2.2 Growth Reaction
In the above example, we have assumed that all of the carbon found in substrate (glucose) is incorporated into cell mass. This does not happen as the cell needs to "oxidize" or respire some of the carbon to produce energy for biosynthesis and maintenance of cellular metabolic machinery. In addition cells may produce extracellular products that accumulate in the broth. Hence we can represent growth as:
The medium is the "food" for the cell. It serves as a source for all elements needed by the cell to grow (or biosynthesis) and for product formation. The compounds carbon dioxide and water on the product side of the reaction above result from oxidation of glucose in the medium.
Since the cellular material contains C, N, P, S, K, Na, Ca, etc, the medium must be formulated to supply these elements in the appropriate form. The above growth reaction can be re-stated as
If we neglect the "others" and assign stoichiometric coefficient for each of the species in the above equation on the basis of one mole of glucose (C-source) consumed, we re-write the above as
where ammonia represents the nitrogen source. We will refer to this reaction as growth reaction.
Note that whatever nitrogen that is supplied in the medium, it is expressed as equivalent nitrogen in the form of ammonia. Cells require nitrogen in both organic and inorganic form. It is common to supply the inorganic nitrogen as salts of ammonium ( e.g. ammonium phosphate ) while the organic nitrogen is usually supplied as amino acids or proteinous extracts which are rich in nitrogen. In most production processes using recombinant cells, glucose is used as the carbon source. However, in the production of low value products, less expensive carbon sources such as molasses ( $ 0.10 / lb) or corn meal ( about $ 0.12 / lb ) are used. Compare this against glucose at $ 1.00 /lb!
The growth reaction derived above is useful in interpreting laboratory data reported in the literature. Because the early work in cell growth were reported by microbiologists, it is necessary for us to learn the terms used by microbiologists to describe growth stoichiometry. We will then relate the above reaction equation to commonly reported cell properties.