Your stomach coats itself in slime to survive its own acid
Gastric juice is built to dismantle protein, and the stomach wall is made of protein. To keep from digesting itself, the stomach secretes mucus and bicarbonate that form a slippery shield against concentrated hydrochloric acid. That is one small step in a long chain that turns a sandwich into molecules small enough to enter the blood.
Breakdown starts before swallowing. Chewing splits food physically, and saliva adds an enzyme, salivary amylase, that begins attacking starch, along with mucus for lubrication and hydrogencarbonate to hold the pH where amylase works best. Roughly 30 percent of starch is turned into double sugars in the mouth. The chewed food, now a small round mass called a bolus, is squeezed down the oesophagus by waves of muscle contraction.
In the stomach, hydrochloric acid provides the acidity the enzyme pepsin needs to cut proteins into shorter chains; in babies and toddlers the juice also contains rennin for milk proteins. Churning keeps mixing everything. When the pyloric valve opens, the resulting slurry, chyme, passes into the duodenum to meet pancreatic enzymes and bile from the liver. About 95 percent of nutrient absorption happens in the small intestine. The colon, mildly acidic at a pH of about 5.6 to 6.9, reclaims water and minerals and absorbs vitamins such as biotin and vitamin K made by resident bacteria. The stomach itself absorbs some water, simple sugar and alcohol.
Not every organism digests inside a tube. External digestion is older in evolutionary terms and most fungi still rely on it, releasing enzymes into their surroundings and absorbing some of what diffuses back. Nearly all spiders do something similar, injecting toxins and digestive chemicals into prey and then consuming the resulting soup. Animals' internal gut is more efficient because it captures more of the products and controls the chemistry better.
Bacteria have their own machinery. Some species of Salmonella, Shigella and Yersinia use a type III secretion system, a molecular syringe first found in Yersinia pestis that injects substances straight into a host cell's interior. Agrobacterium tumefaciens transfers a plasmid into plant cells, reprogramming them to manufacture opines that feed the bacteria and causing crown gall tumours.
Source: Digestion
