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During active photosynthesis in bright light a plant leaf produces more
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Carbohydrates than it needs for generating energy or synthesising precursors
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The excess is converted to
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Sucrose and transported to other parts of the plant to be used as fuel or stored
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In most plants
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Starch is the main storage form
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But in few plants such as sugar beet and sugarcane
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Sucrose is the primary storage form
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The synthesis of sucrose and starch
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Occurs in different cellular compartments (cystoscope and plastids respectively)
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And these processes are coordinated by
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Variety of regulatory mechanisms
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That respond to changes in
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Light level and photosynthetic rate
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Starch like glycogen is a high molecular weight polymer of
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D-glucose in (alpha-1———-4) Linkage
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It is synthesized in choloroplast for
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Temporary storage as one of the stable end products of photosynthesis
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And for long term storage it is synthesized in the
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Amyloplasts of non photosynthetic parts of plants -seedsroots and
tubers |
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The mechanism of glucose activation in starch synthesis is similar to that in
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Glycogen synthesis
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An activated nucleotide sugar , in this case ADP- glucose is formed by
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Condensation of glucose 1-phosphate with ATP in a reaction made essentially irreversible by the presence in plastids of inorganic pyrophosphate
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Starch synthase then transfers glucose residue from
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ADP-glucose to preexisting starch molecules
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Although it has been generally assumed that
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Glucose is added to non-reducing end of starch as in glycogen synthesis
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Evidence now suggest that starch synthase has
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Two equivalent active site
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