Carbohydrates are organic compounds containing carbon, hydrogen and oxygen atoms. The ratio of hydrogen to oxygen is 2:1 in all carbohydrates. Starch, sugar, glycogen and cellulose are carbohydrates. They are classified into three major groups namely:-

  • Monosaccharides
  • Disaccharides and
  • Polysaccharides.


1) Monosaccharides (one molecule sugars)

These are simple sugars which consist of one molecule each. These sugars may contain six or less carbon atoms per molecule. Examples are glucose, fructose both with chemical formula (C6H12O6) and ribose (C5H10O5).


2) Disaccharides (Double molecule sugars)

When two molecules of simple sugar combine and one molecule of water is removed, a disaccharide is formed.

When one molecule of glucose combines with one molecule of fructose, one molecule of sucrose (cane sugar) is formed.

1 molecule of glucose +1 molecule of fructose = 1 molecule of sucrose.

Combination of two molecules of glucose will form one molecule of maltose or malt sugar.

1 molecule of glucose + 1 molecule of glucose = 1 molecule of maltose.

1 molecule of glucose combining with one molecule of galactose forms lactose (milk sugar).

Examples of disaccharides are sucrose (cane sugar), maltose (malt sugar) and lactose (milk sugar).


3) Polysaccharides

Polysaccharides are complex carbohydrates consisting of glucose molecules in chain with general formula of (C6H10O5)n, where letter n shows a large number. Starch, glycogen and cellulose are polysaccharides.

Sources of starch are potato, yam, tapioca, rice, millet, wheat, bread and cassava. These are plant starches. Glycogen is an animal starch produced and stored in the liver and muscles. It is converted to glucose when the need arises.

Cellulose is derived from vegetables and fruits. It forms the cell walls in most plant cells.


Importance of carbohydrates

  • When oxidized, they give energy and heat. The energy is used for work while the heat is used to maintain body temperature in homoiothermic animals.
  • Excess carbohydrates are converted into glycogen thus acting as food reserve only to be used when the need arises.
  • Carbohydrates provide starting materials for the synthesis of proteins, fats, oil and vitamins.