Plant Biochemistry by Caroline Bowsher;Martin Steer;Alyson Tobin;

Author:Caroline Bowsher;Martin Steer;Alyson Tobin;
Language: eng
Format: epub
Publisher: CRC Press LLC

AGPase

3%

Cytosolic marker enzymes

Alcohol dehydrogenase

1.4%

PFP

0.8%

Plastidial marker enzyme

Soluble starch synthase

47%

Alkaline pyrophosphatase

24.4%

Starch formation occurs in water-insoluble starch granules in the plastids

Starch granules are composed of a mixture of amylose and amylopectin (Figures 7.9 and 7.10). A starch granule generally contains 20–30% amylose, 70–80% amylopectin, and, in some cases, up to 20% of phytoglycogen. The granule also contains small amounts of lipid and phosphate. Amylopectin synthesis occurs primarily at the surface of the granule. The granules contain the enzymes for starch biosynthesis and degradation, with granule bound starch synthase the only enzyme exclusively bound to the granule. In many cases multiple isoforms are present that may either be soluble or bound to the granules. The branchpoints within amylopectin allow the short linear chains to pack together efficiently in layers as parallel left-handed double helices. It is this packing that makes the starch granule semicrystalline in structure. The glucose residue at the start of the molecule contains a reducing group and is directed towards the inside of the structure. The glucose residues at the ends of the branches are towards the outside. The packing of neighboring branches of amylopectin forming double helices leads to the crystalline array. In contrast, amylose is present in less crystalline amorphous regions, although its precise location is still unresolved.

The particular characteristics of a starch granule may vary in terms of size and shape with different plant species (Table 7.1). This is largely determined by the amylose content. For example, starch granules in wheat are essentially spherical due to a lower amylose and hence a greater amylopectin content. Starch granules range in size from 1 to 100 μm and may be present as a single or multiple class. For instance, wheat has two classes of granules, one less than 10 μm in diameter and the other 10–20 μm, while potato starch granules are between 5 and 100 μm.

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