Pepsin is secreted by cells called chief cells, found in the lining of the stomach. Pepsin breaks the peptide bonds linking amino acids in the chains found in proteins. This aids in digestion by breaking the protein down into smaller amino acids. The pepsin must be in a pH that is acidic (generally 1-3) and a body temperature of between 30 and 40 degrees Celsius (general body temp is 37).
"Pepsin | Biochemistry." Encyclopedia Britannica Online. Encyclopedia Britannica, n.d. Web. 09 Mar. 2015. <http://www.britannica.com/EBchecked/topic/450873/pepsin>.