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In this chapter, the complexity and the unique nature
of plant carbohydrate metabolism have been presented.
The differences between this form of metabolism in plants
and that in other organisms stem from two factors.
First, unlike other organisms,
plants metabolize carbohydrates in two compartments,
the cytosol and plastids, with most of the biosynthetic
activity occurring in the latter compartment. All plant
tissues have plastids, but the primary metabolic activity
of these organelles varies extensively. The metabolic
activity of a tissue as a whole is determined to a large
extent by the enzyme complement of the plastids it contains.
This characteristic of plant cells has a major impact
on the regulation and characteristics of the carbohydrate
metabolism pathways that generate building blocks for
the biosynthetic activities localized in the plastids.
Carbohydrate metabolism
in the cytosol is connected with plastid metabolism
by a series of carriers in the plastid envelope. Each
plastid type has a different group of carriers, which
usually reflects the nature of the biosynthetic activity
within the organelle. Universally present, however,
is the TPT, which exchanges triose phosphate for inorganic
phosphate. The overall regulation of carbohydrate metabolism
is linked to the activity of this and the other transporters.
The TPT connects the cytosol with the plastid stroma
at about the midpoint of the glycolytic pathway. For
this reason, the regulation of glycolysis differs from
that in other organisms, in which metabolism is regulated
by the supply of nutrients.
Second, all cells in
a plant have to withstand changes in the external environment
without the homeostatic mechanisms that stabilize the
milieu surrounding animal cells. This has led to an
inherent flexibility in plant metabolism reminiscent
of that seen in some microorganisms. Often, several
enzymes can catalyze a single step in a pathway. Certain
of these enzymes appear to bypass principal reactions
involved in carbohydrate metabolism, although mechanisms
for the control of these bypasses have yet to be elucidated.
Some of the most fundamental
aspects of plant metabolism, such as the role of pyrophosphate
and the control of its concentration, are still not
understood. Considerable opportunities for future discovery
await those who read this chapter and wish to pursue
a better understanding of plant carbohydrate metabolism
and its regulation.
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