Starch is a type of polymer known as a polysaccharide that consists of chains of glucose molecules and is synthesized in relatively large amounts by plants. Glucose is a simple type of sugar — or saccharide — known as a monosaccharide. Polysaccharides are built from many monosaccharide units, possibly thousands, joined together. Starch biosynthesis in plants begins with sugars produced by photosynthesis and involves a number of enzymes, or organic catalysts.
Two types of starch are produced by plants. Amylose consists of mostly unbranched chains of glucose molecules, or glucans, typically 1,000 to 4,400 in number. In amylopectin, the chains are multi-branched and generally contain between 10,000 and 100,000 glucans. About 70% of the starch in most plants is in the form of amylopectin, but this can vary somewhat between different species. Plants store starch in the form of granules within cells.
Starch biosynthesis takes place in amyloplasts and also to some extent in chloroplasts. These are both types of plastids — bodies within the plant cell that perform specialized functions. They are thought to have originated as symbiotic blue-green algae that were incorporated into the cells at an early stage in the evolution of plants. Within these plastids, starch molecules are assembled from glucose building blocks. The glucose comes in the form of a glucose-phosphate compound that is an indirect product of photosynthesis.
Molecules of glucose have hydroxyl (OH) groups bonded to carbon atoms. Glucose units bind together when a hydrogen atom is removed from a hydroxyl group on one glucose molecule and a whole hydroxyl group is removed from another, in effect removing water (H2O). The remaining oxygen atom from one molecule then bonds to the carbon atom from which the hydroxyl group was removed on the other — the reaction can be represented as: R-OH + HO-R → R-O-R + H2O, where R stands for the rest of the glucose molecule. In this way, long chains of glucose molecules are built up. This type of bonding between saccharide molecules is known as a glycosidic bond.
The details of the process are, however, more complicated than this — involving a number of enzymes — but can be summarized as follows. The process starts with the combining of glucose-1-phosphate with adenosine triphosphate (ATP) to form adenosine diphosphate glucose (ADP-glucose), catalysed by the enzyme AGPase. ADP-glucose can then add its glucose molecule to an existing glucose molecule, forming a glycosidic bond and thus, through many repetitions of this process, building up an amylose molecule. This reaction is catalysed by starch synthase enzymes. Amylopectin is formed by the action of starch branching enzymes (SBEs) that forge links between existing chains of glucose molecules to create a branched polymer.
The purpose of starch biosynthesis in plants is to provide a store of energy. Glucose, produced by photosynthesis, provides for immediate energy needs, but a reserve of energy to be used when conditions prevent the synthesis of sufficient glucose has a clear survival value. Many plants have evolved to store large quantities of starch in tubers; in potatoes, for example, 60-80% of the dry weight consists of starch. As of 2011, there is a considerable amount of research underway into starch biosynthesis in plants, with a view to increasing the starch production of certain food crops.