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Starches.

Text from Keeling (©1998)

Edible starches are found in a very diverse array of plants, primarily in storage organs such as tubers and seeds. Starch is synthesised in specialised plastids known as amyloplasts and also in chloroplasts. Starch is a major component of the "average" dietary intake of man and animals. Calculated in calories, about four-fifths of the worlds food is provided by three grain crops (maize, wheat and rice) and three tuber crops (potato, yam and cassava). On a dry-weight basis starch is by far the major component of the edible portions of these crops, providing between 60% to 90% of the dry weight. As well as its uses in nutrition, starch is also an important component in manufacturing a wide range of industrial products such as paper, textiles and building materials. Furthermore, chemically modified starch and starch derivatives are used widely throughout industry. World-wide, maize represents the major commercial source of starch, whereas wheat starch is of only minor significance in the starch industry.

Starch plays a central role in metabolism in most higher plants since it serves as the plants major food reserve. It is deposited as water-insoluble granules as a mixture of amylose and amylopectin. In chloroplasts, starch acts as a temporary store during photosynthesis: during the dark the starch is remobilised. Longer-term storage takes place in the reserve organs during one phase of the plant's growth: the starch will be used at another time for germination. In plant storage tissues such as the developing cereal grain, the final stages of starch biosynthesis are confined to a separate metabolic compartment within the cytosol. This intracellular organelle is termed an amyloplast and consists of a starch granule surrounded by plastid stroma which is enclosed by a double membrane.