62
UNIT 1
Organization of the Body
3
Next, let’s examine the three main parts of the cell in greater
detail.
Check Your Understanding
1.
Summarize the four key points of the cell theory.
2.
How would you explain the meaning of a “generalized cell”
to a classmate?
For answers, see Appendix H.
The Cellular Basis of Life
Define cell.
List the three major regions of a generalized cell and their
functions.
Te English scientist Robert Hooke first observed plant cells
with a crude microscope in the late 1600s. Ten, in the 1830s
two German scientists, Matthias Schleiden and Teodor
Schwann, proposed that all living things are composed of cells.
German pathologist Rudolf Virchow extended this idea by con-
tending that cells arise only from other cells.
Since the late 1800s, cell research has been exceptionally
fruitful and provided us with four concepts collectively known
as the
cell theory
:
A
cell
is the basic structural and functional unit of living
organisms. When you define cell properties, you define the
properties of life.
Te activity of an organism depends on both the individual
and the collective activities of its cells.
According to the
principle of complementarity of structure
and function
, the biochemical activities of cells are dictated
by their shapes or forms, and by the relative number of their
specific subcellular structures.
Continuity of life from one generation to another has a cel-
lular basis.
We will expand on all of these concepts as we progress. Let us
begin with the idea that the cell is the smallest living unit. What-
ever its form, however it behaves, the cell is the microscopic
package that contains all the parts necessary to survive in an
ever-changing world. It follows then that loss of cellular homeo-
stasis underlies virtually every disease.
Te trillions of cells in the human body include over 200
different cell types that vary greatly in shape, size, and function
(Figure 3.1)
. Te disc-shaped red blood cells, branching nerve
cells, and cubelike cells of kidney tubules are just a few examples
of the shapes cells take. Cells also vary in length—ranging from
2 micrometers (1/12,000 of an inch) in the smallest cells to over
a meter in the nerve cells that cause you to wiggle your toes. A
cell’s shape reflects its function. For example, the flat, tilelike
epithelial cells that line the inside of your cheek fit closely to-
gether, forming a living barrier that protects underlying tissues
from bacterial invasion.
Regardless of type, all cells are composed chiefly of carbon,
hydrogen, nitrogen, oxygen, and trace amounts of several other
elements. In addition, all cells have the same basic parts and
some common functions. For this reason, it is possible to speak
of a
generalized
, or
composite
,
cell
(Figure 3.2)
.
A human cell has three main parts:
Te
plasma membrane
: the outer boundary of the cell.
Te
cytoplasm
(si
9
to-plazm): the intracellular fluid packed with
organelles
, small structures that perform specific cell functions.
Te
nucleus
(nu
9
kle-us): an organelle that controls cellular
activities. ±ypically the nucleus lies near the cell’s center.
Fibroblasts
Erythrocytes
Epithelial cells
(a) Cells that connect body parts, form linings, or transport
gases
(c) Cell that stores nutrients
(b) Cells that move organs and body parts
(d) Cell that fights disease
Macrophage
Nerve cell
Fat cell
Sperm
(e) Cell that gathers information and controls body functions
(f) Cell of reproduction
Skeletal
muscle
cell
Smooth
muscle cells
Figure 3.1
Cell diversity.
(Note that cells are not drawn to the
same scale.)
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