Transcript Digestion2
Digestion: Day 2 Digestion requires “juice” • Salivary glands – secrete saliva into the mouth which contains salivary amylase; solvent is water • Gastric glands – located in the inner lining of the stomach; secrete mucus, hydrochloric acid and pepsin; solvent is water – Gastric juice is not secreted at all times by the stomach (if it did, it would be wasteful and potentially harmful) but instead gastric juice is secreted at the sight or smell of food – Once food enters the stomach, receptor cells in stomach wall send chemical signals to the brain so that more gastric juice is secreted – Stomach distension produces the hormone gastrin which continues the release of gastric fluid • Pancreas – sends pancreatic juice to the small intestine via duct; juice contains trypsin, amylase, lipase and biocarbonate (to help neutralize acid fluids from stomach); solvent is water • Liver – makes bile then stores in gall bladder; bile moves from gall bladder to small intestine via bile duct. • Bile emulsifies lipids to increase surface area for the action of lipase. • Emulsification can occur b/c bile molecules are polar (have a hydrophilic end and a hydrophobic end) and thus are partially soluble in both lipids and water • Intestinal glands – some cells on inner lining of small intestine are glandular cells; these 1) secrete digestive enzymes that are added to partially digested food or 2) some stay attached to villi – Enzymes that are secreted mix with substrates (food particles) in a molecular “soup” and these enzymes have a short lifespan – Membrane-bound enzymes (e.g. maltase which breaks down maltose into two glucose molecules) have longer life spans and by being located on the inner lining, they are in the perfect spot to allow absorption after breakdown Why doesn’t the alimentary canal digest itself? • Two digest enzymes (pepsin and trypsin) hydrolyze proteins, but they do NOT differentiate ingested proteins from cellular proteins therefore they are initially synthesized in inactive forms known as pepsinogen and trypsinogen • Pepsinogen is secreted into stomach, it mixes with hydrochloric acid which removes extra amino acids; now it is pepsin • Trypsinogen secreted from pancreas but when it mixes in the small intestine a enzyme enterokinase converts it to trypsin Why can’t humans digest cellulose? • No mammals produce the enzyme cellulase necessary to digest cellulose (polysaccharide composed of thousands of glucose molecules linked together); “grazers” have mutualistic bacteria that produce the enzyme for them • Human that eat plant material can’t digest it and therefore it makes its way through and out the alimentary canal • • • • Other Items Humans CANNOT Digest: Lignin – component of plant cell walls Bile pigments – give color to feces Bacteria – normal inhabitants of our digestive tract Intestinal cells – these break off as foods move through the lumen * Note: All of these items, along with cellulose, become a waste product and exit the body as feces What causes stomach ulcers? • Helicobacter pylori enter the stomach and attach to the protective mucus lining of the stomach wall. The bacteria are able to survive in the acid enviro b/c they excrete the enzyme urease which neutralizes the acid by converting urea (lots in stomach due to saliva and gastric juices) into ammonia (pH ~11.5). This creates cloud of safety and inside the mucus lining of the stomach wall, the bacteria cannot be destroyed by immune system. • The H. pylori produce toxins that cause the cells in the lining of the stomach to die leaving holes that can end as ulcers • There has been a correlation between H. pylori infections and