Loading…
Loading…
Loading…
AQA GCSE Chemistry · 8462
AQA 8462 · 4.7.3.3 Check the specification (PDF) (opens in a new tab)
This content is assessed at Higher Tier only.
An amino acid is an organic molecule containing two different . It has an , , and a , .
This combination is important because it allows one amino-acid molecule to react at both ends. Its amino group can react with the carboxyl group of another molecule, while its carboxyl group can react with the amino group of a further molecule. Repeating these reactions builds a long chain, so amino acids act as monomers.
The amino acid specified as an example is glycine, with the formula
The formula shows the two different functional groups clearly: is the amino end and is the carboxyl end. Both groups are present in every glycine molecule.
The carboxyl group of one amino acid reacts with the amino group of another. An is removed from the carboxyl group and an H is removed from the amino group. These atoms combine to form .
The two amino-acid molecules are left joined through a connection, called a peptide link. For two glycine molecules, the first joining step can be represented as
This is a condensation reaction because two molecules join while a small molecule is formed. Each time another amino acid is joined by this process, another peptide link is made and one water molecule is produced.
Two amino acids join by condensation: H and OH form water, and the carbonyl carbon bonds to nitrogen to make a –CO–NH– peptide link. Repetition builds a polypeptide.
The product after only two amino acids have joined is still a short molecule. Crucially, it retains an amino group at one end and a carboxyl group at the other. More amino acids can therefore react at these ends.
Many repeated condensation reactions produce a long polypeptide. For glycine, the chain contains the repeating section
The peptide link is specifically the connection between neighbouring amino-acid residues; it is not the adjacent bond. The overall process is condensation polymerisation because many amino-acid monomers form one long polymer chain while small water molecules are also formed.
A polypeptide does not have to contain only glycine. Different amino acids all possess the required amino and carboxyl functional groups, so different amino acids can be joined within the same chain.
At this level, chains made by combining different amino acids are described as proteins. The central connection is therefore:
amino-acid monomers → condensation polymerisation → polypeptide or protein chain + water.
Each amino acid contains two different functional groups:
Glycine is .
For glycine, the chain section is .
Different amino acids can be combined within the same chain to produce proteins.
Condensation polymerisation: a polymer and a small molecule, usually water, are formed.
Addition polymerisation: no small molecule is formed.
When explaining why an amino acid can polymerise, state that each molecule contains two different functional groups: an amino group and a carboxyl group.
For condensation polymerisation, identify both products: the polypeptide and water. Do not describe the process as addition polymerisation.
If asked how water forms, state that an –OH from a carboxyl group and an H from an amino group are removed.
Learn glycine’s formula accurately: .
When different amino acids are shown, explain that they can still join because each has the required amino and carboxyl functional groups.
It can see this note and what your specification asks for.
Answers are free with an account.
1 free note left this week
Get more free notesYour answer sets when this note comes back for review, or marks it complete.
Was this note helpful?
Log in to tell us whether it helped, or to report a problem.
Last updated
Monomer
A small molecule that can join to other molecules to form a polymer.
Functional group
A group of atoms within a molecule that determines its characteristic chemical reactions.
Amino group
A functional group written as –NH₂ in an amino acid.
Carboxyl group
A functional group written as –COOH in an amino acid.
Condensation polymerisation
A type of polymerisation in which monomers with two functional groups join and a small molecule, usually water, is formed at each new link.
Polypeptide
A polymer chain formed when many amino-acid monomers join by condensation polymerisation.
Peptide link
The –CO–NH– connection between neighbouring amino-acid residues in a polypeptide chain.
Protein
A naturally occurring polymer made from amino acids; different amino acids can occur within the same chain.
Put your knowledge into practice — try past paper questions for Chemistry
Monomer
A small molecule that can join to other molecules to form a polymer.
Functional group
A group of atoms within a molecule that determines its characteristic chemical reactions.
Amino group
A functional group written as –NH₂ in an amino acid.
Carboxyl group
A functional group written as –COOH in an amino acid.
Condensation polymerisation
A type of polymerisation in which monomers with two functional groups join and a small molecule, usually water, is formed at each new link.
Polypeptide
A polymer chain formed when many amino-acid monomers join by condensation polymerisation.
Peptide link
The –CO–NH– connection between neighbouring amino-acid residues in a polypeptide chain.
Protein
A naturally occurring polymer made from amino acids; different amino acids can occur within the same chain.