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Comprising 75% of the biological world and 80%
of all food intakes for human consumption, the most common known human
carbohydrate is starch. The simplest version of a
carbohydrate is a monosaccharide which
possesses the properties of carbon, hydrogen, and oxygen in a 1:2:1 ratio
under a general formula of CnH2nOn where n is
a minimum of 3. Glucose is an example of a monosaccharide
as is fructose. A chain of monosaccharide form to
make a polysaccharide. Such polysaccharides
include pectin, dextran,
and agar. Sugar content is commonly measured in degrees brix. Carbohydrates
or saccharides are the most abundant of the four major classes of biomolecules, which also include proteins, lipids and nucleic acids. They fill numerous roles in
living things, such as the storage and transport of energy
(starch, glycogen)
and structural components (cellulose in plants, chitin in animals). Additionally, carbohydrates and their
derivatives play major roles in the working process of the immune system, fertilization,
pathogenesis, blood clotting,
and development Classification
Monosaccharides are the simplest carbohydrates in that they cannot be hydrolyzed to smaller carbohydrates. The general chemical formula of an unmodified monosaccharide is (CH2O)n, where n is any number of three or greater. Classification of monosaccharides Monosaccharides are classified according to three different characteristics: the placement of its carbonyl group, the number of carbon atoms it contains, and its chiral handedness. If the carbonyl group is an aldehyde, the monosaccharide is an aldose; if the carbonyl group is a ketone, the monosaccharide is a ketose. Monosaccharides with three carbon atoms are called trioses, those with four are called tetroses, five are called pentoses and six are hexoses, and so on. These two systems of classification are often combined. For example, glucose is an aldohexose (a six-carbon aldehyde), ribose is an aldopentose (a five-carbon aldehyde), and fructose is a ketohexose (a six-carbon ketone). Monosaccharides are the major source of fuel for metabolism, being used both as an energy source (glucose being the most important in nature) and in biosynthesis. When monosaccharides are not needed by cells they are quickly converted into another form, such as polysaccharides.
Two joined monosaccharides are called disaccharides and represent the simplest polysaccharides. Examples include sucrose and lactose. They are composed of two monosaccharide units bound together by a covalent bond known as a glycosidic linkage formed via a dehydration reaction, resulting in the loss of a hydrogen atom from one monosaccharide and a hydroxyl group from the other. The formula of unmodified disaccharides is C12H22O11. Although there are numerous kinds of disaccharides, a handful of disaccharides are particularly notable. Sucrose, is the most abundant disaccharide and the main form in which carbohydrates are transported in plants. It is composed of one D-glucose molecule and one D-fructose molecule. The systematic name for sucrose, α-D-glucopyranosyl-(1→2)-D-fructofuranoside, indicates four things:
Lactose, a disaccharide composed of one D-galactose molecule and one D-glucose molecule, occurs naturally in milk. The systematic name for lactose is β-D-galactopyranosyl-(1→4)-D-glucopyranose. Other notable disaccharides include maltose (two D-glucoses linked α-1, 4) and cellulobiose (two D-glucoses linked β-1, 4).
Oligosaccharides and polysaccharides are composed of longer chains of monosaccharide units bound together by glycosidic bonds. The distinction between the two is based upon the number of monosaccharide units present in the chain. Oligosaccharides typically contain between two and nine monosaccharide units, and polysaccharides contain greater than ten monosaccharide units. Definitions of how large a carbohydrate must be to fall into each category vary according to personal opinion. Examples of oligosaccharides include the trisaccharide raffinose and the tetrasaccharide stachyose.Polysaccharides represent an important class of biological polymers. Their function in living organisms is usually either structure or storage related. Starch is used as a storage polysaccharide in plants, being found in the form of both amylose and the branched amylopectin. In animals, the structurally similar but more densely branched glycogen is used instead.
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