Computer artwork of the molecule of Phenylalanine, an essential amino acid. It cannot be synthesised by humans and is instead sourced from the diet. It is a neutral, non-polar amino acid and a precursor for tyrosine, another amino acid involved in the production of the neurotransmitters norepinephrine (noradrenaline), dopamine, and epinephrine (adrenalin). Atoms are represented as colour-coded spheres: carbon (black), hydrogen (white), nitrogen (blue) and oxygen. Illustration.