Cell Metabolism - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 515

10.7. MAJOR METABOLIC PATHWAY
We have now learned the mechanisms of cellular information transmissions. We also
learned how cells uptake nutrients (substrates). The metabolism of substrates is the key in
realizing the passage of the cellular information.
A major challenge in bioprocess development is to select an organism that can efficiently
make a given product (from a particular substrate). Before about 1980 only naturally occur-
ring organisms were available. With the advent of genetic engineering, it is possible to
remove and add genes to an organism to alter its metabolic functions in a predetermined
manner (metabolic engineering). In any case, the bioprocess developer must understand the
metabolic capabilities of natural organisms either to use them directly or to know how to
metabolically engineer them to make a desired, perhaps novel, product, from a particular
substrate. Consequently, we turn our focus toward learning about some essential metabolic
pathways.
Differences in microbial metabolism can be attributed partly to genetic differences and/or
to differences in their responses to changes in their environment. Even the same species may
produce different products when grown under different nutritional and environmental
conditions. The control of metabolic pathways by nutritional and environmental regulation
has become an important consideration in bioprocess engineering. For example, Saccharo-
myces cerevisiae (baker's yeast) produces ethanol when grown under anaerobic conditions.
However, the major product is yeast cells (baker's yeast) when growth conditions are aerobic.
Moreover, even under aerobic conditions, at high glucose concentrations some ethanol
formation is observed, which indicates metabolic regulation not only by oxygen but also
by glucose. This effect is known as the Crabtree effect.
Ethanol formation during baker's yeast fermentation may be reduced or eliminated by
culture with intermittent addition of glucose or by using carbon sources other than sucrose
and glucose that support less rapid growth.
The major metabolic pathways and products of various microorganisms will be briefly
covered in this chapter. Metabolic pathways are subgrouped as aerobic and anaerobic metab-
olism. There are two key concepts in our discussion. Catabolism is the intracellular process of
degrading a compound into smaller and simpler products (e.g. glucose to CO 2 and H 2 O).
Catabolism produces energy for the cell. Anabolism is involved in the synthesis of more
complex compounds (e.g. glucose to glycogen) and requires energy.
10.7.1. Bioenergetics
Living cells require energy for biosynthesis, transport of nutrients, motility, and mainte-
nance. This energy is obtained from the catabolism of carbon compounds, mainly carbohy-
drates. Carbohydrates are synthesized from CO 2 and H 2 O in the presence of light by
photosynthesis. The sun is the ultimate energy source for the life processes on earth. The
only exception is near some thermal vents at the bottom of the ocean, where nonphotosyn-
thetic ecosystems exist independently of sunlight.
Metabolic reactions are fairly complicated and vary from one organism to another.
However, these reactions can be classified into three major categories. A schematic diagram