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B1.2.2—Condensation reactions

Condensation links amino acids by peptide bonds between carboxyl and amine groups, releasing water and forming polypeptide chains at ribosomes.

Syllabus
First assessment 2025
Objective
B1.2.2
Level
HL

Exam analysis

Chance of appearing5%of analysed past papers
Latest appearanceNovember 2021
Most common paperPaper2
Typical marks1–3

Common command terms

  • Draw
  • Label
  • Annotate
  • State
  • Identify

Scoring notes

Common mistake
Calling peptide-bond formation hydrolysis instead of condensation.

Recent exam appearances

November 2021Paper2 ["HL"] · TZ04(a)[ 1 ]B1.2.2—Condensation reactions
November 2020Paper2 ["HL"] · TZ06(a)[ 3 ]B1.2.2—Condensation reactions
May 2019Paper2 ["HL"] · TZ26(b)[ 3 ]B1.2.2—Condensation reactions
November 2018Paper2 ["HL"] · TZ02(a)(i)[ 1 ]B1.2.2—Condensation reactions
November 2017Paper2 ["HL"] · TZ07(a)[ 3 ]B1.2.2—Condensation reactions
Practice this objective

Coverage 2017–2021 · Updated 15 Jul 2026

Peptide Bonds Join Amino Acids

A peptide bond forms when the carboxyl group of one amino acid condenses with the amine group of another, releasing water and creating a covalent C–N link.

amino acid+amino acid→dipeptide+water\text{amino acid}+\text{amino acid}\rightarrow\text{dipeptide}+\text{water}

OH is removed from the carboxyl group and H from the amine group. Repeating condensation extends the polypeptide from its amino (N) terminus toward its carboxyl (C) terminus.

Two amino acids produce one peptide bond and one water molecule; adding a third amino acid produces a tripeptide with two peptide bonds and releases a second water molecule.

A peptide bond is the covalent link in the backbone, not a hydrogen bond. Hydrogen bonds stabilize later folding levels.

Condensation reactions

Assessment in practice

1–3 marks
How it is assessed

This objective is assessed through experimental design, commonly using Draw / Label / Annotate.

Command terms

Draw / Label / Annotate / State / Identify

What earns marks

Build the answer around this relationship: Peptide bonds form by condensation between carboxyl and amine groups.

Watch for

Calling peptide-bond formation hydrolysis instead of condensation.

Representative question

Question 1

[Maximum number: 4]

Draw molecular diagrams to show the condensation reaction between two amino acids to form a dipeptide.

Build And Use Proteins

The core protein story is build -> vary -> function. Amino acids share a backbone but differ in R-groups. Peptide bonds form by condensation between carboxyl and amine groups. Some amino acids must come from diet, or protein synthesis is limited. Twenty coded amino acids create many sequences by type, number, and order. Finally, shape determines function, so denaturation changes performance.

  • Amino acids share an alpha-carbon backbone and vary in R-groups.
  • Peptide bonds form by condensation and release water.
  • Essential amino acids must be obtained from dietary protein.
  • Protein diversity depends on amino acid type, number, and order.
  • Protein shape determines function; denaturation changes shape and function.

Concept essentials

  • Peptide bonds form by condensation between carboxyl and amine groups.
  • Water is released when amino acids join.
  • A dipeptide contains two amino acids linked by one peptide bond.
  • A polypeptide has an amino terminus and a carboxyl terminus.