AQA AS Biology

Revision Notes

1.1.2 Biological Molecules: Reactions

Biological Molecules: Reactions

  • A covalent bond is the sharing of two or more electrons between two atoms
    • The electrons can be shared equally forming a nonpolar covalent bond or unequally (where an atom can be more electronegative δ) to form a polar covalent bond
  • Generally each atom will form a certain number of covalent bonds due to the number of free electrons in the outer orbital e.g. H = 1 bond, C = 4 bonds
  • Covalent bonds are very stable as high energies are required to break the bonds
  • Multiple pairs of electrons can be shared forming double bonds (e.g. unsaturated fats C=C) or triple bonds

Covalent bonds, downloadable IGCSE & GCSE Biology revision notes

Different types of covalent bonds

  • When two monomers are close enough that their outer orbitals overlap this results in their electrons being shared and a covalent bond forming. If more monomers are added then polymerisation occurs (and / or a macromolecule forms)


  • Also known as dehydration synthesis (‘to put together while losing water’)
  • A condensation reaction occurs when monomers combine together by covalent bonds to form polymers (polymerisation) or macromolecules (lipids) and water is removed

Condensation reaction, downloadable IGCSE & GCSE Biology revision notes

Written and symbolic illustrations of the removal of water to form a covalent bond between two or more monomers during a condensation reaction


  • Hydrolysis means ‘lyse’ (to break) and ‘hydro’ (with water)
  • In the hydrolysis of polymers, covalent bonds are broken when water is added

Hydrolysis reaction, downloadable IGCSE & GCSE Biology revision notes

Written and symbolic illustrations of the addition of water to break down covalent bond/s during a hydrolysis reaction

Covalent Bonds in Organic Molecules Table

Covalent bonds in organic molecules table, downloadable AS & A Level Biology revision notes

Exam Tip

You must be able to recognise and identify the location of the covalent bonds in the molecules – note that these molecules may be unfamiliar to you.

Author: Amelia

While studying Biochemistry at Oxford University, Amelia started her own tutoring service, helping to connect Science tutors with students in her local area. Amelia has experience teaching the sciences and Maths at all levels to UK and international students and, as well as being our Biology Lead, designs revision resources for Chemistry.

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