The big idea: Proteins are built from small subunits called amino acids. Two amino acids join together by a reaction called condensation.
What condensation does
- It forms a new covalent bond called a peptide bond.
- It releases one molecule of water (H₂O).
- Two amino acids joined this way make a dipeptide.
- Many joined in a chain make a polypeptide — the basis of every protein.
Condensation joins two amino acids: an –OH from one carboxyl group and an –H from the other amino group leave together as water, and the carbon and nitrogen left behind join by a new peptide bond (CO–NH).
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- Amino acid
- The small subunit (monomer) that proteins are built from. Every amino acid has an amino group (—NH₂) at one end and a carboxyl group (—COOH) at the other.
- Peptide bond
- The covalent bond (CO—NH) that joins two amino acids together. It forms by condensation.
- Condensation
- A reaction that joins two subunits and releases one molecule of water (H₂O) as the new bond forms.
- Dipeptide
- Two amino acids joined by a single peptide bond.
- Polypeptide
- A long chain of many amino acids joined by peptide bonds — the chain that folds into a protein.
Which groups react: The carboxyl group (—COOH) of one amino acid reacts with the amino group (—NH₂) of the next.
This is always the pairing the exam wants: carboxyl meets amino — the two ends that give an amino acid its name.
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When the peptide bond forms, some atoms are removed. They leave as one molecule of water (H₂O) — this is why the reaction is condensation.
Which atoms are removed
- An —OH is taken from the carboxyl group of one amino acid.
- An —H is taken from the amino group of the other.
- Together these make one molecule of water (H₂O).
| Group involved | Which amino acid | What it provides |
|---|---|---|
| Carboxyl group (—COOH) | The first amino acid | Gives up an —OH |
| Amino group (—NH₂) | The second amino acid | Gives up an —H |
| These removed atoms (—OH + —H) | From the two reacting groups | Join to form one water molecule (H₂O) |
| Peptide bond (CO—NH) | Forms between the two amino acids | The new covalent bond that links them |
The atoms that are removed: Exam questions love to ask which atoms are lost when two amino acids join.
The answer: an —OH from one carboxyl group and an —H from the other amino group, which leave together as one water molecule (H₂O).
So a peptide bond forming and a water molecule leaving always happen together.
Breaking the bond: hydrolysis: The reverse reaction is hydrolysis: one water molecule is added across the peptide bond, splitting the dipeptide back into two separate amino acids.
This is how a protein you eat is digested back into amino acids.
Hydrolysis is the reverse: a water molecule is added across the peptide bond, splitting the dipeptide back into two separate amino acids.
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Condensation (joining)
- Forms a peptide bond
- Removes an —OH and an —H → one water molecule out
- Builds a dipeptide (then a polypeptide)
- Carboxyl —COOH reacts with amino —NH₂
Hydrolysis (breaking)
- Breaks a peptide bond
- Adds one water molecule across the bond
- Splits the chain back into amino acids
- Happens during digestion of protein
Counting peptide bonds in a chain: Each peptide bond joins one pair of amino acids, so a single chain always has one fewer bond than amino acids.
A chain of 5 amino acids has 4 peptide bonds; a chain of 100 amino acids has 99.
More than one chain: If a protein is made of more than one chain, count each chain separately, or use the quick rule: total amino acids minus the number of chains.
| Situation | How to count the peptide bonds | Example |
|---|---|---|
| One single chain | Number of amino acids minus 1 (each bond links a pair) | 5 amino acids in 1 chain → 4 peptide bonds |
| More than one chain | Each separate chain has one fewer bond than its amino acids | Two chains of 21 + 30 → 20 + 29 = 49 peptide bonds |
| Quick rule | Total amino acids minus the number of chains | 51 amino acids in 2 chains → 51 − 2 = 49 bonds |
An easy way to remember: Condensation connects (and water comes out). Peptide bonds, like fence panels between posts, are always one fewer than the things they connect.
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The big idea: The word equation is amino acid + amino acid → dipeptide + water. In the drawing, the carbon of one carboxyl group bonds to the nitrogen of the next amine group.
The points to remember
- Draw two amino acids side by side, the carboxyl group of the first facing the amine group of the second.
- Take the OH from that carboxyl group and one H from that amine group: together they leave as water.
- Join the C to the N with a single line: the peptide bond, with C=O on one side and N–H on the other.
- The dipeptide still has a free amine end and a free carboxyl end, so more amino acids can join on.
Remember it as: Carboxyl gives the OH, amine gives the H, and C joins N.
Real example: aspartame: The sweetener aspartame is built from two amino acids, aspartic acid and phenylalanine, joined by one peptide bond: a dipeptide.
How this is tested: Paper 1A asks how amino acids link and which atoms leave. A Calculate item asks you to count peptide bonds.
What you may be asked
- A common multiple-choice item shows two amino acids and asks you to identify how they link to form a dipeptide. The answer is condensation, forming a peptide bond.
- A frequent variant labels the atoms or groups and asks which atoms are removed (an —OH and an —H, leaving as water).
- Or it asks which groups join (the carboxyl —COOH and the amino —NH₂).
- A Calculate item gives an amino-acid count and the number of polypeptide chains and asks for the number of peptide bonds.
Exam-style question — atoms removed and the bond formed
Two amino acids react together to form a dipeptide. State the name of the reaction, identify the atoms that are removed, and name the bond that forms. [3]
How to score all three marks
- Name the reaction. The two amino acids join by condensation.
- Identify the atoms removed. An —OH is removed from the carboxyl group (—COOH) of one amino acid and an —H from the amino group (—NH₂) of the other; together they leave as one water molecule (H₂O).
- Name the bond. The new covalent bond joining the two amino acids is a peptide bond (CO—NH). (Mark 1: condensation. Mark 2: —OH and —H removed / water released. Mark 3: peptide bond.)
Final answer
Condensation: an —OH (from a carboxyl group) and an —H (from an amino group) are removed as one water molecule, and a peptide bond forms between the two amino acids.
Why this scores full marks: Each part is a separate scoring point: the reaction (condensation), the atoms lost (—OH + —H → water), and the bond (peptide).
Saying 'water is lost' is good, but naming which groups the —OH and —H come from makes the answer watertight.
| Group involved | Which amino acid | What it provides |
|---|---|---|
| Carboxyl group (—COOH) | The first amino acid | Gives up an —OH |
| Amino group (—NH₂) | The second amino acid | Gives up an —H |
| These removed atoms (—OH + —H) | From the two reacting groups | Join to form one water molecule (H₂O) |
| Peptide bond (CO—NH) | Forms between the two amino acids | The new covalent bond that links them |
A three-mark Outline in the Paper 2 style:
Outline the general structure of a peptide.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.
And the one-mark word equation:
State the products of a condensation reaction between two amino acids.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.