Proteins can have an effectively infinite variety because 20 amino acids can be joined in chains of many different lengths and in any sequence. Each distinct amino acid sequence may produce a polypeptide with a different structure and function. This content applies to both SL and HL in IB Biology subtopic B1.2.
The Reasoning
A protein begins as a polypeptide, a chain of amino acids joined by peptide bonds. These bonds form through condensation reactions between the amine group of one amino acid and the carboxyl group of another.
Protein variety arises from three main factors:
Factor | How it increases variety |
|---|---|
Number of amino acid types | The genetic code specifies 20 different amino acids that can be incorporated into polypeptides. |
Chain length | Polypeptides can contain from a few to thousands of amino acids. |
Amino acid sequence | The amino acids can occur in any order, so changing even one position creates a different primary structure. |
For a polypeptide containing amino acids, there are theoretically possible sequences if each position can contain any of the 20 amino acids. A chain only 100 amino acids long therefore has possible sequences, an astronomically large number.
At HL, connect this sequence diversity to protein conformation. Amino acids have chemically different R-groups, so changing the sequence changes hydrogen bonding, ionic bonding, disulfide bonding and hydrophobic interactions. This can alter the protein’s three-dimensional shape and therefore its function.
A common misconception is that “infinite variety” means literally every possible protein exists. It means the number of possible polypeptide chains is effectively unlimited biologically; organisms produce only a tiny fraction of them through gene expression.
Exam Technique
For an explain question, link the points causally: 20 amino acids + variable chain length + any sequence → enormous numbers of primary structures → different conformations and functions. Do not simply state that “proteins are made from amino acids”; that does not explain their variety.