Dietary amino acids and the variety of peptide chainsSpec B1.2.3, B1.2.4
In short
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.
Dietary requirements
- Essential amino acids
- Amino acids that cannot be synthesised and must be obtained from food.
- Non-essential amino acids
- Amino acids that can be made from other amino acids.
Proteins in food are hydrolysed into amino acids, which are absorbed and used to build the body's own proteins. If any essential amino acid is missing from the diet, proteins that contain it cannot be made.
Some plant proteins contain only small amounts of certain essential amino acids. Vegan diets therefore need attention: eating a variety of plant protein sources, such as legumes together with cereals, ensures that every essential amino acid is consumed.
An infinite variety of peptide chains
- 20 amino acids are coded for in the genetic code.
- A peptide chain can contain any number of amino acids, from a few to thousands.
- The amino acids can be in any order.
For a chain of n amino acids there are 20ⁿ possible sequences. Even short chains have a vast number of possibilities, so the variety of possible peptide chains is effectively infinite. Only a small fraction are actually made by living things.
Counting possible peptide sequences
How many different tripeptides could be made from the 20 amino acids? How many sequences are possible for a chain of 10 amino acids?
- Each position can be any of 20 amino acids, so the number of sequences is 20ⁿ.
- Tripeptide: 20³ = 20 × 20 × 20 = 8000.
- Ten amino acids: 20¹⁰ ≈ 1.0 × 10¹³.
Answer: 8000 tripeptides; about 1.0 × 10¹³ sequences of 10 amino acids
| Polypeptide | Size | Function |
|---|---|---|
| Insulin | Two chains of 21 and 30 amino acids | Hormone that lowers blood glucose |
| Beta chain of haemoglobin | 146 amino acids | Part of the oxygen-carrying protein in red blood cells |
| Titin | Up to about 34 000 amino acids, the largest known polypeptide | Gives elasticity to muscle |
Written and checked against the IB Biology SL specification · Updated October 2026