Cellular Structure, Proteins, and Metabolism
ells are the structural and functional units of all living organisms. The word
means “a small
chamber.” The human body is composed of trillions of cells, each a microscopic compartment
Figure 3–1
). In this chapter, we briefl y describe the structures found in most of the body’s cells
and list their functions.
Having learned the basic structures that comprise cells, we next turn our attention to how cellular
proteins are produced, secreted, and degraded, and how proteins participate in the chemical reactions
needed for all cells to survive.
Proteins are associated with practically every function living cells perform. One fact is crucial for an
understanding of protein function, and thus the functioning of a living organism: Proteins have a
unique shape or conformation that enables them to bind specifi c molecules on portions of their surfaces
known as binding sites. Thus, this chapter includes a discussion of the properties of protein binding
sites that apply to all proteins, as well as a description of how these properties are involved in one class
of protein functions—the ability of enzymes to accelerate specifi c chemical reactions. We then apply this
information to a description of the multitude of chemical reactions involved in metabolism.
Cell Structure
Microscopic Observations of Cells
The smallest object that can be resolved with a microscope
depends upon the wavelength of the radiation used to illumi-
nate the specimen—the shorter the wavelength, the smaller
the object that can be seen. While a light microscope can
resolve objects as small as 0.2 μm in diameter, an electron
microscope, which uses electron beams instead of light rays,
can resolve structures as small as 0.002 μm. Typical sizes of
cells and cellular components are illustrated in
Figure 3–2
Although living cells can be observed with a light micro-
scope, this is not possible with an electron microscope. To
form an image with an electron beam, most of the electrons
must pass through the specimen, just as light passes through
a specimen in a light microscope. However, electrons can pen-
etrate only a short distance through matter; therefore, the
observed specimen must be very thin. Cells to be observed
with an electron microscope must be cut into sections on the
order of 0.1 μm thick, which is about one-hundredth of the
thickness of a typical cell.
Because electron micrographs, such as the one in
Figure 3–3
, are images of very thin sections of a cell, they
can sometimes be misleading. Structures that appear as separate
objects in the electron micrograph may actually be continuous
Figure 3–1
Cellular organization of tissues, as illustrated by a portion of spleen. Oval, clear spaces in the electron micrograph are blood vessels. Note the
changes in overall appearance of this complex organ, as you move from left (closely compacted cells) to right (loosely arranged cells).
From Johannes A. G. Rhodin,
Histology, A Text & Atlas,
Oxford University Press, New York, 1974.
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