Carbohydrate metabolism has been studied extensively
in animals and microorganisms but has received
far less attention in plants. Researchers, as
well as the authors of most general biochemistry
texts, have assumed that metabolic data from any
model system could be directly applied to plants
because the individual reactions involved in carbohydrate
metabolism appear to be very similar in all organisms.
As our knowledge of metabolism has increased,
however, we have discovered many aspects of plant
carbohydrate metabolism that are unique.
attributes distinguish plants from other organisms
and help explain the singular nature of plant
carbohydrate metabolism. These factors result
in a unique carbohydrate metabolism that only
superficially resembles carbohydrate metabolism
in other organisms.
• Plants are autotrophic.
• Plants are photosynthetic.
• Plants are sessile.
• Plants contain plastids.
• Plants have cell walls.
Plants are autotrophic.
Almost all plants convert simple nutrients such
as carbon dioxide, water, and inorganic ions into
all the intermediates required for the biosynthesis
of nucleic acids, proteins, lipids, and polysaccharides
as well as coenzymes and numerous secondary metabolites.
Thus, in addition to fulfilling the energy requirements
of the cell, plant carbohydrate metabolism must
feed numerous anabolic pathways (Fig.
13.1). This biosynthetic investment affects
the regulation of carbohydrate metabolism.
role of carbohydrate metabolism in the supply
of carbon skeletons for biosynthetic reactions.
G6PDH, glucose-6-phosphate dehydrogenase; 6PGDH,
6-phosphogluconate dehydrogenase; PFK, ATP-dependent
phosphofructokinase; FBPase, fructose-1,6-bisphosphatase;
PFP, pyrophosphate- dependent phosphofructokinase.
Photosynthetic electron transfer
generates ATP and energy-rich reductants used
to assimilate carbon, nitrogen, and sulfur.
Plants are photosynthetic.
Apart from a very few parasitic species, plants
harness light and use its energy to fix and reduce
carbon dioxide (see Chapter 12). The resulting
triose phosphates produced can supply carbon to
leaf cells or be converted to sucrose for export
to other parts of the plant. Other activities
associated with the capture of light energy include
nitrite and sulfate reduction and ammonia assimilation
(see Fig. 13.2 and Chapters
8 and 16). In addition, photosynthesis supplies
many biosynthetic pathways with reducing equivalents
and ATP. Because the processes associated with
light absorption are diurnal, plants must cope
with major variations in the supply of nutrients
during the light and dark periods. This imposes
the need for a flexibility in metabolism that
is not seen in other organisms.
Plants are sessile.
Terrestrial plants are unable to relocate when
faced with physical or chemical stress and must
tolerate changes in temperature; fluctuations
in the availability of light, water, and nutrients;
and herbivory. To ensure survival in unforgiving
circumstances, plant metabolism is highly flexible
(see Box 13.4 for discussion). In addition, plants
expend considerable resources in the production
of defense compounds (see Chapter 24).
plastids. These organ-elles, found only in
plants, perform most of the biosynthetic reactions
that occur in plant cells. Duplication of some
pathways in both cytosol and plastid, and the
requirement for coordination of metabolism in
all parts of the cell, have resulted in the evolution
of several plastid envelope transporters (Fig.
13.3). The properties and functions of these
transporters differ among various tissues and
plastid types. Besides linking the cytosol and
plastid compartments, these transporters also
play important roles in metabolic regulation.
The segregation of synthesis reactions into plastids
presents another problem for the plant cell. Syntheses
require an abundant supply of ATP and reducing
power in the form of NAD(P)H, both of which must
be generated within the plastid. Various mechanisms
have evolved to satisfy this need (Fig.
cell walls. Cell walls are composed of cellulose,
hemicellulose, and lignin (see Chapters 2 and
24). Synthesis of these wall components represents
a major drain on carbohydrates, because the pathways
for the synthesis of wall components can account
for 30% or more of cellular carbohydrate metabolism.
The topic of carbohydrate
metabolism is typically discussed in terms of
discrete anabolic and catabolic pathways. However,
these pathways can be envisioned as a series of
“pools” of metabolic intermediates linked by reversible
enzyme reactions (Fig. 13.5).
Metabolites can be added to or withdrawn from
these pools to serve the needs of the various
Two pools feed
plant carbohydrate metabolism. One is composed
of hexose phosphates, the other of pentose phosphate
pathway intermediates and the triose phosphates
glyceraldehyde 3-phosphate and dihydroxyacetone
phosphate. The direction of flow through these
pools depends on the requirements of the cell
(Fig. 13.6). In photosynthetic
tissues, carbon flux through pathways also is
governed by whether cells are in light or darkness.
Metabolite pools in the cytosolic and plastid
compartments communicate by way of highly specific
carrier proteins (see Fig. 13.3
and Chapter 3).
To highlight the
function of metabolite pools, in this chapter
we will frame some pathways differently from most
biochemistry texts. For example, glycolysis in
animals is typically described as the catabolism
of either glucose or the storage polysaccharide
glycogen to pyruvate. Starch metabolism in plants,
however, is compartmentalized in plastids, and
the complements of glycolytic enzymes present
are different in the cytosol, chloroplasts, and
heterotrophic plastids. When viewed as only one
of many pathways linking the hexose phosphate
and triose phosphate/pentose phosphate pools,
glycolysis is revealed as a set of reactions contributing
to an integrated whole plant metabolism rather
than a discrete pathway to be memorized and quickly
in the inner envelope membrane of plastids exchange
inorganic phosphate (P i ) for carbon metabolites.
In the transport processes illustrated here, photosynthetic
chloroplasts export C 3 intermediates, whereas
the non-photosynthetic plastids import a more
diverse group of sugars and organic acids. All
the transport reactions are reversible, and the
direction of transport is dependent on the concentrations
of P i and metabolites on each side of the envelope.
3-PGA, 3-phosphoglycerate; 2-PGA, 2-phosphoglycerate.
supplying cofactors to plastids. Cofactors such
as NAD(P)H or ATP are required for biosynthetic
pathways. (A) Cofactors can be generated within
the plastid by catabolic pathways located in the
organelle, or by light energy
in the case of the chloroplast. (B) A simple shuttle
system for the transfer of reducing equivalents
into a plastid. Cofactors may be imported from
the cytosol either by direct transport (e.g.,
exchange of ADP for ATP) or via shuttles, as shown
A metabolic pool,
showing four metabolites at equilibrium. Flow
through this pool will be dictated by the addition
of specific metabolites (A), or withdrawal of
intermediates (B and C). In many cases, the metabolic
status of the cell will determine whether a particular
compound (D) enters or leaves the pool.
The use and synthesis
of cofactors (NADH, NADPH, and ATP) in photosynthetic
and nonphotosynthetic plastids. The contribution
of photosynthetic electron transfer and photophosphorylation
to the supply of cofactors in chloroplasts has
not been included. 1,3-BisPGA, 1,3-bisphosphoglycerate.