Starch is a polysaccharide made up of glucose molecules synthesized by plants. Two types of starch, amylose and amylopectin, are stored in granules inside cells. Starch biosynthesis occurs in amyloplasts and chloroplasts, and involves enzymes. The purpose of starch biosynthesis is to provide an energy reserve for plants. Research is underway to increase starch production in food crops.
Starch is a type of polymer known as a polysaccharide made up of chains of glucose molecules and is synthesized in relatively large quantities by plants. Glucose is a simple type of sugar, or saccharide, known as a monosaccharide. Polysaccharides are made up of many monosaccharide units, possibly thousands, joined together. The biosynthesis of starch in plants begins with the 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 numbering from 1,000 to 4,400. 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 slightly between different species. Plants store starch in the form of granules inside cells.
Starch biosynthesis occurs in amyloplasts and, to some extent, also 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 cells at an early stage in plant evolution. Inside these plastids, starch molecules are assembled from glucose building blocks. Glucose comes in the form of a glucose-phosphate compound which is an indirect product of photosynthesis.
Glucose molecules have hydroxyl groups (OH) bonded to carbon atoms. Glucose units bond together when a hydrogen atom is removed from a hydroxyl group on one glucose molecule and an entire hydroxyl group is removed from another, effectively removing water (H2O). The remaining oxygen atom from one molecule then bonds to the carbon atom from which the hydroxyl group has been removed from the other – the reaction can be represented as: R-OH + HO-R → ROR + H2O, where R stands for the remainder of the glucose molecule. In this way, long chains of glucose molecules are formed. This type of bond 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 begins with the combination of glucose-1-phosphate with adenosine triphosphate (ATP) to form adenosine diphosphate glucose (ADP-glucose), catalyzed by the enzyme AGPase. ADP-glucose can then add its glucose molecule to an existing glucose molecule, forming a glycosidic bond and then, through many repetitions of this process, building an amylose molecule. This reaction is catalyzed by starch synthase enzymes. Amylopectin is formed by the action of branched starch enzymes (SBEs) which create bonds between existing chains of glucose molecules to create a branched polymer.
The purpose of starch biosynthesis in plants is to provide an energy reserve. Glucose, produced by photosynthesis, supplies 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 amounts of starch in tubers; in potatoes, for example, 60-80% of the dry weight consists of starch. As of 2011, considerable research is underway on the biosynthesis of starch in plants, with a view to increasing the starch production of some food crops.
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