AQA A Level Biology

Revision Notes

1.1.7 Disaccharides

Disaccharides: Common Examples

  • Monosaccharides can join together via condensation reactions to form disaccharides
    • A condensation reaction is one in which two molecules join together via the formation of a new chemical bond, with a molecule of water being released in the process
    • The new chemical bond that forms between two monosaccharides is known as a glycosidic bond
    • To calculate the chemical formula of a disaccharide, you add all the carbons, hydrogens and oxygens in both monomers then subtract 2x H and 1x O (for the water molecule lost)
  • Common examples of disaccharides include:
    • Maltose (the sugar formed in the production and breakdown of starch)
    • Sucrose (the main sugar produced in plants)
    • Lactose (a sugar found only in milk)
  • All three of the common examples above have the formula C12H22O11

Common Disaccharides and their Monosaccharide Monomers Table

Monosaccharides and disaccharides table, downloadable AS & A Level Biology revision notes

Sucrose formation, downloadable IGCSE & GCSE Biology revision notes, downloadable AS & A Level Biology revision notes

The disaccharide maltose is formed from two α-glucose monomers (sub-units)

Sucrose formation, downloadable AS & A Level Biology revision notes

The disaccharide sucrose is formed from α-glucose and fructose monomers (sub-units)

Exam Tip

Like glucose, galactose and fructose are monosaccharides and actually have the same molecular formula as glucose. However, the atoms that make up these three monosaccharides are arranged in different ways, meaning they each have slightly different molecular structures, giving them slightly different properties.

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