Glycine is the smallest of the amino acids. It is ambivalent, meaning that it can be inside or outside of the protein molecule. In aqueous solution at or near neutral pH, glycine will exist predominantly as the zwitterion.
Alanine is a hydrophobic molecule. It is ambivalent, meaning that it can be inside or outside of the protein molecule. The α carbon of alanine is optically active; in proteins, only the L-isomer is found.
Serine differs from alanine in that one of the methylenic hydrogens is replaced by a hydroxyl group. Serine is one of two hydroxyl amino acids. Both are commonly considered to by hydrophilic due to the hydrogen bonding capacity of the hydroxyl group.
Threonine, an essential amino acid, is a hydrophilic molecule. Threonine is an other hydroxyl-containing amino acid. It differs from serine by having a methyl substituent in place of one of the hydrogens on the β carbon and it differs from valine by replacement of a methyl substituent with a hydroxyl group. Note that both the α and β carbons of threonine are optically active.
Cysteine is one of two sulfur-containing amino acids; the other is methionine. Cysteine differs from serine in a single atom-- the sulfur of the thiol replaces the oxygen of the alcohol. The amino acids are, however, much more different in their physical and chemical properties than their similarity might suggest. Consider, for example, the differences between H2O and H2S. The hydrogen bonding propensity of water is well known and is responsible for many of its remarkable features. Under similar conditions of temperature and pressure, however, H2S is a gas as a consequence of its weak H-bonding propensity. Furthermore, the proton of the thiol of cysteine is much more acid than the hydroxylic proton of serine, making the nucleophilic thiol(ate) much more reactive than the hydroxyl of serine. Cysteine also plays a key role in stabilizing extracellular proteins. Cysteine can react with itself to form an oxidized