Carbohydrates are among the most abundant compounds on earth. They are normally broken down into five major classifications of carbohydrates:
Monosaccharides
Disaccharides
Oligosaccharides
Polysaccharides
Nucleotides
Monosaccharides
The word monosaccharide is derived from mono, meaning "one", and saccharide,
meaning "sugar". The common monosaccharides are glucose, fructose, and
galactose. Each simple sugar has a cyclic structure and is composed of
carbon, hydrogen and oxygen in ratios of 1:2:1 respectively. Although each
sugar mainly exists as a cyclic compound, it is important to note that they are
all in equilibrium to a small extent with their linear forms.
Figure %: Monosaccharides
While galactose and glucose are composed of six-membered rings, fructose has
only five carbon atoms bonded to each other in ring form.
Glucose
Glucose is the main sugar metabolized by the body for energy. The
D-isomer of glucose predominates in nature and it is for this reason that the
enzymes in our body have adapted to binding this form only. Since it is an
important energy source, the concentration of glucose in the bloodstream usually
falls within a narrow range of 70 to 115mg/100 ml of blood. Sources of glucose
include starch, the major storage form of carbohydrate in plants.
Galactose
Galactose is nearly identical to glucose in structure except for one hydroxyl
group on carbon atom number four of the six-sided sugar. Since it differs in
only one position about all six asymmetric centers in the linear form of the
sugar, galactose is known as an epimer of glucose. Galactose is not
normally found in nature in large quantities, however it combines with glucose
to form lactose in milk. After being absorbed by the body, galactose is
converted into glucose by the liver so that it can be used to provide energy for
the body. Both galactose and glucose are very stable in solution because they
are able to adopt chair and boat conformations.
Figure %: Chair and Boat Conformations
These conformations are most stable because their OH groups are pointed away
from the structure, preventing steric hindrance.
Fructose
Fructose is a structural isomer of glucose, meaning it has the same chemical
ormula but a completely different three-dimensional structure. The main
difference is that fructose is a ketone in its linear form while glucose is
an aldehyde. Through an intramolecular addition reaction with the C-5 OH
group, glucose forms a six-membered ring while fructose forms a five-membered
ring as seen in Figure 1. Upon consumption, fructose
is absorbed
and converted into glucose by the liver in the same manner as lactose. Sources
of fructose include fruit, honey and high-fructose corn syrup.
Disacharides
Disaccharides, meaning "two sugars", are commonly found in nature as
sucrose, lactose and maltose. They are formed by a condensation reaction where
one molecule of water condenses or is released during the joining of two
monosaccharides. The type of bond that is formed between the two sugars is
called a glycosidic bond.
