Biomedical Engineering Reference
In-Depth Information
quantitative nutritional requirements of cells need to be determined to optimize growth and
product formation. Nutrients required by cells can be classified into two categories:
(1) Macronutrients are needed in concentrations larger than 10 4 mol/L. Carbon, nitrogen,
oxygen, hydrogen, sulfur, phosphorus, Mg 2 þ ,andK þ are nutrients.
(2) Micronutrients are needed in concentrations of less than 10 4 mol/L. Trace elements such
as Mo þ ,Zn 2 þ ,Cu 2 þ ,Mn 2 þ ,Ca 2 þ ,Na þ , vitamins, growth hormones, and metabolic
precursors are micronutrients.
2.4.1. Macronutrients
Carbon compounds are major sources of cellular carbon and energy. Microorganisms are
classified into two categories on the basis of their carbon source: 1) heterotrophs use organic
compounds such as carbohydrates, lipids, and hydrocarbons as a carbon and energy source;
2) autotrophs use carbon dioxide as a carbon source. Mixotrophs concomitantly grow under
both autotrophic and heterotrophic conditions; however, autotrophic growth is stimulated
by certain organic compounds. Facultative autotrophs normally grow under autotrophic
conditions; however, they can grow under heterotrophic conditions in the absence of CO 2
and inorganic energy sources. Chemoautotrophs utilize CO 2 as a carbon source and obtain
energy from the oxidation of inorganic compounds. Photoautotrophs use CO 2 as a carbon
source and utilize light as an energy source.
The most common carbon sources in industrial fermentations are molasses (sucrose),
starch (glucose, dextrin), corn syrup, and waste sulfite liquor (glucose). In laboratory fermen-
tations, glucose, sucrose, and fructose are the most common carbon sources. Methanol,
ethanol, and methane also constitute cheap carbon sources for some fermentations. In aerobic
fermentations, about 50% of substrate carbon is incorporated into cells and about 50% of it is
used as an energy source. In anaerobic fermentations, a large fraction of substrate carbon is
converted to products and a smaller fraction is converted to cell mass (less than 30%).
Nitrogen constitutes about 10 e 14% of cell dry weight. The most widely used nitrogen sour-
ces are ammonia or the ammonium salts [NH 4 Cl, (NH 4 ) 2 SO 4 ,NH 4 NO 3 ], proteins, peptides,
and amino acids. Nitrogen is incorporated into cell mass in the form of proteins and nucleic
acids. Some organisms such as Azotobacter spp. and the cyanobacteria fix nitrogen from the
atmosphere to form ammonium. Urea may also be used as a nitrogen source by some organ-
isms. Organic nitrogen sources such as yeast extract and peptone are expensive compared to
ammonium salts. Some carbon and nitrogen sources utilized by the fermentation industry
are summarized in Table 2.8 .
Oxygen is present in all organic cell components and cellular water and constitutes about
20% of the dry weight of cells. Molecular oxygen is required as a terminal electron acceptor in
the aerobic metabolism of carbon compounds. Gaseous oxygen is introduced into growth
media by sparging air or by surface aeration.
Hydrogen constitutes about 8% of cellular dry weight and is derived primarily from carbon
compounds, such as carbohydrates. Some bacteria such as methanogens can utilize hydrogen
as an energy source.
Phosphorus constitutes about 3% of cellular dry weight and is present in nucleic acids and
in the cell wall of some gram-positive bacteria such as teichoic acids. Inorganic phosphate
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