Amino Acids, Dipeptides, and Polypeptides Revision Notes | A Level AQA Biology

1. Why Amino Acids Matter

Every enzyme catalysing a reaction in your cells, every antibody defending you from infection, every structural protein holding your tissues together — all of them are built from the same pool of just twenty amino acids. The way those amino acids link together determines everything about what a protein can do.

This lesson unpacks exactly how amino acids join, what bonds form, and how a chain of hundreds of them becomes a functional protein. These ideas underpin almost every other topic in the proteins section of the AQA specification.

2. The Structure of an Amino Acid

Picture a central carbon atom — sometimes called the alpha carbon — with four groups attached to it. On one side sits an amine group ($-NH_2$). On the opposite side sits a carboxyl group ($-COOH$). A hydrogen atom occupies a third position. The fourth attachment is the R group, also called the side chain.

The general formula for an amino acid can be written as: $H_2N - CH(R) - COOH$. The $-NH_2$ is the amine group, $-COOH$ is the carboxyl group, and $R$ represents whichever side chain that particular amino acid carries. When R is a single hydrogen atom, the amino acid is glycine — the smallest of the twenty. When R is a large aromatic ring, the amino acid might be tryptophan.

The key point the AQA specification emphasises is that all twenty common amino acids share the same backbone structure. Their differences arise entirely from the R group. This is what makes the variety of proteins possible from such a small set of building blocks.

3. Forming a Dipeptide: The Condensation Reaction

Now that you know what a single amino acid looks like, here is what happens when two of them meet. The carboxyl group ($-COOH$) of one amino acid reacts with the amine group ($-NH_2$) of a second amino acid. A water molecule ($H_2O$) is released as the two groups bond together. This is a condensation reaction.

The bond that forms between the two amino acids is called a peptide bond ($-CO-NH-$). The resulting molecule — two amino acids joined by one peptide bond — is a dipeptide. Notice that the dipeptide still has a free amine group at one end and a free carboxyl group at the other, which means it can continue to react.

Worked example: Tracing a Condensation Reaction Between Two Amino Acids

Describe, step by step, how two amino acids join to form a dipeptide, identifying the bond formed and the molecule released.

1. Amino acid 1: $-COOH$ (carboxyl group at its right end) Amino acid 2: $-NH_2$ (amine group at its left end) — These two groups are the reactive sites. The carboxyl group of amino acid 1 reacts with the amine group of amino acid 2.
2. $-COOH + H-NH- \rightarrow -CO-NH- + H_2O$ — The -OH from the carboxyl group and the -H from the amine group combine to form water (H2O). This is what makes it a condensation reaction — water is always released.
3. The bond formed is $-CO-NH-$, written as a peptide bond. — This covalent bond links the carbon of the first amino acid's carboxyl group to the nitrogen of the second amino acid's amine group.
4. Two amino acids + condensation reaction $\rightarrow$ dipeptide + $H_2O$ — The product is a dipeptide. It retains a free -NH2 at one end and a free -COOH at the other, so further condensation reactions are possible.

Answer: A condensation reaction between the carboxyl group of one amino acid and the amine group of another releases a water molecule and forms a peptide bond (-CO-NH-), producing a dipeptide.

4. Extending the Chain: Polypeptides

A dipeptide still has a free carboxyl group at one end and a free amine group at the other. Each of those can participate in another condensation reaction with a third amino acid. Add a fourth, a fifth, and so on — each time releasing a water molecule and forming a new peptide bond — and you build a polypeptide.

The AQA specification defines a polypeptide as a chain formed by the condensation of many amino acids. There is no fixed minimum number given in the specification, but in practice polypeptides typically contain tens to hundreds of amino acids. Each additional amino acid adds one more peptide bond and releases one more water molecule.

The pattern in the table is worth memorising: for a chain of n amino acids, there are always n - 1 peptide bonds and n - 1 water molecules released. This relationship comes up in exam questions more often than you might expect.

5. From Polypeptide to Functional Protein

A polypeptide chain is not automatically a functional protein. The AQA specification states that a functional protein may contain one or more polypeptide chains. Haemoglobin, for example, is made of four polypeptide chains (two alpha and two beta chains) that associate together. Insulin consists of two polypeptide chains linked by disulfide bonds. By contrast, lysozyme is a functional protein made of a single polypeptide chain.

The sequence of amino acids in a polypeptide — determined by the order of codons in the gene — dictates how the chain folds and which other chains it associates with. This is why the R groups matter so much: they interact with each other and with the surrounding environment to drive folding into the precise three-dimensional shape that gives the protein its function.

This distinction between a polypeptide and a functional protein is one that examiners test directly. A polypeptide is a structural description — a chain of amino acids joined by peptide bonds. A functional protein is the biologically active molecule, which may require multiple polypeptides to fold and associate correctly before it can carry out its role.

6. Exam Technique and Common Mistakes

Questions on this topic tend to target three areas: the precise language of condensation reactions, the arithmetic of peptide bonds, and the distinction between a polypeptide and a functional protein. Getting the language right is where most marks are won or lost.

Every spec point in this topic connects back to the same core idea: amino acids are monomers with a fixed backbone and a variable R group, and they polymerise through condensation reactions to form dipeptides, then polypeptides, which then fold and associate to become functional proteins. Keep that chain of logic clear in your answers and you will cover every mark available.