title: Sugars: their origin in photosynthesis and subsequent biological interconversions.
abstract: Sugar has been valued as a commodity for thousands of years. Despite its long history in commerce, the biological mechanisms accounting for the production of sugar are rather recent discoveries. The reactions are remarkable. Sugar is produced by all green plants and photosynthetic bacteria in a reaction sequence capable of forming carbon-carbon bonds. The very first steps occur independently of solar energy input, but to sustain the reaction, the products of initial fixation are phosphorylated and undergo a reduction in oxidation state. These steps responsible for phosphorylation and reduction are driven by products generated in the chloroplast upon the absorption of light. At this point, after just a few reactions, the products of photosynthesis have already acquired the attributes characteristic of sugars. Once carbon is stabilized as simple sugars in the chloroplast, the products undergo a sequence of rearrangements to sustain a cycle leading to new carbon dioxide acceptor molecules, and with each turn of the cycle a new carbon atom is introduced into the pool. As the process continues some of the carbon is diverted to synthesize starch within the chloroplast. Sucrose is synthesized in the cytoplasm adjacent to the chloroplast from exported carbohydrate as a diversion from the formation of starch. Sucrose represents the principal transport substance in most plants. Storage starch, cellulose, and other complex cell wall polysaccharides are typically derived from the sugar monomers found in sucrose. Sugars supply all the fixed carbon for synthesis of biological compounds and are fundamental for sustaining the energy flow to all food systems.
authors.affiliation: Department of Vegetable Crops, University of California, Davis, 95616.
authors.forename: D J
authors.initials: DJ
authors.lastname: Nevins
mesh_terms: D000818:Animals; D050260:Carbohydrate Metabolism; D002244:Carbon / Q000378:metabolism; D006801:Humans; D009747:Nutritional Physiological Phenomena; D010084:Oxidation-Reduction; D010766:Phosphorylation; D010788:Photosynthesis; D010944:Plants / Q000378:metabolism
chemical_list: D002244:Carbon