GI digestion and absorption Flashcards
Define digestion
the breakdwon of nutrients into absorbale molecules
Define absoprtion
movement of nuterients, water and electrolytes from the gut lumen into the internal environment
What is the surface of the small intestine arranged in?
circular fold of Keckring villi project from the folds.
What are the surface of villi covered with?
epithelial cells (enterocytes) and mucus secreting cells (goblet cells). Apical surface of epithelial cells are covered by microvilli - brush border
What are the main cabrohydrates in “an avergae, western diet?
- starch
- glycogen]
- cellulse
- disaccharides
What are the main lipids within “an average western diet”?
- triacylglycerols
- phopsholipids
- cholestrol and cholestrol esters
- free fatty acids
- lipid vitamins
Where is the main supply of protein in “an average western diet” from?
ingested
endogenous sources - digestive enzymes and dead cells from GI tract
Name 4 fat soluble vitamins found in a normal diet?
- Vitamin A
- Vitamin D
- Vitamin E
- Vitamin K
Name the main water soluble vitamins in an “average western diet”
- B1/B2/B3/B5/B6/B12
- folate
- biotin
- Vitamin C
Name the main minerals found in an “average western diet”
Ca, Fe, Zn, Mn, Mg, Phosphorous
Name the pathological conditions assoicated with defiency in:
- vitamin D
- vitamin C
- B12
D = rickets
C = scruvy
B12 = anaemia
Overview of digestion and absoprtion in the mouth
very little digestion
small amount of lipid, CHO
almost no absorption (certain drugs)
Overview of digestion and absoprtion in the stomach
some digestion
protein but not very important
almost no absorption (certain drugs)
Overview of digestion and absoprtion in the small intestine
lots
CHO, lipid, protein
vital site of digestion and absorption, (some regional differences between duo, ileum, jejunum)
Overview of digestion and absoprtion in the large intestine
almost none (apart from water), some “indigestible” substances used as fuel by gut flora
What type of carbohydrates can be absorbed?
monosaccharides
Carboydrate digestion in the mouth
- salivary α-amylase
How does salivary α-amylase digest food in the stomach despite the acid pH
enzyme is contained with the bolus of food and this is protected from the pH of the stomach until broken down further
What enzymes act on carbohydrate digestion in the duodenum?
- Pancreatic amylase
- Brush border enzymes (maltase, sucrase & lactase) - act on disaccharides, producing monosaccharides-fructose, glucose & galactose
What are the digestible carbohydrates?
Starch (from plants) & glycogen (from animals):
Linear chains of glucose molecules joined by α 1-4 glycosidic bonds initially digested by amylase
What are indigestible carbohydrates?
Cellulose (from plants):
Linear chains linked by β 1-4 glycosidic bonds. No enzymes in humans to digest cellulose
What are amylases?
free enzymes acting in the GI lumen and digest only internal α-1-4 glycosidic bonds
Describe salivary amylase
secreted from mouth in response to sight & smell of food - of lesser importance than
Describe pancreatic amylase
secreted from pancreas into duodenum
o Produces maltotriose, maltose & α-limit dextrins
Draw a diagram showing the digestion of a polysacchride by pancreatic amylase
What are the products of amylase digested by?
digestedbyoligosaccharidases (attached to the enterocyte mucosal membrane of the brush border of epithelial cells)
Name 2 oligosaccharides and give their main action
- α-glucosidase – cleaves α-1-4 glycosidic bonds to remove single glucose units from the non reducing end of the polymer
- Isomaltase - cleaves α-1-6 glycosidic bonds in the α-limit dextrin oligosaccharides
Draw a diagram showing the action of oligosaccharides on α-limit dextrin
What are the 3 enzymes that result in digestion to disacchardies?
products of amylase and oligosaccharidases are hydrolysed by the disaccharidases which are attached to the brush border membrane
- Maltase – produces glucose
- Sucrase – produces glucose & fructose
- Lactase – produces galactose & glucose
What are the final products of carbohydrate digestion?
glucose, fructose and galactose
What are the 2 methods by which carbohydrates are absorbed?
- Secondary active transport
- SGLT 1 (sodium dependent glucose transporter 1) located on the apical membrane transports glucose and galactose
- Facilitated diffusion
- GLUT 5 (glucose transporter 5) transports fructose across the apical membrane
Draw a diagram indicating the different types and locations of carbohydrate absoprtion
Where does protein digestion begin?
in the stomach with pepsin
completed in the small intestine with pancreatic and brush-border proteases
Action of endopeptidases
hydrolyses the interior peptide bonds of proteins
Action of exopeptidases
hydrolyse one amino acid at a time from one end of the protein
Name the main endopeptidases
- Pepsin (stomach)
- Trypsin (SI)
- Chymotrypsin (SI)
- Elastase (SI)
Action of pepsin
hydrolyses links with tyrosine, D alanine & leucine
- hydrolyses long polypeptide chains into shorter ones
Action of trypsin
hydrolyses links with arginine & lysine
- Trypsin inhibitor - Protection of self
- Small protein, present in pancreatic juice to inhibit any trypsin found prematurely in the pancreatic cells or duct
Action of chymotrysin
hydrolyses links with tyrosine, tryptophan, D alanine, methionine & leucine
Action of elastase
degrades elastin
Name the main exopeptidases
- Carboxypeptidases (C-terminal)
- Aminopeptidases (N-terminal)
What are the products of stomach and pancreatic luminal enzymes?
- Oligopeptides
- Amino acids
What continues protein digestion via brush border enzymes?
- Oligopeptidase
- Aminopeptidase
What are the products of protein digestion of brush border enzymes?
- Amino acids
- Dipeptides
- Tripeptides
Draw a digram highlighting the different types of digestion in the GI tract by proteases
Mechanisms of protein absoption
- Na+-dependant cotransport
- H+-dipeptide cotransport
- H+-tripeptide cotransport
Describe the 4 initial stages of lipid digestion?
- Dietary lipids are hydrophobic (insoluble in water) so causes problems with digestion and abdoprtion
- must be solublised before digestion and absoprtion can occur
- digestion begins in the stomach with the action of lingual and gastric lipases
- digestion is completed in the small intestine with the action of pancreatic enzymes
Types of lipids
- fats/oils
- phospholipids
- cholestrol and cholestrol esters
- fatty acids
3 main types of enzymes involved in lipid digestion
- lipases
- phsophlipases
- cholestrol esterases
- (bile salts also have an important role in digestion/ absoprtion but are not enzymatic)
Describe lipid digestion in the mouth and stomach of TAGs by lipases
o Heat & movements in stomach mix food with lipases lipid emulsion
o Hydrolysis initially slow due to largely separate aqueous/lipid interface
o As hydrolysis proceeds, rate increases due to fatty acids produced acting as surfactants breaking down lipid globules aiding emulsification
o Emulsified fats ejected from stomach to duodenum
Describe lipid digestion in the duodenum
- Pancreatic lipase (-important- the main lipid digestive enzyme)
- Aided by bile salts from gall bladder
- HCO3- neutralises stomach acid suitable pH
Describe the digestion of lipids in the small intestine
- Bile salts, lysolecithin and products of lipid digestion emulsify dietary lipids
- Emulsification produces small droplets of lipids dispersed in an aqueous solution creating a large surface area for pancreatic enzyme digestion
- Pancreatic enzymes (pancreatic lipase, cholesterol ester hydrolase and phospholipase A2) and the protein, colipase are secreted to complete digestion
What are micelles?
small disks with bile salts, phospholipids, fatty acids, cholestrol and mon/di-glycerides
Why do bile salts coat lipids?
to make emulsions
Describe stage 1 of the digestion and absoprtion of lipids
Bile salts from liver coat fat droplets
Describe stage 2 of digestion and absoprtion of lipids
Pamcreatic lipases and colipase break dwon fats into monoglycerides and fatty acids which are stored in micelles
Describe stage 3 of digestion and absoprtion of lipids
- Monoglycerides and fatty acids move out of micelles and enter cells by diffusion
- Cholestrol is transported into cells by a membrane transporter
Describe stage 4 of digestion and absoprtion of lipids
Absrobed fats combine with cholestrol and protein in the intestinal cells to form chylomicrons
Describe stage 5 of digestion and absoprtion of lipids
chylomicrons are released into the lymphatic system
What are mixed micelles?
- Productsoflipiddigestion(cholesterol,monoglycerides, lysolecithin and free fatty acids) are solubilised in mixed micelles (diameter 5 nm)
- Core contains the products of lipid digestion
- Surface coating of bile salts which are amphipathic
What are chylomicrons?
- Chylomicrons (100nmdiameter) have a core of triglycerides and cholesterol ester - phospholipids and apoproteins on the outside (80%/20%)
- Chylomicrons are packaged into secretory vesicles on the Golgi membrane and are exocytosed across the basolateral membrane
- Chylomicrons are too big to enter vascular capillaries but enter the lymphatic capillaries (lacteals) by moving between the endothelial cells that line the lacteals.
- The lymphatic circulation carries the chylomicrons to the thoracic duct which empties into the blood stream
Where does absoprtion of lipids occur?
small intestine
What mechanisms are used to digest and absorb lipids?
- Bile salts form micelles
- Diffusion of products into intestinal cells
- Re-esterification within the cell to triglycerides and cholesterol
- Chylomicrons form in the cell and transfer to lymph
What is the problem with bile salts?
- There aren’t enough bile salts to deal with theaverage meal
- ~3-4g in body, but ~3-15g required per meal
- After use, most reabsorbed from terminal ileum into the liver via enterohepatic circulation (via hepatic portal vein)
Describe absoprtion of water and NaCl is large intestine
Mechanisms complex and not fully understood
- Na+ actively absorbed (in exchange for K+)
- K+ reabsorbed in exchange for H+
- Cl- absorbed (in exchange for HCO3-)
- H2O follows due to osmosis
Controlled by enteric nerve plexi and hormonal control o Aldosterone (↑ water abs)
Bacterial actions in the LI
- Movement thro’ LI very slow bacteria thrive (1011 bacteria / g faeces)
- Formation of certain B vitamins (B12, thiamin, riboflavin)
- Formation of vitamin K
- B and K absorbed passively (probably –mechanisms remain uncertain
- “Digestion”
- Certain bile constituents
- Other “indigestible” matter
Basoprtion of fluid in the SI and LI
- ~ 9L of fluid enters GI tract each day
- (approx. 2.3L from ingestion, the rest from secretion)
- Small intestine reabsorbs ~ 8L
- Large intestine reabsorbs ~ 90% of the last litre by osmosis through cell walls into vascular capillaries inside villi
