Biochemistry,  Biology,  Cell biology


Table of Contents

Carbohydrates are classified into two types on the basis of molecular weight.

Micromolecules – Monosaccharides and Oligosaccharides (Including Disaccharides)

Macromolecules – Polysaccharides

The micromolecules have the molecular weight of < 1000 Daltons whereas the
macromolecules have > 1000 Daltons as molecular weight.

  • Why do we need carbohydrates in our food? Carbohydrates provides about 50-70% of total energy. We need average carbohydrate requirement in an adult is ~ 400g per day.
  • Essential elements in the constitution of carbohydrate – C. H. O
  • General Formula – *Cx (H2O)y (Containing more than one-OH groups). The polyhydroxy aldehydes or ketones can also be called as saccharides
  • 80% of the dry weight of the plant is carbohydrate.

There are three types of carbohydrates:

  1. Monosaccharides
  2. Oligosaccharides
  3. Polysaccharides


They are simplest carbohydrates, with 3 to 7 carbon atoms. All are reducing sugars with a free aldehyde (– CHQ) or ketone (– CO) groups.

  1. 3-carbons – TRIOSES (C3H6O3) – e.g. Glyceraldehyde (aldose) and Dehydroxyacetone (ketose) (Acetic acid (CH3COOH/ C2H4O2)and Lactic acid are not considered as carbohydrates)
  2. 4-carbons – TETROSES (C4H8O4) – e.g. Erythrose – an aldose (forms raw-material forlignin)
  3. 5-carbons – PENTOSES (C5H10O5) – e.g. Ribose (present in RNA, ATP and vitamin B2), Xylose and Arabinose – all aldoses, Ribulose – a ketose.
  4. 6-carbons – HEXOSES (C6H12O6) – e.g. Glucose, Galactose, Mannose (All aldose sugars) and Fructose (Ketose-sugar). Alcohol of mannose, called Mannitol, is found in brown algae.
  5. 7-carbons – HEPTOSES (C7H14O7) – e.g. Glucoheptose (Both Pentoses and Hexoses may occur in Ring form and Open chain)

Glucose – It is called Blood sugar or Grape sugar and occurs in 2-forms, i.e. D-form (Dextro-) and L-form (Levo-). All naturally occurring sugars are in D-form. It is an aldose sugar.

Fructose – It is called Fruit sugar. It is the most common sugar in plants. It is sweetest amongst naturally occurring sugars. It is a ketose sugar


They contain 2 to 10 monosaccharide molecules.

A. Disaccharides – They contain 2-monosaccharides

(i) Maltose – It is malt sugar and is formed during germination of starchy seeds. It is a reducing sugar. It contains 2 α – glucose units with α – 1,4 linkage/glycosidic bond


α-1,4-Glycosidic bond

(ii) Lactose – It is milk sugar. It is also reducing sugar. It contains 1α-glucose and 1β-galactose (with β–1,4-linkage/glycosidic bond). Lactose is maximum in human milk. The galactose, produced from milk digestion, is also the constituent of AgarAgar.

(iii) Sucrose – It is the sugar of sugar cane and sugar beet. It is non-reducing sugar as it does not have free aldehyde or ketose groups. It contains 1 α-glucose and 1 β-fructose units. (α – 1- 2 linkage/glycosidic bond) The equimolecular mixture of glucose and fructose is called Invert Sugar which is sweeter than sucrose.

(iv) Trehalose – It is a disaccharide (α 1-1 linkage) present in micro-organisms. It is also a non-reducing sugar.

(v) Cellobiose – It contains 2 β-glucose units (β 1-4 linkage). It cannot be digested by mammalian enzymes.

B. Trisaccharides – They contain 3 monosaccharides, ex. Raffi nose – 1 glucose + 1
fructose + 1 galactose

C. Dextrin – It is also an oligosaccharide and is formed during starch-digestion.


  • They are formed by joining of Monosaccharides (Monomers) by Glycosidic bonds between 1-4 carbon atoms. (In a polysaccharide chain the right end is called reducing end and the left end is called non-reducing end.
  • They are non-reducing and mostly insoluble in water.

Two types – 1. Storage polysaccharides – e.g. Glycogen and Starch

2. Structural polysaccharides – e.g. Cellulose and Chitin

  1. Glycogen – It is present in animals (also called animal starch). It is a branched chain compound and has about 30 α-glucose units. It gives ‘red colour’ with an iodine solution.
  2. Starch – It is present in plants. The natural starch contains a mixture of amylose (10-20%) and amylopectin (80-90%), latter branched and insoluble in water. It also contains all α-glucose. (Amylose in starch is responsible for ‘deep blue colour’ with iodine)
  3. Cellulose
    • It contains all β-glucose.
    • It is the most abundant organic compound in the biosphere.
    • It is a fibrous polysaccharide and forms cell wall in plants.
    • It forms roughage in human food.
    • It is a straight chain (unbranched) compound.
    • It forms 25 to 50% of wood and about 90% of cotton.
    • Each molecule of cellulose contains about 6000 units of monosaccharides (glucose) and hence is Homoglycan hexosan.
    • Rayon, an artifi cial and regenerated fi bre, is produced from cellulose.

4. Chitin – It forms exoskeleton, mainly in arthropods. It is also present in the cell wall of fungi. Its unit is β-N- Acetylglucosamine. It is a homopolymer. The polysaccharide Agar has more than one type of Monosaccharide units (hence Heteroglycan). The polysaccharide Inulin (Dahlia starch) is a polymer of fructose (Homoglycan) with β-1,2 linkage. 

Conjugated or Complex Carbohydrates

They contain carbohydrate with non-carbohydrate units

  1. Glycoproteins – e.g., Blood antigens, Collagen, Lens protein, Blood protein, Hormones like FSH, LH, TSH, hCG, and in cell membranes.
  2. Glycolipids – e.g., Blood antigens, nerve fibres and cell membranes
  3. Mucopolysaccharides – e.g., Heparin. Hyaluronic acid, Synovial fluid, Vitrous humour, Cell wall in bacteria and Mucilages ( Galactose and mannose). They are also present in Bhindi (Lady’s finger) and Isabgol

Glucose Test – Benedict’s test, Fehling’s test.

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