Biology_Today_-_October_2016

CARBOHYDRATES

Carbohydrates are the most abundant organic molecules in nature. They are produced during photosynthesis of autotrophic plants.
They represent combinations of carbon, hydrogen and oxygen. Although carbohydrates can be generally represented by the empirical

formula Cn(H 2 O)n in which ‘n’ is an integer, some carbohydrates contain additional atoms of sulphur or nitrogen.

Carbohydrates can be aldoses or ketoses, depending upon whether they have an aldehyde or ketone group. In addition to
aldehydes and ketones, the polyhydric alcohols (sugar alcohols or polyols), in which the aldehyde or ketone group has been
reduced to an alcohol group, also occur naturally in foods.

In these molecules, the carbon atom forms chains or rings with two or more hydroxyl group and either an aldehyde or a ketone
group, giving them the technical designation of polyhydroxy aldehydes and ketones.

Classification of Carbohydrates

Carbohydrates are often referred to as saccharides. They are broadly classified into three major groups– monosaccharides,
oligosaccharides and polysaccharides. This categorisation is based on the number of sugar units.

Monosaccharides

Monosaccharides are those sugars that cannot be further hydrolysed into simpler carbohydrates. They may be classified as trioses,

tetroses, pentoses, hexoses or heptoses, depending upon the number of carbon atoms.

Table: Classification of important sugars

Aldoses Ketoses

Trioses (C 3 H 6 O 3 ) Glycerose

(glyceraldehyde)

Dihydroxy-

acetone

Tetroses (C 4 H 8 O 4 ) Erythrose Erythrulose

Pentoses (C 5 H 10 O 5 ) Ribose Ribulose

Hexoses (C 6 H 12 O 6 ) Glucose Fructose

Heptoses (C 7 H 14 O 7 ) Glucoheptose Sedoheptulose

Monosaccharides are linked by glycosidic bond (C-O-C) to form

oligosaccharide or polysaccharide. A molecule of water is produced
at each condensation, it is, therefore, a dehydration synthesis.

Fructose is the sweetest of all naturally occurring sugars. It is also called fruit sugar because of its common occurrence in fruits
(exception grape). Nectar and honey contain fructose. The same is called laevulose (= levulose) because of its laevorotatory nature.

Glucose is blood sugar and common respiratory substrate which is also called grape sugar, corn sugar. It is called dextrose being
dextrorotatory.

Ring Forms of Hexoses and Pentoses

Furanose (pentose)

Pentagon structure with four

carbons and one oxygen

Pyranose (hexose)

Hexagon structure with five

carbons and one oxygen