Final answer:
Amino acids have a common structure and variation is contributed by the R group. There are 20 different types of amino acids, with two main types being polar and nonpolar amino acids.
Step-by-step explanation:
Amino acids, which are the building blocks of proteins, have a common structure. They consist of a central carbon atom bonded to an amino group (-NH₂), a carboxyl group (-COOH), a hydrogen atom, and a variable R group. The R group is what differentiates each amino acid and contributes to amino acid variation. There are 20 different types of amino acids, each with a unique R group.
Two types of amino acids are polar and nonpolar amino acids. Polar amino acids have an R group that can form hydrogen bonds and interact with water. Nonpolar amino acids have an R group that is hydrophobic and does not interact with water.