serine

Serine is an amino acid, one of the small chemical units that living things use to build proteins. Amino acids join together in long chains, and those chains fold into proteins that do much of the work inside cells. Proteins help form body tissues, carry materials, and support chemical reactions that keep organisms alive.

The word comes from the Latin word sericum, meaning "silk," because this amino acid was first isolated from silk. That bit of history is useful because it explains why the name sounds unusual and why silk is often mentioned when people define it.

In biochemistry, serine is often written as Ser or S. It is a polar amino acid, which means it mixes well with water and often appears in parts of proteins that interact with watery environments inside or outside the cell. It also plays an important part in enzyme activity and cell signaling, so it matters not just as a building block but also as an active part of how cells work.

Your body can make serine on its own, so it is called a non-essential amino acid in nutrition. Even so, it is still essential in the everyday sense: cells need it to function properly. You may never use the word in casual conversation, but in biology it refers to a common and important amino acid found throughout living systems.