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Biological Molecules: Reactions (AQA AS Biology): Revision Note

Exam code: 7401

Lára Marie McIvor

Written by: Lára Marie McIvor

Reviewed by: Cara Head

Updated on

Biological molecules: reactions

  • Many biological reactions involve the formation of covalent bonds, which add strength and structure to a molecule

  • Two important reactions occur to form covalent bonds in molecules:

    • Condensation reactions

    • Hydrolysis reactions

Condensation reactions

  • A condensation reaction (also known as dehydration synthesis) occurs when monomers combine by covalent bonds to form polymers (during polymerisation) or macromolecules, and water is removed

Diagram showing polymerisation; monomers become a polymer, removing H2O. Top: written form, bottom: symbolic form with triangle symbols.
Written and symbolic illustrations of the removal of water to form a covalent bond between two or more monomers during a condensation reaction
Table showing organic molecules: carbohydrates, proteins, lipids, nucleic acids with covalent bonds: glycosidic, peptide, ester, phosphodiester, and bond diagrams.
Covalent bonds in organic molecules formed during condensation reactions

Hydrolysis reactions

  • Hydrolysis means ‘lyse’ (to break) and ‘hydro’ (with water)

  • In the hydrolysis of polymers, covalent bonds are broken when water is added

    Diagram showing polymer hydrolysis. Written: Polymer plus H2O forms monomers. Symbolic: Triangles with OH and H groups, showing breakdown into smaller units.
    Written and symbolic illustrations of the addition of water to break down covalent bond/s during a hydrolysis reaction

Examiner Tips and Tricks

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

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    Author
    Reviewer
    Lára Marie McIvor

    Author: Lára Marie McIvor

    Expertise: Biology, Psychology & Sociology Subject Lead

    Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.

    Cara Head

    Reviewer: Cara Head

    Expertise: Biology & Psychology Content Creator

    Cara graduated from the University of Exeter in 2005 with a degree in Biological Sciences. She has fifteen years of experience teaching the Sciences at KS3 to KS5, and Psychology at A-Level. Cara has taught in a range of secondary schools across the South West of England before joining the team at SME. Cara is passionate about Biology and creating resources that bring the subject alive and deepen students' understanding