All living organisms are made of cells and cell
products. This simple statement, called the cell the-
ory, was first proposed more than 150 years ago. You
may think of a theory as a guess or hypothesis, and
sometimes this is so. A scientific theory, however, is
actually the best explanation of all available evidence.
All of the evidence science has gathered so far sup-
ports the validity of the cell theory.
Cellsare the smallest living subunits of a multicel-
lular organism such as a human being. A cell is a com-
plex arrangement of the chemicals discussed in the
previous chapter, is living, and carries out specific
activities. Microorganisms, such as amoebas and bac-
teria, are single cells that function independently.
Human cells, however, must work together and func-
tion interdependently. Homeostasis depends upon the
contributions of all of the different kinds of cells.
Human cells vary in size, shape, and function. Most
human cells are so small they can only be seen with
the aid of a microscope and are measured in units
called micrometers(formerly called microns). One
micrometer 1/1,000,000 of a meter or 1/25,000 of
an inch (see Appendix A: Units of Measure). One
exception is the human ovum or egg cell, which is
about 1 millimeter in diameter, just visible to the
unaided eye. Some nerve cells, although microscopic
in diameter, may be quite long. Those in our arms and
legs, for example, are at least 2 feet (60 cm) long.
With respect to shape, human cells vary greatly.
Some are round or spherical, others rectangular, still
others irregular. White blood cells even change shape
as they move.
Cell functions also vary, and since our cells do not
act independently, we will cover specialized cell func-
tions as part of tissue functions in Chapter 4. Based on
function, there are more than 200 different kinds of
human cells. This chapter is concerned with the basic
structure of cells and the cellular activities common to
all our cells.
CELL STRUCTURE
Despite their many differences, human cells have sev-
eral similar structural features: a cell membrane, a
nucleus, and cytoplasm and cell organelles. Red blood
cells are an exception because they have no nuclei
when mature. The cell membrane forms the outer
boundary of the cell and surrounds the cytoplasm,
organelles, and nucleus.CELL MEMBRANE
Also called the plasma membrane, the cell mem-
braneis made of phospholipids, cholesterol, and pro-
teins. The arrangement of these organic molecules is
shown in Fig. 3–1. The phospholipids are diglyc-
erides, and form a bilayer, or double layer, which
makes up most of the membrane. Phospholipids
permit lipid-soluble materials to easily enter or leave
the cell by diffusion through the cell membrane. The
presence of cholesterol decreases the fluidity of
the membrane, thus making it more stable. The pro-
teins have several functions: Some form channelsor
poresto permit passage of materials such as water or
ions; others are carrier enzymes or transportersthat
also help substances enter the cell. Still other proteins,
with oligosaccharides on their outer surface, are anti-
gens, markers that identify the cells of an individual as
“self.” Yet another group of proteins serves as recep-
tor sitesfor hormones. Many hormones bring about
their specific effects by first bonding to a particular
receptor on the cell membrane, a receptor with the
proper shape. This bonding, or fit, then triggers
chemical reactions within the cell membrane or the
interior of the cell (see Box 10–3 for an illustration
involving the hormone insulin).
Many receptors for other molecules are also part of
cell membranes. These molecules are part of the
chemical communication networks our cells have. An
unavoidable consequence of having so many receptors
for chemical communication is that some pathogens
have evolved shapes to match certain receptors. For
example, HIV, the virus that causes AIDS, just hap-
pens to fit a particular surface receptor on our white
blood cells. When the virus fits in, the receptor
becomes a gateway into the cell, which begins the
takeover of the cell by the virus.
Most often, however, the cell membrane is a bene-
ficial structure. Although the cell membrane is the
outer boundary of the cell, it should already be appar-
ent to you that it is not a static or wall-like boundary,
but rather an active, dynamic one. It is much more like
a line of tollbooths than a concrete barrier. The cell
membrane is selectively permeable; that is, certain
substances are permitted to pass through and others48 Cells