Essential Cell Biology 5th edition

Model Organisms
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(C)
(D)
(A) (B) (E) (F) (G)
the other cell, inverting like a hollow ball to engulf its victim, which can
be almost as large as itself (Figure 1–30B).
Not all protozoans are predators. They can be photosynthetic or carnivorous,
motile or sedentary. Their anatomy is often elaborate and includes
such structures as sensory bristles, photoreceptors, beating cilia, stalklike
appendages, mouthparts, stinging darts, and musclelike contractile bundles.
Although they are single cells, protozoans can be as intricate and
versatile as many multicellular organisms (Figure 1–31). Much remains
to be learned about fundamental cell biology from studies of these fascinating
life-forms.
MODEL ORGANISMS
ECB5 e1.30/1.31
All cells are thought to be descended from a common ancestor, whose
fundamental properties have been conserved through evolution. Thus,
knowledge gained from the study of one organism contributes to our
understanding of others, including ourselves. But certain organisms are
easier than others to study in the laboratory. Some reproduce rapidly and
are convenient for genetic manipulations; others are multicellular but
transparent, so the development of all their internal tissues and organs
can be viewed directly in the live animal. For reasons such as these, biologists
have become dedicated to studying a few chosen species, pooling
their knowledge to gain a deeper understanding than could be achieved if
their efforts were spread over many different species. Although the roster
of these representative organisms is continually expanding, a few stand
out in terms of the breadth and depth of information that has been accumulated
about them over the years—knowledge that contributes to our
understanding of how all cells work. In this section, we examine some
of these model organisms and review the benefits that each offers to
the study of cell biology and, in many cases, to the promotion of human
health.
Figure 1–31 An assortment of protozoans
illustrates the enormous variety within
this class of single-celled eukaryotes.
These drawings are done to different scales,
but in each case the scale bar represents
10 μm. The organisms in (A), (C), and (G) are
ciliates; (B) is a heliozoan; (D) is an amoeba;
(E) is a dinoflagellate; and (F) is a euglenoid.
To see the latter in action, watch Movie 1.6.
Because these organisms can only be seen
with the aid of a microscope, they are also
referred to as microorganisms. (From M.A.
Sleigh, The Biology of Protozoa. London:
Edward Arnold, 1973. With permission from
Edward Arnold.)
Molecular Biologists Have Focused on E. coli
In molecular terms, we understand the workings of the bacterium
Escherichia coli—E. coli for short—more thoroughly than those of any
other living organism (see Figure 1–11). This small, rod-shaped cell normally
lives in the gut of humans and other vertebrates, but it also grows
happily and reproduces rapidly in a simple nutrient broth in a culture
bottle.