Biomedical Engineering Reference
In-Depth Information
classified as a living thing, and so is often called the building block of life. Some organisms,
such as most bacteria, are unicellular (consist of a single cell). Other organisms, such as
humans, are multicellular. (Humans have an estimated 100 trillion or 10
14
cells; a typical
cell size is 10
m
m; a typical cell mass is 1 ng.) The largest known cell is an unfertilized ostrich
egg cell. An average ostrich egg is an oval about 15 cm
13 cm and weighs 1.4 kg.
In 1835, before modern cell theory was developed, Jan Evangelista Purkyn
e observed small
“granules” while looking at plant tissue through a microscope. The cell theory, first developed
in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are
composed of one or more cells, that all cells come from preexisting cells, that vital functions
of an organismoccurwithin cells, and that all cells contain the hereditary information necessary
for regulating cell functions and for transmitting information to the next generation of cells.
Each cell is at least somewhat self-contained and self-maintaining: it can take in nutrients,
convert these nutrients into energy, carry out specialized functions, and reproduce as neces-
sary. Each cell stores its own set of instructions for carrying out each of these activities.
2.1.1. Microbial Diversity
Life is very tenacious and can exist in extreme environments. Living cells can be found
almost anywhere that water is in the liquid state. The right temperature, pH, and moisture
levels vary from one organism to another. Some cells can grow at
20
C (in a brine to
prevent freezing), while others can grow at 120
C (where water is under high enough pres-
sure to prevent boiling). Cells that grow best at low temperatures (below 20
C) are usually
called psychrophiles, while those with temperature optima in the range of 20
e
50
C are mes-
ophiles. Organisms that grow best at temperatures greater than 50
C are thermophiles.
Many organisms have pH optima far from neutrality; some prefer in an environment with
a pH value down to 1 or 2, while others may grow well at pH 9. Some organisms can grow at
both low pH values and high temperatures.
Although most organisms can grow only with the presence of liquid water, others can
grow on barely moist solid surfaces or in solutions with high-salt concentrations.
Some cells require oxygen for growth and metabolism. Such organisms can be termed
aerobic. Other organisms are inhibited by the presence of oxygen and grow only anaerobically.
Some organisms can switch metabolic pathways to allow them to grow under either circum-
stance. Such organisms are facultative.
Often, organisms can grow in environments with almost no obvious source of nutrients.
Some cyanobacteria (formerly called blue-green algae) can grow in an environment with
only a little moisture and a few dissolved minerals. These bacteria are photosynthetic and
can convert CO
2
from the atmosphere into the organic compounds necessary for life. They
can also convert N
2
into NH
3
for use in making the essential building blocks of life.
CO
2
þ
H
O
CH
O
þ
O
/
2
2
2
N
2
þ
3
H
O
NH
3
þ
3
=
2
O
/2
2
2
Cyanobacteria are important in colonizing nutrient-deficient environments. Organisms
from these extreme environments (extremophiles) often provide humanity with important
tools for processes to make useful chemicals and medicinals. They are also key to the
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