thumb|right|270px|Structure of the amylose molecule. thumb|right|270px|Structure of the amylopectin molecule.
Pure starch is a white, tasteless and odorless powder that is insoluble in cold water or alcohol. It consists of two types of molecules: the linear and helical amylose and the branched amylopectin. Depending on the plant, starch generally contains 20 to 25% amylose and 75 to 80% amylopectin. Glycogen, the glucose store of animals, is a more branched version of amylopectin.
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thumb|right|270px|Structure of the amylose molecule. thumb|right|270px|Structure of the amylopectin molecule.
Pure starch is a white, tasteless and odorless powder that is insoluble in cold water or alcohol. It consists of two types of molecules: the linear and helical amylose and the branched amylopectin. Depending on the plant, starch generally contains 20 to 25% amylose and 75 to 80% amylopectin. Glycogen, the glucose store of animals, is a more branched version of amylopectin.
Starch can be used as a thickening, stiffening or gluing agent when dissolved in warm water, giving wheatpaste.
Name
The word "starch" is derived from Middle English sterchen, meaning to stiffen. "Amylum" is Latin for starch, from the Greek "amulon" which means "not ground at a mill". The root amyl is used in biochemistry for several compounds related to starch.
History
Wheat starch paste was used by Egyptians to glue papyrus, to stiffen cloth and during weaving linen. Romans used it also in cosmetic creams, to powder the hair and to thicken sauces. Persians and Indians used it to make dishes similar to wheat halva. In China, with the invention of paper, rice starch was used as a surface treatment of the paper.
Energy store of plants
In photosynthesis, plants use light energy to produce glucose from carbon dioxide. The glucose is stored mainly in the form of starch granules, in plastids such as chloroplasts and especially amyloplasts. Toward the end of the growing season, starch accumulates in twigs of trees near the buds. Fruit, seeds, rhizomes, and tubers store starch to prepare for the next growing season.
Glucose is soluble in water, hydrophilic, binds much water and takes up much space; glucose in the form of starch, on the other hand, is not soluble and can be stored much more compactly.
Since starch is a reserve sugar for the plant, glucose molecules are bound in starch by the easily hydrolyzed alpha bonds. The same type of bond can also be seen in the animal reserve polysaccharide glycogen. This is in contrast to many structural polysaccharides such as chitin, cellulose and peptidoglycan, which are bound by beta-ties and are much more resistant to hydrolysis.
Biosynthesis
Plants produce starch by first converting glucose 1-phosphate to ADP-glucose using the enzyme glucose-1-phosphate adenylyltransferase. This step requires energy in the form of ATP. The enzyme starch synthase then adds the ADP-glucose via a 1,4-alpha glycosidic bond to a growing chain of glucose residues, liberating ADP and creating amylose. Starch branching enzyme introduces 1,6-alpha glycosidic bonds between these chains, creating the branched amylopectin. The starch debranching enzyme isoamylase removes some of these branches. Several isoforms of these enzymes exist, leading to a highly complex synthesis process.
