Biomedical Engineering Reference
In-Depth Information
maintenance of natural cycles and can be used in the recovery of metals from low-grade ores
or in the desulfurization of coal or other fuels. The fact that organisms can develop a capacity
to exist and multiply in almost any environment on Earth is extremely useful.
Not only do organisms occupy a wide variety of habitats, but they also come in a wide
range of sizes and shapes. Spherical, cylindrical, ellipsoidal, spiral, and pleomorphic cells
exist. Special names are used to describe the shape of bacteria. A cell with a spherical or ellip-
tical shape is often called a coccus (plural, cocci); a cylindrical cell is a rod or bacillus (plural,
bacilli); a spiral-shaped cell is a spirillum (plural, spirilla). Some cells may change shape in
response to changes in their local environment. Pleomorphic cells take on at least two
different forms or shapes during their life cycle.
Thus, organisms can be found in the most extreme environments and have evolved
a wondrous array of shapes, sizes, and metabolic capabilities. This great diversity provides
the engineer with an immense variety of potential tools. We have barely begun to learn how
to manipulate these tools.
2.1.2. How Cells are Named
The naming of cells is complicated by the large variety of organisms. A systematic
approach to classifying these organisms is an essential aid to their intelligent use. Taxonomy
is the development of approaches to organize and summarize our knowledge about the
variety of organisms that exist. Although knowledge of taxonomy may seem remote from
the needs of an engineer, it is necessary for efficient communication among engineers and
scientists working with living cells. Taxonomy can also play a critical role in patent litigation
involving bioprocesses.
While taxonomy is concerned with approaches to classification, nomenclature refers to the
actual naming of organisms. For microorganisms, we use a dual name (binary nomencla-
ture). The names are given in Latin or are Latinized. A genus is a group of related species,
while a species includes organisms that are sufficiently alike to reproduce. A common well-
documented gut organism is Escherichia coli. Escherichia is the genus and coli the species.
When writing a report or paper, it is common practice to give the full name when the
organism is first mentioned, but to abbreviate the genus to the first letter in subsequent
discussion, e.g. E. coli. Although organisms that belong to the same species all share the
same major characteristics, there are subtle and often technologically important variations
within species. A strain of E. coli used in one laboratory may differ from that used in
another. Thus, various strains and substrains are designated by the addition of letters
and numbers. For example, E. coli fBr5 will differ in growth and physiological properties
from E. coli K01.
Now that we know how to name organisms, we could consider broader classification up to
the level of kingdoms. There is no universal agreement on how to classify microorganisms at
this level. Such classification is rather arbitrary and need not concern us. However, we must
be aware that there are two primary cell types: eukaryotic and prokaryotic. The primary differ-
ence between them is the presence or absence of a membrane around the cell's genetic
information.
Prokaryotes have a simple structure with a single chromosome (
Fig. 2.1
). Prokaryotic cells
have no nuclear membrane and no organelles (such as mitochondria and endoplasmic
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