Agriculture Reference
In-Depth Information
Starch is formed in the leaves of plants by condensation polymerisation of glucose
with the aid of starch-synthesising enzymes. This process can be written as
follows (simplifi ed):
nC 6 H 12 O 6
enzymes
(C 6 H 10 O 5 )n
+
nH 2 O
[7.2]
glucose
starch
+
water
During active photosynthesis (during the day), the starch is accumulated in the
leaves in the form of tiny granules of about 1 μm in diameter (leaf or transitory
starch). During the night, this leaf starch is partly broken down by enzymes and
transported in the form of sugars (mainly sucrose) to other parts of the plant.
Some of these sugars are re-converted to starch in the seeds, tubers and roots of
various plants (storage starch). It is from these sources that commercial starch is
obtained.
Starch molecules are synthesised in plants from sugars but the true mechanism
for the biosynthesis of amylose and amylopectin is not entirely clear. The enzymes
phosphorylase (P-enzyme), starch synthase and a branching enzyme are, or may
be, involved in starch biosynthesis (Smith 2001). Nowadays most starch scientists
believe that starch synthase is the true chain-lengthening enzyme in normal starch
biosynthesis (Fujita 2006). The branching enzyme is responsible for the synthesis
of the branching points in the amylopectin molecules. The mechanism that
prevents amylose from branching in the obvious presence of the branching
enzyme is still unresolved.
The development of starch granules commences with the accumulation of
poorly organised material of unknown chemical composition. At a certain point
the deposition of a minute amount of insoluble polysaccharide takes place, which
acts as a nucleus for further starch deposition. This nucleus is the botanical centre
(hilum) of the granule, around which the granule is grown. Initial growth gives
nearly spherical granules and as the granules are enlarged they often become
elongated or fl attened. The starch molecular chains grow in an orientation
perpendicular to the growing surface of the starch granule. As the dissolved
glucose units are linked to the growing starch polymer they simultaneously
solidify. During the growing of the starch granule there is an increase in the
proportion of amylose and an increase in molecular size of both amylose and
amylopectin
Starch is a polymeric carbohydrate composed of anhydroglucose units and is
extracted in granular form from the organs of certain plants. Starch granules are
deposited in the seeds, tubers, roots and stem piths of plants, as a reserve food
supply during periods of dormancy, germination and growth. The microscope
reveals that starch is composed of tiny, white granules, ranging from about 1 to
100 μm in diameter. After cellulose, starch is the next most abundant compound
synthesised by plant cells. It is a renewable substance and a new supply of starch
is grown annually. The size and shape of the granules are peculiar to starch from
specifi c botanical sources. The structures of the most common starches are set out
in Fig. 7.1 .
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