STOMACH

=Stomach= The stomach is a muscle lined bag that stores food while the digestive process begins. [image:http://i.imgur.com/9DMGD5H.png?1] '''Physical Digestion:''' Muscles lining the stomach contract and relax churning the food (mixing it up) and breaking it into smaller pieces. This increases the '''surface area''' that is exposed to digestive enzymes. '''Chemical Digestion:''' Special cells lining the stomach secrete ''gastric juice'' that contains a strong ''protease enzymes'' (pepsin) and ''hydrochloric acid'' that gives gastric juice a pH of 1-2 (highly acidic). The resulting soupy mixture of partially digestive food and gastric juices is called ''chyme''. The low pH of the stomach aids digestion and helps to kill bacteria. ==Protease Enzymes== [image:http://i.imgur.com/CEiJyAw.png] Protease enzymes break down proteins into soluble amino acids which are small enough to be absorbed into the bloodstream. Protease enzymes secreted by the stomach lining function best in acidic environments. For instance a Pepsin (a powerful protease) has an optimum pH of around 2. Other digestive enzymes such as amylase (in saliva) will denature (lose their structure and function) at such a low pH. ==Ruminants== [image:http://i.imgur.com/Iu8Yy3M.png?2] Ruminants are a special group of '''herbivore''' mammals. Ruminants have large four-chambered stomachs that house specialised bacteria that help break down (ferment) plant material. Plant material is often rich in '''cellulose''' a tough carbohydrate that is considered a form of dietary fibre (indigestible portion of food) because mammals don't produce the enzymes that are required to break it down. In ruminants, the bacteria housed in the stomach produce ''cellulase'' an enzyme that breaks down cellulose into sugars. Ruminant stomaches have a relatively neutral pH as the highly acidic gastric juices found in other animals would kill the bacteria needed to digest cellulose rich plant materials. Because plant material is often tough and fibrous, ruminants often regurgitate ("throw-up") partially digested food (called cud) so that that it can be re-chewed.