amino acid
/ uh-MEE-noh AS-id /
If a protein is a sentence, amino acids are its letters. There are twenty common kinds in living things, and stringing them together in different orders spells out every protein your body uses — from the keratin in your hair to the enzymes digesting your lunch.
Each amino acid is built on the same central plan: a central carbon holding an amino group (basic), a carboxyl group (acidic), a hydrogen, and one variable part called the side chain or R-group. That side chain is the only thing that differs between the twenty, and it gives each amino acid its character — some side chains are water-loving, some water-fearing, some carry charge, some are tiny, some are bulky. Those personalities are what later push a protein chain to fold into a precise shape.
Because all twenty share the same two reactive ends (amino and carboxyl), the cell can link any amino acid to any other in a uniform way, like train cars that all couple the same. That uniformity is what makes proteins possible: a single assembly line can stitch the twenty letters into any order the genetic code dictates, and the order decides everything about the finished protein.
Glycine has the smallest side chain — just a hydrogen — while tryptophan has a big bulky ring; swap one for the other in a protein and the protein may fold differently or stop working.
Twenty side chains, twenty personalities — the alphabet every protein is written in.
Essential amino acids are not more important than the rest; the word just means your body cannot make them, so you must get those particular ones from food.