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MD3002 - GI Physiology > GI digestion and absorption > Flashcards

GI digestion and absorption Flashcards

1
Q

Define digestion

A

the breakdwon of nutrients into absorbale molecules

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2
Q

Define absoprtion

A

movement of nuterients, water and electrolytes from the gut lumen into the internal environment

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3
Q

What is the surface of the small intestine arranged in?

A

circular fold of Keckring villi project from the folds.

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4
Q

What are the surface of villi covered with?

A

epithelial cells (enterocytes) and mucus secreting cells (goblet cells). Apical surface of epithelial cells are covered by microvilli - brush border

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5
Q

What are the main cabrohydrates in “an avergae, western diet?

A
  • starch
  • glycogen]
  • cellulse
  • disaccharides
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6
Q

What are the main lipids within “an average western diet”?

A
  • triacylglycerols
  • phopsholipids
  • cholestrol and cholestrol esters
  • free fatty acids
  • lipid vitamins
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7
Q

Where is the main supply of protein in “an average western diet” from?

A

ingested

endogenous sources - digestive enzymes and dead cells from GI tract

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8
Q

Name 4 fat soluble vitamins found in a normal diet?

A
  • Vitamin A
  • Vitamin D
  • Vitamin E
  • Vitamin K
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9
Q

Name the main water soluble vitamins in an “average western diet”

A
  • B1/B2/B3/B5/B6/B12
  • folate
  • biotin
  • Vitamin C
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10
Q

Name the main minerals found in an “average western diet”

A

Ca, Fe, Zn, Mn, Mg, Phosphorous

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11
Q

Name the pathological conditions assoicated with defiency in:

  • vitamin D
  • vitamin C
  • B12​
A

D = rickets

C = scruvy

B12 = anaemia

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12
Q

Overview of digestion and absoprtion in the mouth

A

very little digestion

small amount of lipid, CHO

almost no absorption (certain drugs)

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13
Q

Overview of digestion and absoprtion in the stomach

A

some digestion

protein but not very important

almost no absorption (certain drugs)

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14
Q

Overview of digestion and absoprtion in the small intestine

A

lots

CHO, lipid, protein

vital site of digestion and absorption, (some regional differences between duo, ileum, jejunum)

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15
Q

Overview of digestion and absoprtion in the large intestine

A

almost none (apart from water), some “indigestible” substances used as fuel by gut flora

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16
Q

What type of carbohydrates can be absorbed?

A

monosaccharides

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17
Q

Carboydrate digestion in the mouth

A
  • salivary α-amylase
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18
Q

How does salivary α-amylase digest food in the stomach despite the acid pH

A

enzyme is contained with the bolus of food and this is protected from the pH of the stomach until broken down further

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19
Q

What enzymes act on carbohydrate digestion in the duodenum?

A
  • Pancreatic amylase
  • Brush border enzymes (maltase, sucrase & lactase) - act on disaccharides, producing monosaccharides-fructose, glucose & galactose
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20
Q

What are the digestible carbohydrates?

A

Starch (from plants) & glycogen (from animals):
Linear chains of glucose molecules joined by α 1-4 glycosidic bonds initially digested by amylase

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21
Q

What are indigestible carbohydrates?

A

Cellulose (from plants):
Linear chains linked by β 1-4 glycosidic bonds. No enzymes in humans to digest cellulose

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22
Q

What are amylases?

A

free enzymes acting in the GI lumen and digest only internal α-1-4 glycosidic bonds

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23
Q

Describe salivary amylase

A

secreted from mouth in response to sight & smell of food - of lesser importance than

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24
Q

Describe pancreatic amylase

A

secreted from pancreas into duodenum

o Produces maltotriose, maltose & α-limit dextrins

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25
Q

Draw a diagram showing the digestion of a polysacchride by pancreatic amylase

A
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26
Q

What are the products of amylase digested by?

A

digestedbyoligosaccharidases (attached to the enterocyte mucosal membrane of the brush border of epithelial cells)

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27
Q

Name 2 oligosaccharides and give their main action

A
  • 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
28
Q

Draw a diagram showing the action of oligosaccharides on α-limit dextrin

A
29
Q

What are the 3 enzymes that result in digestion to disacchardies?

A

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
30
Q

What are the final products of carbohydrate digestion?

A

glucose, fructose and galactose

31
Q

What are the 2 methods by which carbohydrates are absorbed?

A
  • 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
32
Q

Draw a diagram indicating the different types and locations of carbohydrate absoprtion

A
33
Q

Where does protein digestion begin?

A

in the stomach with pepsin

completed in the small intestine with pancreatic and brush-border proteases

34
Q

Action of endopeptidases

A

hydrolyses the interior peptide bonds of proteins

35
Q

Action of exopeptidases

A

hydrolyse one amino acid at a time from one end of the protein

36
Q

Name the main endopeptidases

A
  • Pepsin (stomach)
  • Trypsin (SI)
  • Chymotrypsin (SI)
  • Elastase (SI)
37
Q

Action of pepsin

A

hydrolyses links with tyrosine, D alanine & leucine

  • hydrolyses long polypeptide chains into shorter ones
38
Q

Action of trypsin

A

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
39
Q

Action of chymotrysin

A

hydrolyses links with tyrosine, tryptophan, D alanine, methionine & leucine

40
Q

Action of elastase

A

degrades elastin

41
Q

Name the main exopeptidases

A
  • Carboxypeptidases (C-terminal)
  • Aminopeptidases (N-terminal)
42
Q

What are the products of stomach and pancreatic luminal enzymes?

A
  • Oligopeptides
  • Amino acids
43
Q

What continues protein digestion via brush border enzymes?

A
  • Oligopeptidase
  • Aminopeptidase
44
Q

What are the products of protein digestion of brush border enzymes?

A
  • Amino acids
  • Dipeptides
  • Tripeptides
45
Q

Draw a digram highlighting the different types of digestion in the GI tract by proteases

A
46
Q

Mechanisms of protein absoption

A
  • Na+-dependant cotransport
  • H+-dipeptide cotransport
  • H+-tripeptide cotransport
47
Q

Describe the 4 initial stages of lipid digestion?

A
  1. Dietary lipids are hydrophobic (insoluble in water) so causes problems with digestion and abdoprtion
  2. must be solublised before digestion and absoprtion can occur
  3. digestion begins in the stomach with the action of lingual and gastric lipases
  4. digestion is completed in the small intestine with the action of pancreatic enzymes
48
Q

Types of lipids

A
  1. fats/oils
  2. phospholipids
  3. cholestrol and cholestrol esters
  4. fatty acids
49
Q

3 main types of enzymes involved in lipid digestion

A
  • lipases
  • phsophlipases
  • cholestrol esterases
  • (bile salts also have an important role in digestion/ absoprtion but are not enzymatic)
50
Q

Describe lipid digestion in the mouth and stomach of TAGs by lipases

A

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

51
Q

Describe lipid digestion in the duodenum

A
  • Pancreatic lipase (-important- the main lipid digestive enzyme)
  • Aided by bile salts from gall bladder
  • HCO3- neutralises stomach acid  suitable pH
52
Q

Describe the digestion of lipids in the small intestine

A
  1. Bile salts, lysolecithin and products of lipid digestion emulsify dietary lipids
  2. Emulsification produces small droplets of lipids dispersed in an aqueous solution creating a large surface area for pancreatic enzyme digestion
  3. Pancreatic enzymes (pancreatic lipase, cholesterol ester hydrolase and phospholipase A2) and the protein, colipase are secreted to complete digestion
53
Q

What are micelles?

A

small disks with bile salts, phospholipids, fatty acids, cholestrol and mon/di-glycerides

54
Q

Why do bile salts coat lipids?

A

to make emulsions

55
Q

Describe stage 1 of the digestion and absoprtion of lipids

A

Bile salts from liver coat fat droplets

56
Q

Describe stage 2 of digestion and absoprtion of lipids

A

Pamcreatic lipases and colipase break dwon fats into monoglycerides and fatty acids which are stored in micelles

57
Q

Describe stage 3 of digestion and absoprtion of lipids

A
  • Monoglycerides and fatty acids move out of micelles and enter cells by diffusion
  • Cholestrol is transported into cells by a membrane transporter
58
Q

Describe stage 4 of digestion and absoprtion of lipids

A

Absrobed fats combine with cholestrol and protein in the intestinal cells to form chylomicrons

59
Q

Describe stage 5 of digestion and absoprtion of lipids

A

chylomicrons are released into the lymphatic system

60
Q

What are mixed micelles?

A
  • 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
61
Q

What are chylomicrons?

A
  • 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
62
Q

Where does absoprtion of lipids occur?

A

small intestine

63
Q

What mechanisms are used to digest and absorb lipids?

A
  • 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
64
Q

What is the problem with bile salts?

A
  • 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)
65
Q

Describe absoprtion of water and NaCl is large intestine

A

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)

66
Q

Bacterial actions in the LI

A
  • 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
67
Q

Basoprtion of fluid in the SI and LI

A
  • ~ 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

MD3002 - GI Physiology (4 decks)

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