From Cellular Function to Industrial Products
11
the formation of several by-products, with carbon dioxide being a typical by-product formed,
especially in respiring cells. Here we will simply refer to by-products formed as a result of the
catabolism as
metabolic products.
They include many different types o f compounds, some of
significant industrial interest, e.g., ethanol or lactic acid. The cells may also secrete other
metabolic products that have other functions, e.g., acting as hormones or toxins for other cells
(antibiotics), and even though the biosynthesis of these may not be a direct result of cellular
growth
we will
also
refer to
them
as
metabolic
products.
Finally,
cells
may
secrete
macromolecules that have specialized functions, e.g. insulin by pancreatic cells or hydrolytic
enzymes that can degrade complex polymers like starch and xylanases (some of which represent
industrial products of significant value).
In order to ensure cell growth the necessary nutrients (or substrates) must be available. Nutrients
can roughly be divided into: 1) carbon source, 2) energy source, 3) nitrogen source, 4) minerals,
and 5) vitamins. The energy source ensures supply of the necessary Gibbs free energy for cell
growth, and often the carbon and energy source are identical. The most typical carbon and energy
source is glucose, but carbohydrates like maltose, sucrose, dextrins or starch are also frequently
used. Many cells may also use organic acids, e.g., acetic acid or lactic acid, polyols, e.g. glycerol,
or alcohols, e.g. ethanol, as carbon and energy source. A few microorganisms grow on
hydrocarbons and on C 0
2
(as do the plant cells), but using H
2
as energy source. In section 2.2.2
we discuss typical media used in industrial fermentation processes and will there return to the
cellular requirements for different nutrients.
Figure 2.1
Overview of reactions and processes involved in cellular growth and product formation.
Substrates are taken up by the cells by transport processes and converted into precursor metabolites via
fueling reactions. The precursor metabolites are converted to building blocks that are polymerized to
macromolecules. Finally macromolecules are assembled into cellular structures like membranes,
organelles etc. that make up the functioning cell. Precursor metabolites and building blocks may be
secreted to the extracellular medium as metabolites, or they may serve as precursors for metabolites that
are secreted. The cell may also secrete certain macromolecules - primarily proteins that can act as
hydrolytic enzymes, but some cells may also secrete polysaccharides.
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