6.1 Digestion and Absorption Flashcards
Draw a labelled diagram of the digestive system with the key structures, show relationship of pancreatic duct, bile duct and small intestine
mouth - voluntary control of eating and swallowing, mechanical digestion of food by chewing nad mixing with saliva, which contains lubricatns and enzymes that start starch digestion
esophagus - movemnet of food by peristalsis to stomach
sotmach - churning and mixing with secreted water and acid which kills foreign bacteria and other pathogens in food, plus initial stages of portein digestion
small intestine - final stnges of digestion of lipids, carb, proteins, nucleic acdis, neutralization of stomach acid, absorption of nutrients
pancreas - secretion of lipase, amylase and protease
liver - secretion of surfactants in bile to break up lipid droplets
gall bladder - storage and regulated release of bile
large intestine - re-absorption of water, further digestion of carbs by symbiotic bacteria, formation and storage of feces
what is the structure of the small intestine?
serosa - outer layer
muscle layers - longitudinal muscles and then circular muscles
sub-mucosa - tissue layer with blood and lymph vessels
mucosa - the lining of the small intestine with the epithelium that abosrbs nutrients on its inner surface
describe perstalsis
this is the contraction of circular and longitudinal muscles layers of the small intestines mixing food with enzymes as it moves along the gut
smooth muscle used not striated (involuntary) controlled not be brain but by the entric nervous system
short cells not elongated fibres
contiuous moderate force with short periods of vigorous contraction
happens behind food to stop it moving backwards
food moves down esophagus in one continuous peristaltic wave and only in one direction - away from the mouth
during vomiting the abdominal muscles are used not the esophagus muscles
in the intestine peristalsis is very slow to allow absorption
what is pancreatic juice
the pancreas has two types of gland cells secreting insulin and glucagon to the blood
the pancreas responds to food, also hormone produced by the gut and the enteric nervous system
ducts wihtin the pancreas merge into larger ducts finally forming one pancreatic duct throguh which about 1 liter of pancreatic juice is secreted a day
the rest of the pancreas makes
- amylase
- lipases
- proteases
digestion in the small intestine
things that happen
- starch to maltose by amylase
- triglycerides to fatty acids and glycerol or fatty acids and monogylcerids by lipase
- phospholipids to fatty acids, glycerol and phosphate by phospholipase
- proteins and polypeptides to shorter peptides by protease
- nucleases to DNA and RNA into nucleotides
- maltase to maltose to glucose
- lactase to lactose to glucose and galactose
- sucrase to sucrose to glucsoeand fructose
- dipeptidases to dipeptides to amino acids
- exopeptidases are proteases turn peptides into amino acids by removing single amino acids at a time from carboxy or amino terminal until there is only a dipeptide
some things are not digested e.g. cellulose goes into teh large inytestine as one of tjhe main components of dietary fibres
some enzymes are immobilised in the walls of the small intestine (in the epitehlium cells) the are active and continue to be active even when teh epitehelium cells are abraded and mix with the food
where do villli work
increase the surface area of epithelium cells
small inestinge is ~7 meters long and 25 - 30 mm wide with fold to increase surface area
villi are on teh mucosa, between 0.5 and 1.5 mm long and as many as 40 per square mm
increase the surface aresa by about 10
what is the job of villi
epithelium must form a barrier to harmful substances and permeable for nutrients
villi absorb
- glucose, fructose, galactose and other monosaccarides
- all 20 amino acids
- fatty acids, monoglycerides, glycerol
- bases from nucleioties
- minerals ions
- vitamins e.g. ascorbic acid (vitamin c)
some bad substances are absorbed but then removed from blood and detoxified by the liver - some harmless but unwanted substances are also absorbed e.g. food colourings - pass out in urine
some bacteria pass through but are killed by phagocytes in liver
see page 284 for diagram - different methods of absorption in the small intestine of things such as glucose, NA + and triglycerides
triglycerides
- broken down into fatty acids and monoglycerides and then absorbed by simple diffusion (monoglycerides) and and facillitated diffusion (fatty acids) by fatty acid transporters proteins on the microvilli
- inside the epitehlium cells fatty acids comibne to produce the triglcyerides again which cannot diffuse back out
- they then coalesce with cholesterol to form droplets of 0.2 micrometers which become coated in phospholipids and proteins
- these are released by exocytosis through teh plasma membrane on teh inner side of the epithelium vilus cell where they enter teh lacteal and are carried away in the lymph or enter the blood capillaries in the villi
na and glucose
- sodium potassium pumps in teh inward facing part of the plasma membrane pump sodium ions by active transport into teh interstitial spaces inside teh villus and potassium ion sin the opposite direction , therefore tehre is a low sodium conc in epithelium cells
- sodium-glucose co transporter proteins in teh microvilli transfer a na and glucose from teh intestinal lumen to teh cytoplasma of epithelium cells - passive faciliatated duffusion but needs to conc gradient of na
- glucose channels allow teh glucose to move by faciliated diffusion from teh cytoplasm to teh interstital spaces inside the villus amd on into the blood capillaries in teh villus
how is starch digested
hydrolysis by amylase makes
- maltose (2) whihc is digested my maltase into (1 and 1)
- dextrins (branch) which is digested by dextrinase into (4x 1)
- maltotriose (3) which is digested by glucosidase into (2 and 1)
