BIOL 1030 – TOPIC 19 LECTURE NOTESTopic 19: Digestive System (Ch. 41)I. Function – break down food into organic molecules small enough to be used by cellsA. carbohydrates à monosaccharides (simple sugars) [major energy source]B. proteins à amino acids [minor energy source; major building blocks]C. lipids à fatty acids and glycerol [major energy source; major building blocks]D. also absorbed: various vitamins and minerals, and waterII. Types of digestion in animalsA. intracellular only – no digestive cavity1. cells take in and break down their own food2. found in spongesB. extracellular – digestive cavity or tube (true digestive system)1. mechanical digestion: may include some type of grinding mechanism (if this occurs, it typically happens first)2. chemical digestion (hydrolysis – breaking polymers by adding water)- carbohydrates: amylases- proteins: acid digestion, plus proteases- lipids: emulsification by bile; lipases3. incomplete digestive system- one opening (serves as mouth and anus)- gastrovascular cavity with no specialization- examples: jellyfish, some flatworms4. complete digestive system- two openings (mouth and anus) with mostly one-way passage of food- digestive tract (alimentary canal) typically broken into specialized regions- examples: earthworms, vertebratesC. none – some parasites absorb food already digested by others (example: tapeworms)III. Food sourcesA. herbivores – eat plants (and sometimes photosynthetic protists) exclusively (example: cows); food tougher to digest due to cellulose1 of 5BIOL 1030 – TOPIC 19 LECTURE NOTESB. carnivores – eat animals (and sometimes protists and bacteria) exclusively (example: cats) – food typically easier to digestC. omnivores – eat both plants and animals (example: humans) – must have adaptable digestive systemIV. Vertebrate digestive systemA. mouth1. all jawed vertebrates except birds have teeth- carnivores have more pointed, sharp teeth for capture, cutting, tearing, and shearing; meat needs little chewing- herbivores have large, flat teeth for grinding plant material- omnivores have both types, sharp in front and flat in back2. food is moistened and lubricated with saliva (also some digestion begins)- saliva made by three pairs of salivary glands- empty through ducts into mouth- contain amylase (start starch digestion)- tongue mixes food with salivaB. swallowing – a reflex under some voluntary control1. pressure on back of mouth triggers swallowing2. in mammals, soft palate elevates to seal off nasal cavity3. pressure on pharynx stimulates receptors to signal swallowing center in brain4. swallowing center signals muscles to contract, raising larynx to and closing glottis with epiglottis, sealing off trachea5. net result is food kept out of respiratory tract, instead directed to esophagusC. esophagus1. upper portion skeletal muscle, used to push food toward stomach as part of swallowing reflex2. lower 2/3 smooth muscle; propel food toward stomach in rhythmic peristaltic waves3. passage to stomach controlled by a sphincter (a ring of smooth muscle at the junction of two organs that constricts passage between the organs when contracted)D. stomach1. saclike portion of digestive tract, may be subdivided and further specialized- interior considered continuous with body exterior; is convoluted, allowing stomach volume to expand considerably as needed- from exposed inner layer out: epithelium, mucosa (connective tissue), then complex layer with muscles, blood vessels, and nerves2 of 5BIOL 1030 – TOPIC 19 LECTURE NOTES2. epithelial cells secrete mucus, protecting and lubricating stomach and facilitating food movement3. function: acid digestion of proteins- parietal cells in mucosa secrete HCl à gastric juice in stomach has pH of ~2- chief cells in mucosa secrete pepsinogen, activated to pepsin (a very active protease) at low pH- result is acid denaturing of proteins plus hydrolysis by pepsin- acid also kills most bacteria- no significant digestion of carbohydrates or lipids in stomach- no major food absorption in stomach (only water, alcohol, some drugs like aspirin)- mixture of gastric juice and food called chyme4. ulcers – problems with acid damaging the epithelial cells- rarely in stomach (gastric ulcers); >90% are duodenal ulcers (first segment of small intestine)- most involve Helobacter pylori bacteria weakening protective mucus layer; these respond to antibiotics5. leaving the stomach- pyloric sphincter controls movement from stomach to small intestine- only small amounts of chyme can be passed along at one time for efficient digestionE. small intestine – site of terminal digestion and absorption1. primary organ of digestion; 6 m long in humans2. divided into three segments: duodenum (first 25 cm in humans), jejunum, then ileum3. active digestion mainly in duodenum, food absorption into bloodstream mainly in jejunum and ileum4. digestion involves- pancreatic juice (from pancreas): bicarbonate (neutralizes stomach acid), amylases, proteases, and lipases- bile from liver (emulsifies fat) – may be temporarily stored in a gall bladder5. specializations of epithelium for absorption- small projections called villi, with cytoplasmic projections called microvilli- greatly increases absorptive surface area (over 300 m2 in humans)- brush border enzymes for final disaccharide digestion- active transport of monosaccharides and amino acids; passed across to blood capillaries- food in blood first taken to liver (main body filter for toxic substances)- free fatty acids and monoglycerides converted to triglycerides in epithelial cells, then combined with proteins to make chylomicrons that are passed to lymphatic capillaries (enter bloodstream at veins in the neck)3 of 5BIOL 1030 – TOPIC 19 LECTURE NOTES6. exit into large intestine through ileocecal valve (no sphincter)7. overall food and water absorption is very efficient- human daily intake: about 800 grams of solids, and a total solid plus food volume of 2 L- add to that about 7 L added by body during digestion, or 9 L total volume- at end, only 50 grams of solids and 100 mL of liquid leave as feces- fluid absorption 99% efficiency; if faulty, rapid dehydration occursF. large intestine (colon)1. actually shorter than small intestine, but wider, with no villi- first segment is cecum; in some, this is expanded and serves as a fermentation chamber or storage (human cecum notexpanded; narrow, vestigial appendix at end with no function)- has