Microbes and Metabolism 43level, provides an important avenue for the control of pollution and the mitigation
of possible eutrophication of aquatic environments. The cycle itself, and some of
the implications arising, are discussed later in this chapter.
C 3 and C 4 plants
Plants for which the reaction catalysed by rubisco is the first point of entry of
atmospheric carbon dioxide into carbohydrate metabolism are termed C 3 plants
due to the product of rubisco being two molecules of 3-phosphoglycerate which
contains three carbons. This is the typical route for temperate organisms. An
alternative to direct carboxylation for introducing carbon dioxide into the Calvin
cycle used by some tropical plants, is the Hatch–Slack pathway illustrated in
Figure 2.10. In this case the first step of entry for atmospheric carbon dioxide is
by carboxylation of phosphoenolpyruvate byphosphoenolpyruvate carboxylase
to produce the four-carbon molecule, oxaloacetate. Hence, plants able to use
this pathway are termed C 4 plants. The oxaloacetate is part of a cycle which
carries the carbon dioxide into the bundle-sheath cells and so away from the
surface of the plant, to where the oxygen concentration is lower. Here the carbon
dioxide, now being carried as part of malate is transferred to rubisco thus releasing
pyruvate which returns to the mesophyll cells at the surface of the plant where it
is phosphorylated at the expense of ATP to phosphoenolpyruvate ready to receive
the next incoming carbon dioxide molecule from the atmosphere.
The overall effect is to fix atmospheric carbon dioxide, transfer it to a site of
lower oxygen concentration compared with the surface of the plant, concentrate
it in the form of malate and then transfer the same molecule to rubisco where
it enters the Calvin cycle. Although the Hatch–Slack pathway uses energy, and
therefore may seem wasteful, it is of great benefit to plants growing in the
warmer regions of the globe The reason for this is that the enzyme involved
in carbon dioxide fixation in C 4 plants namelyphosphoenolpyruvate carboxy-
lasehas a very high affinity for carbon dioxide and does not use oxygen as
a substrate, contrasting with rubisco. The result of this competition between
oxygen and carbon dioxide for binding to rubisco is the futile process of pho-
torespiration, described in the next section. The affinity of carbon dioxide for
rubisco falls off with increasing temperature and so in a tropical environment,
the efficiency of rubisco to fix carbon dioxide is low. In this situation, the dis-
advantage in using energy to operate the Hatch–Slack pathway is more than
compensated for by the advantage of being able to fix carbon dioxide efficiently
at elevated temperatures. So advantageous is this that much research is being
directed to transferring the capability to operate the Hatch–Slack pathway into
selected C 3 plants.
In the broadest sense of environmental biotechnology, the potential maximisa-
tion of solar energy usage, either as a means to the remediation of contamination
or to reduce potential pollution by, for example, excessive fertiliser demand,
could be of considerable advantage. Hence, appropriately engineered C 3 plants