AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
Dipeptides are formed by the condensation of two amino acids. Polypeptides…
Full specification wording
General properties of proteins: Dipeptides are formed by the condensation of two amino acids. Polypeptides are formed by the condensation of many amino acids.
Two amino acids joined by one condensation reaction form a dipeptide, with one peptide bond and one molecule of water released.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 3 of 9
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Two amino acids joined by one condensation reaction form a dipeptide, with one peptide bond and one molecule of water released. Many amino acids joined the same way form a polypeptide. The arithmetic is worth learning: n amino acids give n-1 peptide bonds and n-1 water molecules, so a polypeptide of 100 residues required 99 condensation reactions. Hydrolysis reverses this exactly, breaking the peptide bond and using a molecule of water. A dipeptide therefore has one peptide bond; a tripeptide has two; a chain of n residues has n-1. Note that a polypeptide is not the same as a protein: a protein is a folded, functional molecule, which may consist of one or more polypeptide chains.
What earns the marks
- A dipeptide being two amino acids joined by condensation, forming one peptide bond.
- A polypeptide being many amino acids joined by condensation.
- Calculating that n amino acids form n-1 peptide bonds and release n-1 water molecules.
- Stating that hydrolysis breaks a peptide bond and uses a molecule of water.
- Distinguishing a polypeptide (a chain of amino acids) from a protein (a folded, functional molecule).
Common mistakes and how to put them right
Saying a dipeptide contains two peptide bonds - a dipeptide of two amino acids has one peptide bond.
Stating that ten amino acids release ten molecules of water rather than nine - the number of water molecules equals the number of peptide bonds, which is n-1.
Using polypeptide and protein as if they always mean the same thing - a protein is a folded, functional molecule and may contain more than one polypeptide chain.
Forgetting that hydrolysis is the reverse of condensation and requires a molecule of water to break the peptide bond.
Miscounting peptide bonds in a tripeptide - a tripeptide has two peptide bonds, not three.
Exam tips
- Do the n-1 calculation on paper rather than in your head; the off-by-one error is the single most common slip in this topic.
- Reserve the word protein for a folded, functional molecule and use polypeptide for the chain itself.
- When asked about hydrolysis, state that it is the reverse of condensation and that water is used to break the peptide bond.
Try the lesson
The start of the General properties of proteins: Dipeptides are formed by the condensation of two amino acids. Polypeptides are formed by the condensation of many amino acids. lesson, made from checked teaching notes for this specification point.
Have a go first
Which type of reaction joins two amino acids together to form a dipeptide?
Listen
Two amino acids joined by one condensation reaction form a dipeptide, with one peptide bond and one molecule of water released. Many amino acids joined the same way form a polypeptide. The arithmetic is worth learning: n amino acids give n-1 peptide bonds and n-1 water molecules, so a polypeptide of 100 residues required 99 condensation reactions. Hydrolysis reverses this exactly, breaking the peptide bond and using a molecule of water. A dipeptide therefore has one peptide bond; a tripeptide has two; a chain of n residues has n-1. Note that a polypeptide is not the same as a protein: a protein is a folded, functional molecule, which may consist of one or more polypeptide chains. A strong answer earns marks for these points. a dipeptide being two amino acids joined by condensation, forming one peptide bond. a polypeptide being many amino acids joined by condensation. calculating that n amino acids form n-1 peptide bonds and release n-1 water molecules. stating that hydrolysis breaks a peptide bond and uses a molecule of water. Exam tip: Do the n-1 calculation on paper rather than in your head; the off-by-one error is the single most common slip in this topic.
Key words
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dipeptides
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polypeptides
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condensation arithmetic
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hydrolysis
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See every definition
- dipeptides
- Formed by the condensation of two amino acids, with one peptide bond and one molecule of water released.
- polypeptides
- Formed by the condensation of many amino acids; a chain of amino acids, not the same as a folded, functional protein.
- condensation arithmetic
- n amino acids give n-1 peptide bonds and n-1 water molecules, so a polypeptide of 100 residues required 99 condensation reactions.
- hydrolysis
- Reverses condensation exactly, breaking the peptide bond and using a molecule of water.
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