24 Environmental Biotechnology
the energy requirements of the cell balanced against the need for certain metabolic
intermediates, pyruvate, or its derivatives may now be reduced by accepting the
hydrogen from NADH and so continue on a fermentation route or it may be
decarboxylated to an acetyl group and enter the TCA cycle. The overall energy
balance of glycolysis is discussed later when considering chemical cellular energy
production in more detail.
TCA cycle
Pyruvate decarboxylation produces the acetyl group bound to Coenzyme A, ready
to enter the TCA cycle otherwise named Kreb’s citric acid cycle in tribute to the
scientist who discovered it. Not only is this cycle a source of reduced cofactors
which ‘fuel’ electron transport and thus, the synthesis of ATP, but it is also a great
meeting point of metabolic pathways. Cycle intermediates are constantly being
removed or replenished. During anaerobic fermentation, many of the reactions
seen in the TCA cycle are in operation even though they are not linked to
electron transport.
Glyoxalate cycle
This is principally the TCA cycle, with two additional steps forming a ‘short
circuit’, involving the formation of glyoxalate from isocitrate. The second reaction
requires the addition of acetyl CoA to glyoxalate to produce malic acid and thus
rejoin the TCA cycle. The purpose of this shunt is to permit the organism to
use acetyl CoA, which is the major breakdown product of fatty acids, as its sole
carbon source.
Macromolecules – description and degradation
Lipids
This class of macromolecules (see Figure 2.2) includes the neutral lipids which
are triacylglycerols commonly referred to as fats and oils. Triacylglycerols are
found in reservoirs in micro-organisms as fat droplets, enclosed within a ‘bag’,
called a vesicle, while in higher animals, there is dedicated adipose tissue, com-
prising mainly cells full of fat. These various fat stores are plundered when energy
is required by the organism as the degradation of triacylglycerols is a highly exer-
gonic reaction and therefore a ready source of cellular energy. Gram for gram,
the catabolism of these fats releases much more energy than the catabolism of
sugar which explains in part why energy stores are fat rather than sugar. If this
were not the case the equivalent space taken up by a sugar to store the same
amount of energy would be much greater. In addition, sugar is osmotically active
which could present a problem for water relations within a cell, should sugar be
the major energy store.
Triacylglycerols comprise a glycerol backbone onto which fatty acids are ester-
ified to each of the three available positions. They are insoluble in an aqueous