Pepsin is an enzyme -- specifically, it's a proteolytic enzyme, meaning it helps digest proteins. Cells in the stomach secrete pepsin to help you digest the protein that you consume in food. Specialized cells in the intestine then absorb the digestion products of the protein into the bloodstream, and your cells take them up from there.
Enzymes, like pepsin, are biological catalysts. This means that they're chemicals that help reactions in the body take place faster than they otherwise would, but they themselves aren't consumed in the reaction. Specifically, pepsin helps you digest protein. It's secreted by specialized cells in the lining of the stomach called chief cells, explains Dr. Lauralee Sherwood in her book "Human Physiology." Pepsin itself is a protein, but in contrast to the proteins that you eat and digest, pepsin has biological activity in the stomach.
Technically, you wouldn't need pepsin to digest the protein that you eat. Proteins are made up of small building block molecules called amino acids, explain Drs. Reginald Garrett and Charles Grisham in their book "Biochemistry." The bonds between amino acids -- called peptide bonds -- break when they're exposed to water and acid. Because your gastric juices are made of mostly water, and the stomach secretes acid, you have all the ingredients you need to digest protein. However, in the absence of pepsin, this reaction would be very slow.
One of the body's challenges in producing pepsin is keeping it from digesting the cells where it's made. Cells are made up of mostly protein, meaning that if a cell produces pepsin, it's at risk for being digested by the pepsin. As such, your chief cells produce pepsin in an inactive precursor form, called pepsinogen, explain Drs. Garrett and Grisham. The pepsinogen is activated once it reaches the stomach, so it doesn't harm the cells that produce it.
All biological enzymes, pepsin included, function best in a narrow range of temperature and acidity level. Enzymes exposed to environments outside their preferred temperature or acidity -- also called pH -- range don't function. Most body enzymes function best at body temperature and neutral pH, but because pepsin functions in the stomach, it's designed by the cells to work best at very low, or acidic, pH, explain Drs. Mary Campbell and Shawn Farrell in their book "Biochemistry."
Types of Bonds
In actuality, pepsin can't break all the bonds in a protein molecule. There are many different types of amino acids that are bonded together to make a protein, and pepsin specializes in breaking bonds between very large amino acids. Other proteolytic enzymes, including chymotrypsin and trypsin, work on the bonds between other types of amino acids. Together, pepsin and the other proteolytic enzymes break the proteins you eat into their constituent amino acids.