The big idea: Proteins are built from small subunits called amino acids.
Two amino acids join together by a reaction called condensation, which forms a new covalent bond called a peptide bond and 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.
When the peptide bond forms, some atoms are removed.
An —OH is taken from the carboxyl group of one amino acid, and an —H is taken from the amino group of the other. Together these make one molecule of water (H₂O) — this is why the reaction is condensation.
| 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.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
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.
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 |
A memory hook: 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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How this is tested: On Paper 1A 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 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.
IB-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 |