Peptides are compounds that are formed by linking one or more amino acids with a covalent bond. These compounds are classified as polymers, because they typically link together in long chains. All animals on Earth have them in their body, and in a way, they are one of the building blocks of life. When a chain gets especially long, it turns into a protein. Peptides and proteins represent a wide world of possibilities, and many molecular biologists spend years researching the functions of individual ones to learn more about how the body works.
When discussing peptides, a lot of scientific terminology tends to get thrown around, and it can help to know what various terms mean. A covalent bond is a type of chemical bond that occurs when atoms share electrons. The specific type of covalent bond formed in these compounds is known as a peptide bond or amide bond, and it forms when the carboxl group of one amino acid attaches to another. Carboxyl groups are clusters of carbon, oxygen, and hydrogen molecules, in case you're curious.
The classification of these compounds as polymers is sometimes confusing to people who are not familiar with this use of the term. While many people mean “plastics” when they use this word, in chemistry, a polymer is any sort of repeating chain connected with covalent bonds. Polymers can get extremely complex, as one might imagine.
A peptide can perform a wide range of functions in the body, depending on which amino acids are involved. Some can regulate hormones, for example, while others can have an antibiotic function. The body is also equipped to break down and reuse these compounds; if a person eats meat, for example, the enzymes in his intestines break down the protein at its amide bonds to create an assortment of peptides that may be digested or excreted, depending on the needs of the body.
The dividing line between a peptide and a protein is somewhat fluid. Proteins are much more complex, because they are so much longer, and most proteins are folded into complex structures to accommodate all of their amino acids. As a general rule of thumb, if more than 50 amino acids are involved, the compound is a protein, while shorter chains are considered peptides.