What is the key difference among the 20 amino acids that are used to make proteins?

The side groups present in amno acids makes the 20 amino acids different from each other.

Explanation:

Amino acids are used by each cell of living body in order to build the proteins  required to survive. Amino acids consist of two-carbon bond, one is carboxyl group (COO-) and teh other carbon connected to the amino group.

There are total 50 amino acids out of which only 20 are used to make proteins in our body. The side groups present in these 20 amino acids amke them different from each other and used to make protein. Amino acid made up of carboxyl group -COOH , an amine group -NH2 , and atom of H, and variable radical -R.

The side groups are named as polar or non-polar groups or sometimes also called "R" groups. The side goups are responsible to interact with the environment. The polar and nonpolar chemical properties of amino acid allow them to become  hydrophilic (Polar) and hydrophobic (non-polar).

Example: Serine - C3H7NO3 (side chain of HOH2C- )

               Asparagine - C4H8N2O3 (side chain of H2N - CO- CH2-)

Hence, the "R" group is responsible for the variation in 20 amino acids.

answer; a muscle cell, another neuron and a of the above