Biomedical Engineering Reference
In-Depth Information
5S, 7S, 18S, and 28S. Ribosomal RNA accounts for a large fraction of total RNA. In E. coli,about
85% of all RNA is rRNA, while tRNA represents about 12% and mRNA represents 2
e
3%.
2.4. CELL FEED
A cell's chemical composition differs greatly from its environment. A cell must selectively
remove desirable compounds from its extracellular environment and retain other
compounds within itself. A semipermeable membrane is the key to this selectivity. Since
the cell differs so greatly in composition from its environment, it must expend energy to
maintain itself away from thermodynamic equilibrium. Thermodynamic equilibrium and
death are equivalent for a cell.
All organisms except viruses and prions contain large amounts of water (about 80%).
About 50% of the dry weight of cells is protein, largely enzymes (proteins that act as cata-
lysts). Nucleic acid content (which contains the genetic code and machinery to make
proteins) varies from 10% to 20% of dry weight. (However, viruses may contain nucleic
acids up to 50% of their dry weight.) Typically, the lipid content of most cells varies
between 5% and 15% of dry weight. However, some cells accumulate PHB up to 90%
of the total mass under certain culture conditions. In general, the intracellular composition
of cells varies depending on the type and age of the cells and the composition of the
nutrient media. Typical compositions for major groups of organisms are summarized in
Tabl e 2. 7
.
Most of the products formed by cells are produced as a result of their response to environ-
mental conditions, such as nutrients, growth hormones, and ions. The qualitative and
TABLE 2.7
Chemical Analyses, Dry Weights, and the Populations of Different Microorganisms
Obtained in Culture
Typical
population
in culture,
numbers/l
Composition, % dry weight
Typical dry
weight of
the culture, g/l Comments
Nucleic
acid
Organism
Protein
Lipid
10
11
e
10
12
50
e
90
50
e
50
<
Viruses
1
0.005
*
Viruses with a lipoprotein sheath
may contain 25% lipid.
10
11
Bacteria
40
e
70
13
e
34
10
e
15
2
0.2
e
29
PHB content may reach 90%.
10
12
2
Filamentous
fungi
10
e
25
1
e
3
2
e
7
30
e
50
Some Aspergillus and Penicillium
sp. contain 50% lipid.
10
11
Yeast
40
e
50
4
e
10
1
e
6
1
e
4
10
e
50
Some Rhodotorula and Candida sp.
contain 50% lipid.
10
10
Small
unicellular
algae
10
e
60
(50)
1
e
5 (3)
4
e
80
(10)
4
e
8
4
e
9
Figures in ( ) are commonly found
values but the composition varies
with the growth conditions.
* For a virus of 200 nm diameter.
Source: S. Aiba, A. E. Humphrey, and N. F. Millis, Biochemical Engineering, 2d ed., University of Tokyo Press, Tokyo, 1973.
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