Digestion
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- the digestive system functions to break down the macromolecules found in the diet into usable forms for the body
- digestion is the process by which large chunks of food are broken down into molecule-sized pieces
- products of digestion are used for energy, building blocks, and as enzymes and/or coenzymes
- the digestive system is a long tube that extends from the mouth to the anus, and functions to perform four functions:
1. ingestion (eating)
2. movement of the food through the tract, which is accomplished by peristalsis (smooth muscle contractions)
3. digestion ? the breakdown of food by both mechanical (chewing and scrunching) and chemical (enzymatic) means
4. absorption ? the nutrients are absorbed from the digestive system into the blood and lymph from the small and large intestines
5. elimination ? defecation ? the elimination of indigestable/unabsorbed materials from the body
- the digestive system consists of two basic groups ? the GI tract (the tube) and the accessory structures
- the GI tract is a tube from 6.5 to 9 m long in adults
Mouth
- the mouth is where mechanical and chemical digestion begins
- the food is moistened by saliva produced by the salivary glands(we produce 1.7 L per day), which moistens the food to make it easier to swallow, and also contains the first digestive enzymes:
- amylase (enzyme names end in ase) which begins the breakdown of starch into maltose (a disaccharide) and dextrins (short glucose chains), and maltase, which breaks maltose into glucose
Swallowing
- the tongue is used to push the food to the back of the mouth ? this area is called the pharynx, the presence of the food initiates the swallowing reflex
- the epiglottis, a small flap of tissue will close over the trachea, preventing food from entering the lungs
- the bolus of food is then swallowed, and travels down the 25 cm oesophagus to the stomach, helped by peristalsis (you can swallow while standing on your head!)
- at the bottom of the oesophagus is the lower oesophageal sphincter (or cardiac sphincter) ? a small ring of smooth muscle that keeps food in the stomach
Stomach
-
the stomach holds up to 2L (?4L) of food, and will hold food for from
- There are three important chemicals in the stomach involved in digestion:
- HCl(aq) ? secreted by the parietal cells of the stomach, lowers the pH of the stomach to 2.0, allowing the activation of pepsin to pepsinogen
- Pepsin ? secreted as an inactive precursor, pepsinogen (cannot be active or it would digest cellular proteins) by the peptic (chief) cells. It is activated by the lowered pH in the stomach (which changes the shape of the enzyme)
- mucous ? secreted by the mucous (goblet) cells, protects the stomach endothelium from the acid and enzymes
- Two other secretions:
- gastrin ? a hormone that stimulates gastric secretions (which is stimulated by the presence of proteins in the stomach), and relaxes the pyloric sphincter to allow stomach emptying
- rennin ? found principally in children, it is involved in the digestion of milk ? slowing its emptying from the stomach
- physical digestion - the acid along with the peristaltic motions of the stomach contents helps to break up tissues
- the pepsin will break up proteins into smaller polypeptides called peptones (protein digestion will be completed in the small intestine)
- most materials are not absorbed in the stomach, but some drugs (notably aspirin), some water, electrolytes, alcohol and some are absorbed in the stomach
- the highly acidic stomach contents, called chyme is emptied into the small intestine a bit at a time through the pyloric sphincter
Small Intestine
- measures up to 7m in length, but only 2.5 cm in diameter, divided into three sections
- duodenum ? 25 cm long, where most digestion occurs
-
focusing on absorption
jejunum ? 3 m long
- ileum ? 4 m long
- the majority of digestion and absorption (90%) takes place here ? lipid digestion, continuation of carbohydrate and protein digestion
- the accessory organs of digestion, the pancreas, gall bladder and the liver secrete their juices into the duodenum to aid in digestion
- the small intestine, in order to better digest and absorb digested materials is highly folded to increase surface area (by 600x); the folds are called villi, which are covered in small cytoplasmic projections called microvilli, the structure of the small intestine (from the outer surface in) includes:
- villi/microvilli ? surface (epithelial) cells designed for absorption
- lacteal ? vessel projection of the lymphatic system, designed to absorb fats
- capillaries ? vessels of the circulatory system, for absorption of all other nutrients for immediate transport to the liver for processing
- as the chyme enters the duodenum, its acidity and contents stimulates a number of hormones? release, which in turn stimulate the secretion of digestive and protective chemicals from accessory organs and the intestinal epthelium itself
hormones:
- enteric gastrin ? stimulated by partially digested proteins to cause more gastric juice secretion and promote stomach emptying
- cholecystokinin (CCK) ? stimulated by partially digested proteins and irritants in the chyme, stimulates pancreatic enzyme release and bile from the gallbladder
- secretin ? in response to acids in the duodenum, inhibits the secretion of gastric juice, stimulates sodium bicarbonate release by the pancreas and stimulates bile secretion by the liver
accessory organs:
Pancreas
- a soft tubular gland that lies just behind the stomach, and is connected to the duodenum by two ducts
- has both a exocrine (secretory) and endocrine (hormonal) function
- its exocrine functions are to secrete digestive enzymes and sodium bicarbonate to neutralize the stomach acid and establish a pH of 7.1-8.2 which will not only neutralize the enzyme pepsin, but activate the pancreatic enzymes
- enzymes:
- pancreatic amylase ? digest carbohydrates ? maltose
- trypsin ? protein digestion: peptones ? small polypeptides (activated by enterokinase, secreted by the intestinal wall)
- chymotrypsin ? protein digestion: small polypeptides ? peptides (activated by trypsin)
- carboxypeptidase ? digests polypeptides ? amino acids
- lipase ? digests fats into fatty acids and glycerol
- ribonuclease ? digests RNA to nucleotides
- deoxyribonuclease ? digests DNA to nucleotides
Liver
- the liver has over 500 functions, three being fundamental:
- production of bile
- storage of glucose in the form of glycogen (? fat if glycogen limits exceeded), conversion of galactose and fructose to glucose
- detoxification of the blood (makes enzymes to break down toxins; ex: alcohol, caffeine, nicotine, barbiturates, poisons, excess hormones)
- deamination of amino acids ? removing nitrogen, producing ammonia and eventually urea (excreted by the kidneys)
- it receives two separate blood supplies
- via the portal vein ? bringing freshly absorbed nutrients from the small intestine
- via the hepatic artery ? bringing oxygenated blood from the lungs/heart
- bile
- each day, the liver secretes between 800 ? 1000 mL of bile
- bile is stored in the gallbladder for release on demand into the small intestine
- consists of water, bile salts, cholesterol and bile pigments (made from bilirubin ? yellow in colour)
- bile acts as an emulsifier, breaking large fat globules into small ones, allowing lipase more surface area for digestion
Gallbladder
- is lodged in one of the lobes of the liver
- a light muscular bag that stores and releases bile
- why store bile? We eat large quantities of fats at a time, so having a store of bile is useful but can lead to problems ? bile salts can crystallize in the gallbladder forming gallstones
back
to digestion?.
- the combination of hormones and the presence of certain foods leads to the secretion of pancreatic enzymes, bile, and intestinal enzymes (maltase, lactase, sucrase and enterokinase), all of which are active in the slightly basic pH of the duodenum
- the villi of the small intestine are specialized to absorb the molecules of digestion:
- large surface area
- transport proteins on the epithelial cells move aa, glucose, water soluble vitamins, etc. by facilitated diffusion and active transport into the capillary system, then on to the liver immediately for processing
- electrolytes by diffusion, water by osmosis
- fatty acids and glycerol are absorbed into the epithelial cells and are repackaged as triglycerides
- being hydrophobic, they are packaged into protein packs to enable their transport through the body
- the packaged fats are absorbed into the lacteals in the interior of the villi, and are transported through the body?s lymphatic system
- fat soluble vitamins will move with fats into lacteals for absorption in the body
Large
Intestine
- about 1.5 m in length, a diameter of 6.5 cm.
- at the junction of the small and large intestine, the ileum exists the appendix, which has no real function given our diet
- the large intestine has three main functions:
- absorption of water and electrolytes
- production of feces ? consisting of water, inorganic salts, cells from the GI tract, bacteria, bacterial decomposition and undigested food
- housing bacteria, that will use remaining unabsorbed nutrients or undigested carbohydrates to make vitamin K and B vitamins for us to absorb (as well as methane gas)
- Lab ? pH and enzyme activity
- amylase & starch (use Benedict?s test for sugars as products)
- pepsin & protein (Biuret test ? more violet, more peptide bonds)
-
Digestive System Websites
http://kidshealth.about.com/kids/kidshealth/msub41.htm
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by: Admin
Total views: 2249
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Date: Sat, 31 Mar 2007 Time: 12:00 AM
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Source: http://members.shaw.ca/zsearle/bio20IB/bio20IB.htm
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