How does starch form

Unlike humans, plants are not able to eat food in order to meet their energy needs, instead they have to make their energy by photosynthesis. As every GCSE student can tell you, photosynthesis is the process through which light energy is converted into either chemical energy or sugar. When it is converted to sugar, that is in turn used by the plant for things like respiration, growth and reproduction. Some of the sugar is also stored for use later, by being converted into starch.

Plants make, and store temporary supplies of starch in their leaves, which they use during the night when there is no light available for photosynthesis. Many plants, including crop plants like wheat and potatoes, also make starch in their seeds and storage organs their grains and tubers , which is used for germination and sprouting. But what exactly is starch? Starch is a chain of glucose molecules which are bound together, to form a bigger molecule, which is called a polysaccharide.

There are two types of polysaccharide in starch:. When coiled in this fashion, amylose has just enough room in its core to accommodate an iodine molecule. The characteristic blue-violet color that appears when starch is treated with iodine is due to the formation of the amylose-iodine complex.

This color test is sensitive enough to detect even minute amounts of starch in solution. Figure 5. A molecule of amylopectin may contain many thousands of glucose units with branch points occurring about every 25—30 units Figure 5. The helical structure of amylopectin is disrupted by the branching of the chain, so instead of the deep blue-violet color amylose gives with iodine, amylopectin produces a less intense reddish brown. These branch points occur more often in glycogen. Dextrins are glucose polysaccharides of intermediate size.

The shine and stiffness imparted to clothing by starch are due to the presence of dextrins formed when clothing is ironed. Because of their characteristic stickiness with wetting, dextrins are used as adhesives on stamps, envelopes, and labels; as binders to hold pills and tablets together; and as pastes. Dextrins are more easily digested than starch and are therefore used extensively in the commercial preparation of infant foods.

The complete hydrolysis of starch yields, in successive stages, glucose:. In the human body, several enzymes known collectively as amylases degrade starch sequentially into usable glucose units. Glycogen is the energy reserve carbohydrate of animals. Like starch in plants, glycogen is found as granules in liver and muscle cells. This is a preview of subscription content, log in to check access. French D Organization of Starch Granules.

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