Microbes and Metabolism - Environmental Biotechnology: Theory and Application - page 28

such as glucose, are joined together to form macromolecules, they are called
polysaccharides. Examples of these are glycogen in animals, and cellulose in
plants. In nature, the sugars usually occur as ring structures and many have the
general formula, C(H 2 O) n , where carbon and water are present in equal propor-
tion. Catabolism of glucose has been described earlier in this chapter. As stated
earlier, the resulting metabolite from a given carbon source, or the presence of
specific enzymes, can be diagnostic of an organism. Whether or not the enzymes
of a particular route are present can help to identify a microbe, and carbohydrate
metabolism is frequently the basis of micro-organism identification in a Public
Health laboratory. Glucose enters the glycolytic pathway to pyruvate, the remain-
der of which is determined in part by the energy requirements of the cell and in
part by the availability of oxygen. If the organism or cell normally exists in an
aerobic environment, there is oxygen available and the pyruvate is not required
as a starting point for the synthesis of another molecule, then it is likely to enter
the TCA cycle. If no oxygen is available, fermentation, defined later in this chap-
ter, is the likely route. The function of fermentation is to balance the chemical
reductions and oxidations performed in the initial stages of glycolysis.
Production of Cellular Energy
Cellular energy is present mainly in the form of ATP and to a lesser extent,
GTP (Figure 2.4) which are high energy molecules, so called because a large
amount of chemical energy is released on hydrolysis of the phosphate groups.
The energy to make these molecules is derived from the catabolism of a food,
or from photosynthesis. A food source is commonly carbohydrate, lipid or to a
lesser extent, protein but if a compound considered to be a contaminant can enter
a catabolic pathway, then it can become a 'food' for the organism. This is the
basis of bioremediation. The way in which energy is transferred from the 'food'
molecule to ATP may take two substantially different routes. One is cytoplasmic
synthesis of ATP which is the direct transfer of a phosphate group to ADP,
storing the energy of that reaction in chemical bonds. The other involves a fairly
complicated system involving transfer of electrons and protons, or hydrogen ions,
which originated from the oxidation of the 'food' at some stage during its passage
through the catabolic pathways. The final sink for the electrons and hydrogen
ions is oxygen, in the case of oxidative phosphorylation, to produce water. This
explains the need for good aeration in many of the processes of environmental
biotechnology, where organisms are using oxidative phosphorylation as their
main method for synthesising ATP. An example of this is the activated sludge
process in sewage treatment. However, many microbes are anaerobes, an example
being a class of archaea, the methanogens, which are obligate anaerobes in that
they will die if presented with an oxygenated atmosphere. This being the case,
they are unable to utilise the oxidative phosphorylation pathways and so instead,
operate an electron transport chain similar in principle, although not in detail.