In the general structure of an amino acid, which component makes each amino acid unique?

The bonds found within hair that help organize protein chains in order to build hair are called:

In humans, which amino acid is the most abundant in the body (by total free and protein-bound content)?

In the general structure of an amino acid, which component varies among different amino acids and makes each one unique?

In proteins, each amino acid differs from another primarily in which structural component?

In proteins, each amino acid differs from the others primarily in which structural feature?

Which functional groups are present in the chemical structure of every standard amino acid?

In all standard amino acids, which functional groups are always present on the alpha carbon (in their non-modified form)?

In the context of amino acid structure, what primarily makes each of the 20 standard amino acids unique?