Organization of the Body
Next, let’s examine the three main parts of the cell in greater
Check Your Understanding
Summarize the four key points of the cell theory.
How would you explain the meaning of a “generalized cell”
to a classmate?
For answers, see Appendix H.
The Cellular Basis of Life
List the three major regions of a generalized cell and their
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
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
, 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-
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
. 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
A human cell has three main parts:
: the outer boundary of the cell.
to-plazm): the intracellular ﬂuid packed with
, small structures that perform speciﬁc cell functions.
kle-us): an organelle that controls cellular
activities. ±ypically the nucleus lies near the cell’s center.
(a) Cells that connect body parts, form linings, or transport
(c) Cell that stores nutrients
(b) Cells that move organs and body parts
(d) Cell that fights disease
(e) Cell that gathers information and controls body functions
(f) Cell of reproduction
(Note that cells are not drawn to the