<span> They can be classified according to the core structural functional groups' locations as </span><span>alpha- (α-), beta- (β-), gamma- (γ-) or delta-(δ-)</span><span> amino acids; other categories relate to </span>polarity<span>, </span>pH<span> level, and side-chain group type (</span>aliphatic<span>, </span>acyclic<span>, </span>aromatic, containing hydroxyl orsulfur<span>, etc.). In the form of </span>proteins<span>, amino acids comprise the second-largest component (water is the largest) of human </span>muscles<span>, </span>cells<span> and other </span>tissues.[5]<span> Outside proteins, amino acids perform critical roles in processes such as </span>neurotransmitter<span> transport and </span>biosynthesis<span>.</span>
