Amino acids (HT only) — AQA GCSE Chemistry
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Amino acids (HT only) explained
Amino acids (HT only) covers 2 specification points, including Amino acids have two different functional… and Amino acids react by condensation….
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Open a point below for its explanation and revision guidance.
Amino acids have two different functional groups in a molecule.
An amino acid molecule contains two different functional groups: an amine group, –NH₂, and a carboxylic acid group, –COOH. Because these groups are different, an amino acid can react with another amino acid in two ways. The amine group of one molecule can react with the carboxylic acid group of another, forming an amide link and releasing water. This allows amino acids to join into chains called polypeptides or proteins. The presence of two different functional groups, rather than two identical ones, means a single type of amino acid can polymerise with itself, and different amino acids can also combine in any order.
Amino acids react by condensation polymerisation to produce polypeptides.
Amino acids are small molecules that each contain an amine group and a carboxylic acid group. In condensation polymerisation, the amine group of one amino acid reacts with the carboxylic acid group of another, releasing a small molecule such as water and forming an amide link. Repeating this process builds a long chain called a polypeptide. Because each amino acid can join to two neighbours, the polymer grows in two directions. For example, glycine joining with alanine releases water and forms a dipeptide; further additions extend the chain. The polypeptide backbone is a repeating sequence of amide links, and the side groups vary according to which amino acids were used. This is a higher-tier topic, so you should be able to identify the link formed and explain why the reaction is a condensation polymerisation.
Your focus
- Identify the amine and carboxylic acid groups in an amino acid.
- Explain how different functional groups allow amino acids to polymerise.
- Describe the formation of an amide link with loss of water.
Show all 6 objectives
- Identify the amine and carboxylic acid groups in a given amino acid structure.
- Describe the condensation reaction between two amino acids, including the small molecule released.
- Explain how repeating condensation produces a polypeptide chain with amide links.
Amino acids (HT only) exam tips
Marking Points
- An amino acid contains an amine group, –NH₂.
- An amino acid contains a carboxylic acid group, –COOH.
- The two functional groups are different from each other.
- The amine group of one amino acid reacts with the carboxylic acid group of another.
- An amide link forms and water is eliminated.
- Amino acids can join to form polypeptides or proteins.
- Amino acids contain both an amine group and a carboxylic acid group.
- The amine group of one amino acid reacts with the carboxylic acid group of another.
- A small molecule, commonly water, is eliminated during each linking step.
- The new bond formed between the amino acids is an amide link.
- Repeating the condensation reaction produces a long-chain polypeptide.
- The reaction is classified as condensation polymerisation because a small molecule is released as the polymer forms.
Examiner Tips
- 💡Name both functional groups precisely: amine and carboxylic acid.
- 💡Show the amide link and the eliminated water when explaining polymerisation.
- 💡Use the term polypeptide or protein for the polymer formed from amino acids.
- 💡Name the two functional groups present in an amino acid before describing the reaction.
- 💡State clearly that a small molecule is eliminated, and identify it as water in the common case.
- 💡Use the term amide link when describing the bond formed between amino acid units.
- 💡Link the release of the small molecule to the definition of condensation polymerisation.
Common Mistakes
- Stating that amino acids contain two identical functional groups: correct this by naming the amine and carboxylic acid groups as different.
- Confusing the amine group with the amide link: correct this by noting that the amine group is a reactant and the amide link is the bond formed.
- Forgetting that water is eliminated when amino acids join: correct this by linking amide formation to condensation.
- Thinking that amino acids join without releasing anything: the error is omitting the small molecule; the correction is that condensation polymerisation releases a small molecule such as water at each link.
- Confusing the amide link with an ester link: the error is naming the wrong functional group; the correction is that amine plus carboxylic acid forms an amide link.
- Believing that only one type of amino acid can be used: the error is assuming a single monomer; the correction is that different amino acids can be combined, giving different side groups along the chain.