CIE A Level Biology (9700) 2019-2021

Revision Notes

2.3.1 Amino Acids & the Peptide Bond

Amino Acids & the Peptide Bond


  • Proteins are polymers (and macromolecules) made of monomers called amino acids
  • The sequence, type and number of the amino acids within a protein determines its shape and therefore its function
  • Proteins are extremely important in cells because they form all of the following:
    • Enzymes
    • Cell membrane proteins (eg. carrier)
    • Hormones
    • Immunoproteins (eg. immunoglobulins)
    • Transport proteins (eg. haemoglobin)
    • Structural proteins (eg. keratin, collagen)
    • Contractile proteins (eg. myosin)

Amino acid

  • Amino acids are the monomers of proteins
  • There are 20 amino acids found in proteins common to all living organisms
  • The general structure of all amino acids is a central carbon atom bonded to:
    • An amine group -NH2
    • A carboxylic acid group -COOH
    • A hydrogen atom
    • An R group (which is how each amino acid differs and why amino acid properties differ e.g. whether they are acidic or basic or whether they are polar or non-polar)

Amino acid structure, downloadable AS & A Level Biology revision notes
The generalised structure of an amino acid

Peptide bond

  • In order to form a peptide bond a hydroxyl (-OH) is lost from a carboxylic group of one amino acid and a hydrogen atom is lost from an amine group of another amino acid
  • The remaining carbon atom (with the double-bonded oxygen) from the first amino acid bonds to the nitrogen atom of the second amino acid
  • This is a condensation reaction so water is released. The resulting molecule is a dipeptide
  • When many amino acids are bonded together by peptide bonds the molecule formed is called a polypeptide. A protein may have only one polypeptide chain or it may have multiple chains interacting with each other
  • During hydrolysis reactions polypeptides are broken down to amino acids when the addition of water breaks the peptide bonds

Dipeptide formation, downloadable AS & A Level Biology revision notes
Amino acids are bonded together by covalent peptide bonds to form a dipeptide in a condensation reaction

Exam Tip

You will be expected to recognise whether an unfamiliar molecule is an amino acid or protein so look for the functional groups (amine and carboxyl). When asked to identify the location of the peptide bond, look for where nitrogen is bonded to a carbon which has a double bond with an oxygen atom, note the R group is not involved in the formation of a peptide bond.

Author: Catherine

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