American Journal of Clinical Nutrition, Vol 61, 915S-921S, Copyright © 1995 by The American Society for Clinical Nutrition, Inc
Sugars: their origin in photosynthesis and subsequent biological interconversions
DJ Nevins
Department of Vegetable Crops, University of California, Davis, 95616.
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.
