Porth's Essentials of Pathophysiology, 4e

15

Cell Structure and Function

C h a p t e r 1

Ion

Ions

–60 mV

–45 mV

A

Ion

Ligand

Ions

Ligand

Receptor

Receptor

FIGURE 1-12. Gated ion channels that open in response to specific stimuli. (A) Voltage-gated channels are controlled by a change in the membrane potential. (B) Ligand-gated channels are controlled by binding of a ligand to a receptor.

B

molecules in the cell membrane by osmosis without actually dissolving in the region occupied by the fatty acid side of the chains. Osmosis is regulated by the con- centration of nondiffusible particles on either side of the membrane, with water moving from the side with the lower concentration of particles to the side with the higher concentration. The cell membranes of most cells also contain transmembrane proteins, called aquapo- rins , that function as water channels. Aquaporins are especially abundant in cells that must transport water at particularly high rates, such as certain cells of the kidney. ActiveTransport The process of diffusion describes particle movement from an area of higher concentration to one of lower concentration, resulting in an equal distribution of per- meable substances across the cell membrane. Sometimes, however, different concentrations of a substance are needed in the intracellular and extracellular fluids. For example, to function, a cell requires a higher intracel- lular concentration of potassium ions than is present in the extracellular fluid, while maintaining a much lower concentration of sodium ions than the extracellular fluid. In these situations, energy is required to pump the ions “uphill” or against their concentration gradient. When cells use energy to move ions against an electrical or chemical gradient, the process is called active trans- port . Two types of active transport systems exist: pri- mary active transport and secondary active transport. Primary Active Transport. Among the substances that are transported by primary active transport are sodium, potassium, calcium, and hydrogen ions. The active transport system studied in the greatest detail is the sodium/potassium (Na + /K + )-adenosine triphosphatase (ATPase) membrane pump. The Na + /K + −ATPase mem- brane pump moves sodium from inside the cell to the extracellular region, where its concentration is approxi- mately 14 times greater than inside; the pump also returns potassium to the inside, where its concentration is approximately 35 times greater than it is outside the cell. If it were not for the activity of the Na + /K + −ATPase

membrane pump, the osmotically active sodium parti- cles would accumulate in the cell, causing cellular swell- ing because of an accompanying influx of water. Secondary Active Transport. Secondary active trans- port mechanisms harness the energy derived from the pri- mary active transport of one substance, usually sodium, for the cotransport of a second substance. For example, when sodium ions are actively transported out of a cell by primary active transport, a large concentration gradient develops (i.e., high concentration on the outside and low on the inside). This concentration gradient represents a large storehouse of energy because sodium ions are always attempting to diffuse into the cell. Similar to facilitated diffusion, secondary transport uses membrane transport proteins. These proteins have two binding sites: one for sodium and the other for the substance undergoing sec- ondary transport. Secondary active transport systems are classified into two groups: cotransport, or symport systems, in which sodium and the solute are transported in the same direction, and countertransport, or antiport systems, in which sodium and the solute are transported in the opposite directions. An example of cotransport occurs in the intestine, where the absorption of glucose and amino acids is coupled with sodium transport. Vesicular Transport Vesicular transport is a mechanism in which materials are transported in membrane-bound vesicles. There are two types of vesicular transport: endocytosis , in which materials are moved into a cell in a vesicle formed from the cell membrane, and exocytosis , in which materials are moved out of a cell by fusion of a vesicle with the cell membrane. Endocytosis is the process by which cells engulf mate- rials from their surroundings. In the process, the mate- rial is progressively enclosed in small portions of the cell membrane, which first invaginates (folds inward) and then pinches off to become an endocytotic vesicle. If the vesicle is small (>150 nm in diameter), the process is called pinocytosis , and the vesicle is called a pinocytotic vesicle ; if the vesicle is large (>250 nm in diameter), the process is called phagocytosis , and the vesicle is called