AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
A condensation reaction between two amino acids forms a peptide bond.
Full specification wording
General properties of proteins: A condensation reaction between two amino acids forms a peptide bond.
A peptide bond forms when the carboxyl group of one amino acid reacts with the amine group of the next.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 2 of 9
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A peptide bond forms when the carboxyl group of one amino acid reacts with the amine group of the next. A hydroxyl group leaves the carboxyl group and a hydrogen atom leaves the amine group, so one molecule of water is eliminated and a carbon-nitrogen bond joins the two residues: a condensation reaction. In a cell this happens on a ribosome during translation, and it always joins the backbone groups, never the R groups. The resulting chain therefore has a repeating backbone with side chains projecting from it, and it has direction, with a free amine group at one end and a free carboxyl group at the other. Hydrolysis reverses this process, requiring water to break the bond.
What earns the marks
- State that amino acids are joined by peptide bonds.
- State that the bond is formed by a condensation reaction.
- Identify the reacting groups as the carboxyl group of one amino acid and the amine group of the next.
- State that one molecule of water is released per peptide bond formed.
- State that peptide bonds are hydrolysed, using water, when the polypeptide is broken down into amino acids.
Common mistakes and how to put them right
Saying the peptide bond forms between the R groups of two amino acids.
Put it right: State that the peptide bond forms between the carboxyl and amine groups of the backbone, not the R groups.
Writing that water is added when the peptide bond forms, rather than eliminated.
Put it right: Specify that water is eliminated (or released) during a condensation reaction, whereas it is added during hydrolysis.
Naming the bond as a glycosidic or ester bond in a protein question.
Put it right: Ensure you use 'peptide bond' for proteins, reserving glycosidic for carbohydrates and ester for lipids.
Using 'broken down' about a polypeptide without naming the peptide bond or the hydrolysis.
Put it right: Use precise terminology by stating that peptide bonds are hydrolysed.
Exam tips
- Always explicitly state both 'condensation reaction' and 'peptide bond' when describing the joining of amino acids, as both terms are essential for a complete biological description.
- Remember that the peptide bond is a backbone bond, and that R group interactions belong to tertiary structure.
Try the lesson
The start of the General properties of proteins: A condensation reaction between two amino acids forms a peptide bond. lesson, made from checked teaching notes for this specification point.
Have a go first
What type of reaction joins two amino acids together to form a peptide bond?
Listen
A peptide bond forms when the carboxyl group of one amino acid reacts with the amine group of the next. A hydroxyl group leaves the carboxyl group and a hydrogen atom leaves the amine group, so one molecule of water is eliminated and a carbon-nitrogen bond joins the two residues: a condensation reaction. In a cell this happens on a ribosome during translation, and it always joins the backbone groups, never the R groups. The resulting chain therefore has a repeating backbone with side chains projecting from it, and it has direction, with a free amine group at one end and a free carboxyl group at the other. Hydrolysis reverses this process, requiring water to break the bond. A strong answer earns marks for these points. State that amino acids are joined by peptide bonds. State that the bond is formed by a condensation reaction. Identify the reacting groups as the carboxyl group of one amino acid and the amine group of the next. State that one molecule of water is released per peptide bond formed. A common slip: Saying the peptide bond forms between the R groups of two amino acids. Instead, State that the peptide bond forms between the carboxyl and amine groups of the backbone, not the R groups. Exam tip: Always explicitly state both 'condensation reaction' and 'peptide bond' when describing the joining of amino acids, as both terms are essential for a complete biological description.
Key words
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peptide bond
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condensation reaction
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hydrolysis
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carboxyl and amine groups
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See every definition
- peptide bond
- A carbon-nitrogen bond that joins two amino acids, formed when the carboxyl group of one reacts with the amine group of the next.
- condensation reaction
- A reaction in which a hydroxyl group leaves the carboxyl group and a hydrogen atom leaves the amine group, so one molecule of water is eliminated.
- hydrolysis
- The process that reverses the joining of amino acids, requiring water to break the bond.
- carboxyl and amine groups
- The backbone groups that are always joined, never the R groups, leaving a free amine group at one end and a free carboxyl group at the other.
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