also can change its rate of transport, depending on the level of sodium and potassium leak (which, for example, temporarily increases during a nerve impulse or muscle contraction when there is a transient increase in membrane leak for sodium and potassium). Another example of active transport includes a potassiumhydrogen pump found in stomach cells, which is used to form hydrochloric acid. Some substances that cannot move across the plasma membrane by any other means are transported by vesicular transport . Endocytosis (“into a cell”) describes vesicular transport where particles are brought into a cell by engulfing or enclosing them within small membranous vesicles . Once a vesicle is formed, it detaches from the plasma membrane and moves into the cytoplasm, where it often fuses with a cellular organelle that contains digestive enzymes (Figure 3.5). This mechanisms is well suited for the transport of relatively large substances, such as bacteria or dead body cells and is called phagocytosis (Figure 3.6), a term that literally means “cell eating.” Phagocytosis is routinely conducted by certain white blood cells called phagocytes. On the other hand, pinocytosis (“cell drinking”) is commonly used by cells to take in liquids that contain dissolved proteins or fats. Exocytosis (“out of a cell”) refers to vesicular transport where particles are eliminated from cells (Figure 3.5). Products to be secreted are first packaged into small membrane sacs (vesicles). The sacs migrate to the plasma membrane and fuse with it. This mechanism is often used to secrete hormones, mucus, and other cell products, or to eject certain cellular wastes.