Question

What are carbohydrates? Define small carbohydrates and discuss its types.

Answer 1

Hint: Small molecules are those molecules which are having low molecular weight. Such molecules are usually soluble in water. Biomolecules are the polymeric giant molecule. Their monomeric form can be considered as small molecules.

Complete Answer:
(I) Carbohydrates: Carbohydrates are called hydrates of carbon as most of them possess a general formula Cn(H2O)n. As per modern definition Carbohydrates are the molecules which possess multiple hydroxyl groups and either of aldehyde or ketone groups. Thus they are defined as “Polyhydroxy aldehyde or polyhydroxy ketone”
Carbohydrates are classified on the basis of the number of monomeric subunits in one molecule. They are classified as monosaccharide, oligosaccharide and polysaccharide

(II) Small Carbohydrates: Small carbohydrates are those carbohydrates which possess low molecular weight. They are also called sugars as they are sweet in taste. They include monosaccharide, Derived monosaccharide and Disaccharide
Monosaccharide: They are the simplest monomeric form of the carbohydrates which can’t be further hydrolysed. One molecule of monosaccharide can have 3- 7 carbon atoms.

On the basis of number of carbon atoms they are classified into following types:
1. Trioses: They have 3 carbons. Example: Dihydroxyacetone and Glyceraldehyde.
2. Tetroses: They have 4 carbon atoms. Example: Erythrose and Erythrulose.
3. Pentoses: They have 5 carbon atoms. Example: Ribose, Ribulose
4. Hexose: They have 6 carbon atoms. Example: Glucose, Galactose , Fructose etc
5. Heptose: They have 7 carbon atoms. Example sedoheptulose
On the basis of presence of aldehyde and ketone group they are also classified as Aldoses (example Glucose) and Ketoses (Example Fructose)
Derived Monosaccharides: They are derived from the monosaccharide. From Glucose sugars N-acetyl glucosamine (NAG), Gluconic acid is derived. From Ribose deoxyribose is derived.

Disaccharide: They include two monosaccharide joined together by a Glycosidic bond. It include:
(i) Maltose- Disaccharide to two glucose
(ii) Lactose- Disaccharide of Glucose and Fructose
(iii) Sucrose-Disaccharide of Glucose and Fructose

Additional information: Small carbohydrates can also be classified as reducing or non-reducing sugars on the basis of whether they reduce copper in Benedict and Fehling solution. All Monosaccharides are Reducing sugars. All Disaccharides are also reducing sugar except Sucrose and Trehalose.

Note: Polysaccharides will not be considered as Small carbohydrates, they are polymers, giant molecules with very high molecular weight.