CELLULAR FORM
AND FUNCTION
Mitochondria (blue) and rough endoplasmic reticulum (orange)
in a pancreatic cell (SEM)
CHAPTER OUTLINE
3.1 Concepts of Cellular Structure 79
•
Development of the Cell Theory 79
•
Cell Shapes and Sizes 79
•
Basic Components of a Cell 81
3.2 The Cell Surface 82
•
The Plasma Membrane 83
•
The Glycocalyx 87
•
Microvilli, Cilia, and Flagella 88
DEEPER INSIGHTS
3.1
Clinical Application: Calcium
Channel Blockers 86
3.2
3.3
Clinical Application: Cystic Fibrosis 90
Clinical Application: Familial
Hypercholesterolemia 99
3.4
Evolutionary Medicine: Mitochondria­
Evolution and Clinical Significance 1 1 0
3.3 Membrane Transport 91
•
Filtration 91
•
Simple Diffusion 91
•
Osmosis 93
•
Osmolarity and Tonicity 94
•
Carrier-Mediated Transport 95
•
Vesicular Transport 98
3.4 The Cell Interior 1 01
•
The Cytoskeleton 1 01
•
Organelles 1 02
•
Inclusions 1 09
Study Guide 111
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Module 2: Cells and Chemistry
78
PART ONE
80
Organization of the Body
Squamous
Cuboidal
Columnar
Polygonal
Stellate
Spheroidal
Discoid
Fusiform (spindle-shaped)
Fibrous
FIGURE 3.1 Common Cell Shapes.
•
•
Columnar-distinctly taller than wide, such as the
sizes is the micrometer (j.tm), formerly called the micron­
PolygonaiS-having irregularly angular shapes with
one-millionth (lb-6) of a meter, one-thousandth (10-3) of
four, five, or more sides.
•
Stellate6-having multiple pointed processes pro­
jecting from the body of a cell, giving it a somewhat
starlike shape. The cell bodies of many nerve cells
are stellate.
•
Spheroidal to ovoid-round to oval, as in egg cells
and white blood cells.
•
Discoid-disc-shaped, as in red blood cells.
•
Fusiform7 (FEW-zih-form)-spindle-shaped; elongated,
with a thick middle and tapered ends, as in smooth
muscle cells.
•
The most useful unit of measurement for designating cell
inner lining cells of the stomach and intestines.
Fibrous-long, slender, and threadlike, as in skeletal
muscle cells and the axons (nerve fibers) of nerve cells.
a millimeter. (See appendix D for units of measurement.)
The smallest objects most people can see with the naked
eye are about 100 fLm, which is about one-quarter the size
of the period at the end of this sentence. A few human
cells fall within this range, such as the egg cell and some
fat cells, but most human cells are about 10 to 15 fLm wide.
The longest human cells are nerve cells (sometimes over a
meter long) and muscle cells (up to 30 em long), but both
are usually too slender to be seen with the naked eye.
There is a limit to how large a cell can be, partly due
to the relationship between its volume and surface area.
The surface area of a cell is proportional to the square of
its diameter, while volume is proportional to the cube
of its diameter. Thus, for a given increase in diameter,
volume increases much more than surface area. Picture a
Some of these shapes refer to the way a cell looks in typi­
cuboidal cell 10 fLm on each side (fig 3.2). It would have
cal tissue sections, not to the complete three-dimensional
a surface area of 600 fLm2 (10 fLm X 10 fLm X 6 sides) and
shape of the cell. A cell that looks squamous, cuboidal, or
a volume of 1,000 fLm3 (10 X 10 X 10 fLm). Now, suppose
columnar in a tissue section, for example, usually looks
it grew by another 10 fLm on each side. Its new surface
polygonal if viewed from its upper surface.
area would be 20 fLm X 20 fLm X 6
=
volume would be 20 X 20 X 20 fLm
2,400 fLm2, and its
=
8,000 fLm3. The
20 fLm cell has eight times as much protoplasm need­
'poly
6stell
'fusi
=
=
=
many; gon angles
star; ate characterized by
spindle; form
shape
=
=
=
ing nourishment and waste removal, but only four times
as much membrane surface through which wastes and
nutrients can be exchanged. A cell that is too big cannot
CHAPTER 3
79
Cellular Form and Function
congealed and acquired a membrane and nucleus. This
idea of spontaneous generation-that living things arise
Rr11�hinn l J n
o
from nonliving matter-was rooted in the scientific
To adequately understand the structure of the cell surface, it is essen­
tial that you understand phospholipids and their amphiphilic nature
ple common sense that decaying meat turned into
maggots, stored grain into rodents, and mud into frogs.
(p. 64) as well as glycolipids and glycoproteins (p. 62).
0
thought of the times. For centuries, it seemed to be sim­
T he proteins of cell membranes have a great variety of functions. To
Schwann and his contemporaries merely extended this
understand those depends on an acquaintance with the functions
idea to cells. The idea of spontaneous generation wasn't
of proteins in general (p. 68) and how protein function depends on
discredited until some classic experiments by French
tertiary structure (p. 67).
microbiologist Louis Pasteur in
1859.
By the end of the
nineteenth century, it was established beyond all reason­
able doubt that cells arise only from other cells.
ll organisms, from the simplest to the most complex, are
A
The development of biochemistry from the late nine­
composed of cells-whether the single cell of a bacterium or
teenth to the twentieth century made it further apparent
the trillions of cells that constitute the Bhuman body. These
that all physiological processes of the body are based on
cells are responsible for all structural and functional properties of
cellular activity and that the cells of all species exhibit
a living organism. A knowledge of cells is therefore indispensable
remarkable biochemical unity. Thus emerged the general­
to any true understanding of the workings of the human body,
izations that constitute the modern cell theory:
the mechanisms of disease, and the rationale of therapy. Thus,
1.
this chapter and the next one introduce the basic cell biology
of the human body, and subsequent chapters expand upon this
All organisms are composed of cells and cell products.
2. The cell is the simplest structural and functional
unit of life. There are no smaller subdivisions of a
information as we examine the specialized cellular structure and
cell or organism that, in themselves, are alive. An
function of specific organs.
enzyme molecule, for example, is not alive, although
the life of a cell depends on the activity of numerous
enzymes.
Concepts of Cellular Structure
3.
4.
Expected Learning Outcomes
ancestry to the same original cells.
5.
cell theory;
Cells come only from preexisting cells, not from
nonliving matter. All life, therefore, traces its
When you have completed this section, you should be able to
a. discuss the development and modern tenets of the
An organism's structure and all of its functions are
ultimately due to the activities of its cells.
Because of this common ancestry, the cells of all
species have many fundamental similarities in their
b. describe cell shapes from their descriptive terms;
chemical composition and metabolic mechanisms.
c. state the size range of human cells and discuss factors
Cell Shapes and Sizes
that limit their size;
d. discuss the way that developments in microscopy have
We will shortly examine the structure of a generic cell,
changed our view of cell structure; and
but the generalizations we draw should not blind you to
e. outline the major components of a cell.
the diversity of cellular form and function in humans.
There are about 200 kinds of cells in the human body,
with a variety of shapes, sizes, and functions.
Development of the Cell Theory
Cytology,1 the scientific study of cells, was born in
1663
when Robert Hooke observed the empty cell walls of cork
and coined the word cellulae ("little cells") to describe
Descriptions of organ and tissue structure often refer
to the shapes of cells by the following terms (fig.
•
3.1):
Squamous3 (SQUAY-mus)-a thin, flat, scaly shape,
them. Soon he studied thin slices of fresh wood and saw
often with a bulge where the nucleus is, much like
living cells "filled with juices"-a fluid later named pro­
the shape of a fried egg "sunny side up." Squamous
toplasm.2 Two centuries later, Theodor Schwann studied
cells line the esophagus and form the surface layer
(epidermis) of the skin.
a wide range of animal tissues and concluded that all
animals are made of cells.
Schwann and other biologists originally believed
that cells came from nonliving body fluid that somehow
1 cyto = cell; logy = study of
'proto = first; plasm = formed
•
Cuboidal4 (cue-BOY-dul)-squarish-looking in
frontal tissue sections and about equal in height
and width; liver cells are a good example.
3squam = scale; ous = characterized by
4cub = cube; oidal = like, resembling