Proteins are made up of amino acids. These are the subunits that link together to form long chains, much like beads on a string.

Here's a breakdown:

* Amino Acids: These are the building blocks of proteins. There are 20 different types of amino acids found in proteins. Each amino acid has a central carbon atom bonded to:

* An amino group (-NH2)

* A carboxyl group (-COOH)

* A hydrogen atom (-H)

* A side chain (R-group) - This is the part that varies between different amino acids and gives them unique properties.

* Peptide Bonds: Amino acids are linked together by peptide bonds. These bonds form between the carboxyl group of one amino acid and the amino group of the next.

* Polypeptide Chain: A chain of amino acids linked by peptide bonds is called a polypeptide chain.

* Protein: A protein is one or more polypeptide chains folded into a specific three-dimensional shape. This shape is crucial for the protein's function.

So, to answer your question directly, the subunits that make up proteins are called amino acids.