Environmental Engineering Reference
In-Depth Information
4
H ETEROTROPHIC C ONVERSIONS
Consumer Bioenergetics
For the main factor in the nature of an animal is much
more the final cause than the necessary material. ...If any
person thinks the examination ...of the animal kingdom
an unworthy task, he must hold in like scorn the study of
man.
Aristotle (384-322 B . C . E .), Parts of Animals
this means that enzymes must first destroy the structure
of biopolymers.
This is done by severing glycoside bonds of complex
sugars (to produce the constituent monosaccharides,
with glucose dominant), and amide bonds of proteins
(amino acids are then used for protein synthesis), and by
hydrolyzing triglyceride bonds in lipids (into glycerol and
fatty acids). There are three basic metabolic strategies:
aerobic glycolysis, anaerobic fermentation, and dissimila-
tory anaerobic oxidation. Oxygen is the most common
electron acceptor. Most heterotrophs use the highly
exergonic ( 2870 kJ/mol) oxidation of biomass, which
produces water and CO 2 . Some anaerobic organisms
convert carbon compounds to lactate or ethanol.
These fermenters include both bacteria (e.g., yogurt-
making Lactobacillus) and fungi (e.g., Saccharomyces,
the yeast responsible for alcoholic fermentation and leav-
ened bread), and they gain just 197 kJ/mol by those
oxidations.
Heterotrophic life, precisely because of its total depen-
dence on photosynthesis, has evolved countless adapta-
tions to cope with environmental challenges, diffused
into nearly every conceivable niche, and eventually
resulted in the emergence of global civilization. Auto-
trophs take care of themselves by reducing atmospheric
CO 2 and producing a vast variety of complex organic
compounds. Heterotrophs, incapable of ab initio synthe-
sis of complex molecules, use those compounds (directly
as herbivores and detritivores, indirectly as carnivores) to
energize their growth and activity. Heterotrophic cells
usually absorb carbon in relatively simple molecules, and
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