Microbes and Metabolism - Environmental Biotechnology: Theory and Application - page 43

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 by phosphoenolpyruvate 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 pyru-
vate 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 namely, phosphoenolpyruvate carboxylase has 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 photorespiration,
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 disadvantage 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 in either
role offer major advantages in solar efficiency, which, in temperate climes, could
provide significant environmental benefits.
Photorespiration
Returning to synthesis of carbohydrate by the Calvin Cycle, as mentioned above,
the first step is the carboxylation of the five carbon sugar, ribulose diphosphate