There are 20 amino acids that are considered common to all living systems, which means they are used by all known organisms in the process of protein synthesis. These are:

* Nonpolar (hydrophobic):

* Glycine (Gly, G)

* Alanine (Ala, A)

* Proline (Pro, P)

* Valine (Val, V)

* Leucine (Leu, L)

* Isoleucine (Ile, I)

* Methionine (Met, M)

* Phenylalanine (Phe, F)

* Tryptophan (Trp, W)

* Polar (hydrophilic):

* Serine (Ser, S)

* Threonine (Thr, T)

* Cysteine (Cys, C)

* Tyrosine (Tyr, Y)

* Asparagine (Asn, N)

* Glutamine (Gln, Q)

* Acidic (negatively charged):

* Aspartic acid (Asp, D)

* Glutamic acid (Glu, E)

* Basic (positively charged):

* Lysine (Lys, K)

* Arginine (Arg, R)

* Histidine (His, H)

These 20 amino acids are encoded by the genetic code and are used to build the proteins that carry out essential functions in all living organisms.