OCR · A-Level · Biology
Course version: H420 · Version from Sept 2025
(l) the synthesis and breakdown of dipeptides and polypeptides, by the…
Full specification wording
Biological molecules: (l) the synthesis and breakdown of dipeptides and polypeptides, by the formation and breakage of peptide bonds
A dipeptide forms when the carboxyl group of one amino acid reacts with the amine group of another in a condensation reaction, releasing one molecule of water and creating a peptide bond between…
Checked against the OCR H420 specification (opens in a new tab) on .
- Point 12 of 20
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A dipeptide forms when the carboxyl group of one amino acid reacts with the amine group of another in a condensation reaction, releasing one molecule of water and creating a peptide bond between the two residues. Repeating this condensation links many amino acids into a polypeptide, with a peptide bond at each join. Breakdown is the reverse: hydrolysis adds water across each peptide bond, splitting the chain back into amino acids. Remember that the bond joins the carbon of one carboxyl group to the nitrogen of the next amine group, and that the number of water molecules released equals the number of peptide bonds formed.
What earns the marks
- States that condensation joins the carboxyl group of one amino acid to the amine group of another, releasing water.
- Identifies the peptide bond as the link formed between the two amino acid residues.
- Explains that a dipeptide contains two amino acids joined by one peptide bond, while a polypeptide contains many.
- States that hydrolysis adds water to break peptide bonds, reversing condensation.
- Links the number of water molecules released or used to the number of peptide bonds formed or broken.
Common mistakes and how to put them right
Saying condensation releases carbon dioxide.
Put it right by stating condensation releases water.
Thinking hydrolysis forms peptide bonds.
Put it right by stating hydrolysis breaks peptide bonds by adding water, while condensation forms them by removing water.
Believing the peptide bond forms between two R groups.
Put it right by stating it forms between the carboxyl carbon of one amino acid and the amine nitrogen of the next.
Exam tips
- Link the term to the water: condensation removes water, hydrolysis adds water.
- Count peptide bonds as one fewer than the number of amino acids in a single chain.
- Check which groups are joined when identifying a peptide bond in a diagram.
Try the lesson
The start of the Biological molecules: (l) the synthesis and breakdown of dipeptides and polypeptides, by the formation and breakage of peptide bonds lesson, made from checked teaching notes for this specification point.
Have a go first
Two amino acids are joined together to form a dipeptide. Which type of reaction forms the bond between them?
Listen
A dipeptide forms when the carboxyl group of one amino acid reacts with the amine group of another in a condensation reaction, releasing one molecule of water and creating a peptide bond between the two residues. Repeating this condensation links many amino acids into a polypeptide, with a peptide bond at each join. Breakdown is the reverse: hydrolysis adds water across each peptide bond, splitting the chain back into amino acids. Remember that the bond joins the carbon of one carboxyl group to the nitrogen of the next amine group, and that the number of water molecules released equals the number of peptide bonds formed. A strong answer earns marks for these points. States that condensation joins the carboxyl group of one amino acid to the amine group of another, releasing water. Identifies the peptide bond as the link formed between the two amino acid residues. Explains that a dipeptide contains two amino acids joined by one peptide bond, while a polypeptide contains many. States that hydrolysis adds water to break peptide bonds, reversing condensation. A common slip: Saying condensation releases carbon dioxide. Instead, stating condensation releases water. Exam tip: Link the term to the water: condensation removes water, hydrolysis adds water.
See it
Key words
Say what each word means before you turn the card.
Condensation
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Peptide bond
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Dipeptide
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Hydrolysis
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See every definition
- Condensation
- A reaction in which the carboxyl group of one amino acid reacts with the amine group of another, releasing one molecule of water.
- Peptide bond
- The link formed between two amino acid residues, joining the carbon of one carboxyl group to the nitrogen of the next amine group.
- Dipeptide
- A molecule containing two amino acids joined by one peptide bond.
- Hydrolysis
- The addition of water across each peptide bond, splitting the chain back into amino acids and reversing condensation.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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