GI PHYSIOLOGY III - DIGESTION Flashcards
TRUE/FALSE: enterocytes in the small intestine make digestive enzymes?
true
what are the main nutrients that undergo chemical digestion?
carbohydrates, proteins and lipids
what does carbohydrates store?
- polysaccharides
- large complex of monosaccharides
what are the ingested carbohydrates?
starch and glycogen
- long chains of glucose
disaccharides
- sucrose (glu + fruc)
- lactose (glu + galac)
- maltose (glu + glu)
limited amount of monosaccharides
glucose
what bond joins the chains of glucose in starch and glycogen?
alpha 1-4 glycosidic bond
what is the main source of energy for body?
carbohydrates
which are the essential amino acids that can not be synthesised?
histidine, leucine, lysine
which are the sources of protein?
- 50% from diet
- 50% endogenous proteins like enzymes and immunoglobulins
what is an important source of energy but not essential
lipids
what are the fat soluble vitamins?
ADEK
what are the ingested lipids mainly are?
triglycerides: glycerol back bone with 3 fatty acids attached
what are the average amount consume of the nutrients that needed chemical digestion per day?
- carbs: 250-800g
- proteins: 70-100g
- lipids: 100-150g
what is the role of chemical digestion?
reduces the size of nutrients allow them to be absorbed
what is the role of mechanical digestion that assist chemical digestion?
mechanical digestion breaks up food, increases surface area available for chemical digestion
TRUE/FALSE: digestive enzymes are extracellular
true
list the optimal pH for each type of digestive enzyme?
- salivary and small intestinal enzymes: alkaline
- gastric: acidic
what are the 2 stages of chemical digestion?
- luminal digestion
- contact digestion
what is luminal digestion and where does it occur?
- initial digestion involving enzymes secreted into lumen
- salivary glands, stomach, small intestine - pancreatic
what is contact digestion and where does it happen?
- completes digestion before absorption, involves enzymes produced by enterocytes and attached to brush border of enterocytes
- in small intestine
what happens during luminal digestion of carbs?
- salivary and pancreatic amylase
- polysaccharides converted to oligosaccharides and disaccharides
what happens during contact digestion of carbs?
- involves brush border enzymes: disaccharidases
- disaccharides converted to monosaccharides
what happens during luminal digestion of proteins?
- pepsin in stomach
- trypsin and chymotrypsin to break down peptide bonds in small intestine
- carboxypeptidase acts as COOH terminus in small intestine
- convert proteins to polypeptides
where is pepsin, trypsin, chymotrypsin and carboxypeptidase secreted?
- pepsin: precursors secreted by chief cells in stomach
- trypsin, chymotrypsin, carboxypeptidase: pancreas
what occurs during contact digestion of protein?
many types of enzymes attached to brush borders, involved peptidase to convert polypeptides to individual amino acids
where does fat digestion happen and does it involves both stage of digestion like carbs and proteins?
- in lumen of small intestine
- NO contact digestion
what is the main digestive enzyme of lipid digestion? what are the minor ones?
- pancreatic lipase main
- lingual lipase and gastric lipase minor
why is lipid digestion harder than carbs and proteins’?
because normally, digestive enzymes would dissolve in aqueous luminal fluid, however fat is insoluble in water and being seperated from the aqueous fluid
what are the stages of lipids digestion? what does it mean?
- emulsification: motility
- stabilisation using bile salts
- digestion/hydrolysis using enzymes
- formation of micelles using bile salts
what does emulsification do to the lipid molecule and how does that assist the digestion?
- motility breaks up lipid droplets into small droplets
- increase surface area for digestion
where does lipid emulsification occur in?
- stomach: retropulsion as simple emulsion
- small intestine: segmentation with bile salt stabilised the droplets
where does stabilisation happen in?
small intestine
describe structure of bile salt and what does it do to the lipid droplets?
- have a hydrophobic and negative charged hydrophilic side
- the hydrophobic site + phospholipid attach to the droplets and stabilise as well as reduce the size of the emulsion droplets to increase the surface area
where does hydrolysis occur in?
small intestine at surface of emulsion droplets
what are the enzymes involved in hydrolysis stage of lipid digestion? where are they secreted from? what do they do?
- lipase and colipase
- colipase anchors lipase to surface of droplet
- lipase converts triglycerides to monoglyceride and free fatty acid
however, after hydrolysis, monoglyceride and fatty acid are still insoluble inwater, therefore, they needed to be kept in solution through a formatioin of _______________
micelles
what is micelles, describe the structure of micelle?
- small droplets around 4-6nm diameter
- consist of 20-30 molecules of: bile salts, fatty acids and monoglycerides
what does amphipathic means
TRUE/FALSE: bile solubilises the products of lipid digestion
- amphipathic = have both hydrophylic and hydrophobic site
- true (remember bile is not a lipolytic enzyme, it just assist the enzymes)
