C h a p t e r 1
Cell Structure and Function
17
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).
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
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)
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.
■■
Electrical potentials, which are measured
in volts, describe the ability of separated
electrical charges of opposite polarity
(+ 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).
