Porth's Essentials of Pathophysiology, 4e

17

Cell Structure and Function

C h a p t e r 1

more permeable to potassium than sodium, the resting membrane reflects the diffusion of potassium ions. The Na + /K + -ATPase membrane pump, which removes three Na + from inside the cell while returning two K + to the inside, assists in maintaining the resting membrane potential. During an action potential, the cell membrane becomes more permeable to sodium, causing its polarity to change so that it is positive on the inside and negative on the outside (discussed in Chapter 34).

■■ Electrical potentials, which are measured in volts, describe the ability of separated electrical charges of opposite polarity

Tissues In the preceding sections, we discussed the individual cell, its metabolic processes, and mechanisms of signal- ing and communication. Although cells have similari- ties, their structures and functions vary according to the specific needs of the body. For example, muscle cells are specialized to perform different functions from skin cells or nerve cells. Groups of cells that are closely associated in structure and have common or similar functions are called tissues . Four categories of tissue exist: (1) epithe- lial, (2) connective, (3) muscle, and (4) nervous. These tissues do not exist in isolated units but in association with each other and in variable proportions, forming different structures and organs of the body. This sec- tion of the chapter provides a brief overview of the cells in each of the four tissue types, the structures that hold these cells together, and the extracellular matrix in which they live. Embryonic Origin ofTissueTypes After conception, the fertilized ovum undergoes a series of divisions, ultimately forming different cell types that comprise the various tissues of the body. The forma- tion of different, more specialized types of cells and ( text continues on page 20 ) (+ and −) to do work. In regard to cells, the oppositely charged particles are ions, and the barrier that separates them is the cell membrane. There are two main factors that alter membrane potentials and excitability: the difference in concentration of ions on the inside and outside of the membrane and the permeability of the membrane to these ions. An equilibrium potential is one in which there is no net movement of a particular ion across a membrane because the diffusion and electrical forces generated by the movement of the ions are exactly balanced. The resting membrane potential ( – outside and + inside) is essentially a potassium equilibrium potential that results from the selective permeability of the membrane to the potassium ion and the large difference in potassium concentration that exists between the inside and the outside of the membrane. During an action potential, the cell membrane becomes highly permeable to sodium, causing it to depolarize and reverse its polarity (− inside and + outside).

SUMMARY CONCEPTS

■■ Cells communicate with each other by chemical messenger systems. Chemical messengers bind to receptors on or near the cell surface. There are three classes of cell surface receptor proteins: G protein-linked, enzyme-linked, and channel-linked. G protein–linked receptors rely on a class of molecules called G proteins that function as an on–off switch to convert external signals (first messengers) into internal signals (second messengers). Enzyme-linked receptors have intrinsic enzyme activity or rely on enzymes that are closely associated with the receptor they activate. One type of enzyme-linked receptor is widely used in hormonal control of cell function and involves the activation of the enzyme adenylyl cyclase, which catalyzes the formation of cAMP, a second messenger that has multiple effects inside the cell. Activation of ion channel– linked receptors (e.g., by neurotransmitters) may trigger signaling to transiently open or close ion channels formed by integral proteins in the cell. ■■ Substances that enter or leave the cell must cross the cell membrane. Diffusion is a process by which substances such as ions move from areas of greater concentration to areas of lesser concentration until reaching a uniform distribution. Facilitated diffusion is a passive process, in which molecules that cannot normally pass through the cell’s membranes do so with the assistance of a carrier molecule. Active transport requires the cell to expend energy in moving ions against a concentration gradient.The Na + /K + -ATPase membrane pump is the best-known type of active transport. Vesicular transport is a mechanism in which a cell encloses extracellular material in a membrane-bound vesicle.There are two types of vesicular transport: endocytosis, in which materials are brought into the cell by invagination of the cell membrane to form a vesicle, and exocytosis, in which materials are exported from the cell by fusion of a vesicle with the cell membrane.