. La biochimie des plantes et des animaux, une introduction. La biochimie. 67 glucides l'amidon est la principale substance de réserve des plantes supérieures et est pro- duits au cours de la photosynthèse. Il est stocké dans la plante et parfois concentrées dans les racines, les tubercules et les graines. L'amidon se produit souvent dans des grains dont la taille et l'apparence sont caractéristiques de la source végétale. Ces grains peuvent éclater sur le chauffage pour donner une solution colloïdale ou en gel, en fonction de la concentration. L'amidon mole supé- eux-mêmes sont grandes et de deux types assez différents de sorte que l'amidon peut être fractionnée. Une fractio

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. Biochemistry of plants and animals, an introduction. Biochemistry. CARBOHYDRATES 67 Starch is the major reserve substance of higher plants and is pro- duced during photosynthesis. It is stored throughout the plant and occasionally concentrated in roots, tubers, and seeds. Starch often occurs in grains whose size and appearance are characteristic of the plant source. These grains may burst on heating to yield a colloidal solution or gel, depending on the concentration. The starch mole- cules themselves are large and of two types differing enough so that starch may be fractionated. One fraction called amylose is composed of several hundred a-u-glucose subiuiits connected at the one and four positions to form a long thread-like molecule without branches. Amy- lose is readily dispersed in water and gives a typical blue color with iodine but does not form the characteristic gel or starch paste. The other fraction of starch is amylopectin, which gives a purplish color with iodine and is difficult to disperse in water. However, when dispersed in sufficient concentrations, the amylopectin forms the typi- cal starch gel. As in amylose, the compound is made up only of glucose with most of the linkages connecting one and four positions. However, amylopectin has a branched structure with the branches at- tached through 1-6 links. The naturally occurring molecules seem to differ in size but range up to perhaps 50, 000 glucose subunits. Both amylose and amylopectin are hydrolyzed in acid solution to D-glucose. If hydrolysis is interrupted early, large fragments are formed called dextrins. These materials possess adhesive properties and are used as adhesives on paper products. Starch breaks down into dextrins on heating, especially in the presence of moisture. The re- sulting dextrins then confer stiffness to clothes that have been starched and ironed. Enzymes called amylases catalyze the hydrolysis of starch to high yields of maltose, revealing the structural similarity between

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