A level Biology exam revision resources written by A level Examiners


Disaccharides are composed of two monosaccharides joined by a glycosidic bond. They are:

  • soluble in water
  • sweet tasting
  • most are non-reducing sugars

Disaccharides are formed when two monosaccharides are joined together by a condensation reaction forming a glycosidic bond and a molecule of water (H2O). The formation of a disaccharide is shown in figure 5 below:

Figure 5

This shows two Biochemistry-glucose molecules joining together to form the disaccharide maltose. Because this bond is between the carbon number one, on the first glucose molecule, and carbon number four on the other molecule, the bond formed is referred to as a 1-4 glycosidic bond. Bonds between other carbon atoms are possible, leading to different shapes of molecule.

There are a number of disaccharides which are considered in A level biology. Maltose (which is shown in figure 5) is the disaccharide formed when starch is digested by the enzyme amylase. Sucrose is the main transport sugar in plants and lactose is found in the milk of mammals.

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