Basic formula (CH2O)n
Glucose n = 6 C6 H12 O6
The most abundant monosaccharide is the six carbon glucose. Glucose
is the major fuel for most organisms and the basic building block of the
most abundant polysaccharides, such as starch and cellulose. In the biosphere,
there is probably more carbohydrate than all other organic matter combined,
largely due to the abundance in the plant world of two polymers of glucose:
starch and cellulose.
MONOSACCHARIDES
Individual sugar molecules are called monosaccarides and they come
in chains ranging from 3 to 7 carbons long. (Fig.
1)
Each sugar has one double bonded oxygen either on the terminal carbon
(aldehyde) or an interior carbon (ketone). (Fig.
1)
Molecules that have the same composition and basic structure but differ
in the orientation of groups about chiral centers are called isomers.
(Fig. 1)
Monosaccharides can exist as a linear form, or if more than 5 carbons
long it can form circular structures.(Fig.
2) (Fig. 3)
When circularicing, the oxygen from the aldehyde or ketone can assume
either an a or b
orientation. (Fig. 2) (Fig.
3)
Monosaccharides serve as the principle source of cellular energy
3D-interactive
monosaccharide models. (Note: to view interactively on your Web
browser, the free Chemscape
Chime plug-in viewer must be installed in your browser's plug-in folder.)
Oligosaccharide - from two to ten monosaccharide units joined
in glycosidic linkage.
Two monosaccharides can be joined by dehydration synthesis which
involves the removal of a molecule of water and the formation of a glycosidic
bond (Fig. 4) .
Polysaccharides - contain many monosaccharide units joined in
long linear or branched chains. Most polysaccharides contain recurring
monosaccharide units of only a single kind or two alternating kinds.
Glycogen and starch are composed entirely of glucose molecules in the
a configuration in which the principal linkage
is between the C1 of one unit and the C4
of the second unit (Fig. 4). In addition,
both glycogen and one form of starch (amylopectin) contain
occasional a (1-6) linkages, in which carbon
1 of one glucose is joined to carbon 6 of a second glucose. These linkages
lead to the formation of branches (Fig.
6) .
Cellulose in contrast has quite a distinct function as the principal structural component of the plant cell wall. Cellulose is also composed entirely of glucose molecules which are in the beta linkage and cellulose is unbranched (Fig. 5) . This b (1-4) linkage causes cellulose to form long extended chains that pack side by side to form fibers of great mechanical strength.
Starch is the chief form of fuel storage in most plants, whereas cellulose is the main extracellular structural component of the rigid cell walls and the fibrous and woody tissues of plants.
Glycogen, which resembles starch in structure, is the chief storage carbohydrate in animals.
In addition, some polysaccharides and shorter polymers of sugars act as markers for a variety of cell recognition processes, including the adhesion of cells to their neighboring cells and the transport of proteins to appropriate intracellular locations.