Peptides vs proteins

If a peptide and a protein are made of the same thing, why bother making the distinction between the two?

The difference is mostly just in length. The rough idea is that a chain of about 50 amino acids or fewer is a peptide and anything longer is a protein. It is a fuzzy line and there are no hard rules on this, so don't worry about an exact number. Insulin sits right around the border and people refer to it as both a peptide and a protein.

The distinction is useful to you for practical reasons. Short chains are simpler to manufacture, which is part of why peptides became so widely available. They also behave differently in the body. A large protein taken as a pill usually gets shredded by your stomach before it does anything, which is why most peptides are injected rather than swallowed. More on this in the next module.

The short version is that on one hand, a protein is large and folded into a working shape. On the other hand, a peptide is a small, simple chain. When you hear "peptide," think small and usually injected.