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Medicine LibreTexts

1.07: Carbohydrates

The name “carbohydrate” tells you something about the composition of these “hydrated carbon” compounds. They contain carbon, hydrogen and oxygen and like water (H2O), there are always twice as many hydrogen atoms as oxygen atoms in each molecule. Carbohydrates are a large and diverse group that includes sugars, starches, glycogen and cellulose. Carbohydrates in the diet supply an animal with much of its energy and in the animal’s body, they transport and store energy.

Carbohydrates are divided into three major groups based on size: monosaccharides (single sugars), disaccharides (double sugars) and polysaccharides (multi sugars).

Monosaccharides are the smallest carbohydrate molecules. The most important monosaccharide is glucose which supplies much of the energy in the cell. It consists of a ring of 6 carbon atoms with oxygen and hydrogen atoms attached.

Disaccharides are formed when 2 monosaccharides join together. Sucrose (table sugar), maltose, and lactose (milk sugar), are three important disaccharides. They are broken down to monosaccharides by digestive enzymes in the gut.

Polysaccharides like starch, glycogen and cellulose are formed by tens or hundreds of monosaccharides linking together. Unlike mono- and di-saccharides, polysaccharides are not sweet to taste and most do not dissolve in water.




Disaccharide (Lactose)

  • Starch is the main molecule in which plants store the energy gained from the sun. It is found in grains like barley and roots like potatoes.
  • Glycogen, the polysaccharide used by animals to store energy, is found in the liver and the muscles that move the skeleton.
  • Cellulose forms the rigid cell walls of plants. Its structure is similar to glycogen, but it can’t be digested by mammals. Cows and horses can eat cellulose with the help of bacteria which live in specialised parts of their gut.

Anatomy and physiology of animals-Polysaccharides.jpg


  • Ruth Lawson (Otago Polytechnic; Dunedin, New Zealand)