Starch

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Structure

Starch is a polysaccharide with the basic monosaccharide being glucose. the glucose molecules are joined together by glycosidic bonds straight chains are made up of alpha 1,4 glycosidic bonds while branching chain are made up of 1-6 glycosidic bonds.The branching chains are made up of alpha1-6 glyosidic bonds. Within the glucose chain structure there are terminal reducing sugars and non-reducing sugars at the end of each branch. The terminal reducing sugar is where the c1 of that final sugar within the chain is unattached. This enables the sugar to form analdehyde. The reducing aldehyde end enables the sugar to reduce other molecules, while it itself is oxidised. The non-reducing end at each branch, c1 is bonded via a glyosidic bond this means it is unable to be form the open aldehyde as a result unable to be oxidised or therefore reduced other sugars. The overall structure being high branched means that starch has an open structure where there are many non-reducing terminals by which enzymes can hydrolyse the polysaccharide by acting on the non-reducing terminals and then successively releasing sugars from each end. Referring back to the main role of starch being a source of storage molecule of glucose, the branching enables rapid hydrolysis of the glyosidic bonds and therefore release of many glucose molecules as a result of the branching providing multiple site of attack for the enzyme glycogen phosphorolase. Glycogen phospholases phosphorylates the sugar to produce phosphorylated glucose. Starch is made up of two fractions: amylose and amylopectin. Natural starches are made up of about 20% amylose and 80% amylopectin. Amylose is soluble in hot water whereas amylopectin is insoluble. Amylose has no branching chains, but instead is made up of only 1-4 glycosidic bonds. This causes the amylose to form a helix shape. Amylopectin however contains both straight chains and branching chains.

Testing

Starch can be tested for using iodine solution. If starch is present then a dark blue-black colour will be seen. This dark blue-black colour is due to the iodine reacting with the amylose helix.


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