Proteins — IB Diploma Biology SL
IB Biology B1.2: amino acids, peptide bonds, diet and the effect of pH and temperature on protein structure (denaturation).
Revision notes
3 short notes, in the order of the specification. Each one in short:
Amino acids and peptide bonds
PracticalAn amino acid has a central alpha carbon atom bonded to an amine group, a carboxyl group, a hydrogen atom and a variable R-group. In a condensation reaction the carboxyl group of one amino acid reacts with the amine group of another, forming a peptide bond and releasing water. Repeating this produces a polypeptide.
Essential amino acids cannot be synthesised by the body and must be obtained from food, while non-essential amino acids can be made from other amino acids. The genetic code codes for 20 amino acids, and a chain can have any number of them, from a few to thousands, in any order, so the variety is effectively infinite.
Denaturation is a change in the three-dimensional structure of a protein that is usually permanent, so the protein can no longer carry out its function. High temperatures make the molecule vibrate until weak bonds holding its shape break. Extremes of pH change the charges on R-groups, breaking ionic and hydrogen bonds. Peptide bonds are not broken.
Exam questions
4 exam-style questions (11 marks), each with its mark scheme.
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9 cards: flip them, mark what you knew, and practise the rest.
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The whole of molecules (form and function) on one page, so you can see where this subtopic fits.
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Quick check questions
Which four groups are bonded to the alpha carbon of an amino acid?
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An amine group, a carboxyl group, a hydrogen atom and an R-group.
What is an essential amino acid?
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One that cannot be synthesised by the body and must be obtained from food.
What is denaturation?
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A usually permanent change in the three-dimensional structure of a protein, so it can no longer carry out its function.
Frequently asked questions
What is the difference between essential and non-essential amino acids?
Essential amino acids cannot be synthesised by the body, so they must be obtained from food. Non-essential amino acids can be made in the body from other amino acids. Some plant proteins are low in certain essential amino acids, so vegans eat a variety of plant protein sources to get all of them.
Why do proteins denature at high temperatures?
Proteins denature at high temperatures because heat makes the molecule vibrate more until hydrogen bonds and other weak interactions holding its three-dimensional shape break. The protein unfolds and loses its function. Peptide bonds are not broken, so the amino acid sequence is unchanged, but the change in shape is usually permanent.
How does pH affect protein structure?
A change in pH changes the charges on R-groups, because amine and carboxyl groups gain or lose hydrogen ions. Ionic bonds between oppositely charged R-groups break, and hydrogen bonding changes, so the protein's three-dimensional shape alters. Outside its stable pH range a protein denatures and stops functioning, for example an enzyme losing its active site shape.
Written and checked against the IB Biology SL specification · Updated October 2026