Endocytosis and Exocytosis

Endocytosis is the process by which cells engulf materials from their surroundings. It includes pinocytosis and phagocytosis. Pinocytosis involves the ingestion of small solid or fluid particles. The particles are engulfed into small, membrane-surrounded vesicles for movement into the cytoplasm. The process of pinocytosis is important in the transport of proteins and strong solutions of electrolytes (see Fig. 4-16).

Phagocytosis literally means cell eating and can be compared with pinocytosis, which means cell drinking. It

 

UNIT II Cell Function and Growth

Outside

Inside

FIGURE 4-17 Secondary active transport systems. Symport or co-transport (top) carries the transported solute (s) in the same direc­tion as the Na+ ion. Antiport or counter-transport carries the solute and Na+ in the opposite direction. (Rhoades R.A., Tanner G.A. [1996]. Medical physiology. Boston: Little, Brown)

involves the engulfment and subsequent killing or degra­dation of microorganisms and other particulate matter. During phagocytosis, a particle contacts the cell surface and is surrounded on all sides by the cell membrane, form­ing a phagocytic vesicle or phagosome. Once formed, the phagosome breaks away from the cell membrane and moves into the cytoplasm, where it eventually fuses with a lysosome, allowing the ingested material to be degraded by lysosomal enzymes. Certain cells, such as macrophages and polymorphonuclear leukocytes (neutrophils), are adept at engulfing and disposing of invading organisms, dam­aged cells, and unneeded extracellular constituents (see Chapter 20).

Receptor-mediated endocytosis involves the binding of substances such as low-density lipoproteins to a receptor on the cell surface. Binding of a ligand (i.e., a substance with a high affinity for a receptor) to its receptor normally causes widely distributed receptors to accumulate in clathrin-coated pits. An aggregation of special proteins on the cyto-plasmic side of the pit causes the coated pit to invaginate and pinch off, forming a clathrin-coated vesicle that carries the ligand and its receptor into the cell.

Exocytosis is the mechanism for the secretion of intra-cellular substances into the extracellular spaces. It is the reverse of endocytosis in that a secretory granule fuses to the inner side of the cell membrane, and an opening oc­curs in the cell membrane. This opening allows the con­tents of the granule to be released into the extracellular fluid. Exocytosis is important in removing cellular debris and releasing substances, such as hormones, synthesized in the cell.

 

During endocytosis, portions of the cell membrane become an endocytotic vesicle. During exocytosis, the ve­sicular membrane is incorporated into the plasma mem­brane. In this way, cell membranes can be conserved and reused.

Ion Channels

The electrical charge on small ions such as Na⁺ and K⁺ makes it difficult for these ions to move across the lipid layer of the cell membrane. However, rapid movement of these ions is required for many types of cell functions, such as nerve activity. This is accomplished by facilitated diffusion through selective ion channels. Ion channels are integral proteins that span the width of the cell membrane and are normally composed of several polypeptides or pro­tein subunits that form a gating system. Specific stimuli cause the protein subunits to undergo conformational changes to form an open channel or gate through which the ions can move (Fig. 4-18). In this way, ions do not need to cross the lipid-soluble portion of the membrane but can remain in the aqueous solution that fills the ion channel. Ion channels are highly selective; some channels allow only for passage of sodium ions, and others are se­lective for potassium, calcium, or chloride ions. Specific in­teractions between the ions and the sides of the channel can produce an extremely rapid rate of ion movement. For example, ion channels can become negatively charged, promoting the rapid movement of positively charged ions. The plasma membrane contains two basic groups of ion channels: leakage channels and gated channels. Leakage channels are open even in the unstimulated state, whereas gated channels open and close in response to specific stim­uli. Three types of gated channels are present in the plasma membrane: voltage-gated channels, which have electrically operated gates that open when the membrane potential changes beyond a certain point; ligand-gated channels, which have chemically operated gates that respond to specific receptor-bound ligands, such as the neurotransmitter acetyl-choline; and mechanically gated channels, which open or close in response to such mechanical stimulations as vibra­tions, tissue stretching, and pressure.