Amino Acids and Proteins
Practise Amino Acids and Proteins for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. A separate (triple) chemistry topic: amino acids have both an amino and a carboxylic acid group and join by condensation to form proteins.
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Topic overview
Amino acids have two different functional groups in the same molecule: an amino group and a carboxyl group. This is a Triple-only topic.
Because of this they can react by condensation polymerisation to produce polypeptides. Glycine is the simplest amino acid, and it produces the polypeptide often written as poly(glycine).
Different amino acids can be combined in the same chain, and the resulting molecule is a protein. Water is released each time two amino acids join, which is why this is condensation rather than addition polymerisation.
Revision notes
Structure
Amino acids contain two different functional groups: an amino group and a carboxyl group.
Having two different groups in one molecule is what allows them to join to each other in long chains.
Forming polypeptides
Amino acids react by condensation polymerisation, joining to form polypeptides.
A molecule of water is released each time two amino acids join. Glycine is the simplest amino acid.
Proteins
Different amino acids can be combined in the same chain, producing a protein.
The order of amino acids determines the protein's shape and function, which links this topic directly to biology.
Key points
- Amino acids have two different functional groups.
- They contain an amino group and a carboxyl group.
- They react by condensation polymerisation.
- They form polypeptides.
- Water is released as they join.
- Different amino acids in a chain form a protein.
Worked examples
Example 1
Name the two functional groups present in an amino acid. [2 marks]
Model answer
An amino group (1 mark) and a carboxyl group (1 mark).
Example 2
Explain why amino acids join by condensation polymerisation rather than addition. [2 marks]
Model answer
Amino acids have two different functional groups rather than a carbon-carbon double bond (1 mark), and a molecule of water is released each time two amino acids join (1 mark).
Example 3
Explain the difference between a polypeptide and a protein. [2 marks]
Model answer
A polypeptide is a chain formed from amino acids joined together (1 mark). A protein is formed when different amino acids are combined in the same chain (1 mark).
Common mistakes
Saying amino acids join by addition.
They join by condensation, releasing water.
Naming only one functional group.
Both the amino and the carboxyl group are required.
Forgetting the water molecule.
It is what makes this condensation polymerisation.
Saying all polypeptides are proteins.
A protein contains different amino acids combined in the same chain.
Exam tips
- Name both functional groups when asked.
- Mention the water released to identify condensation.
- Learn glycine as the simplest example.
- Remember this is a Triple-only topic.
Key terms
- Amino acid
- A molecule with an amino group and a carboxyl group.
- Polypeptide
- A chain of amino acids joined together.
- Protein
- A chain containing different amino acids.
- Condensation polymerisation
- Polymerisation releasing a small molecule.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.