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Gastrointestinal > Digestion and Absorption > Flashcards

Digestion and Absorption Flashcards

1
Q

What is digestion?

A

The breakdown of nutrients into absorbable molecules.

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2
Q

What is absorption?

A

Movement of nutrients, water and electrolytes from the gut lumen into the internal environment.

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3
Q

Describe the structure of the intestinal mucosa.

A
  • Surface of the small intestine is arranged in circular folds of keckring.
  • Villi project from the folds
    • Surface of the villi are covered with epithelial cells (enterocytes) with mucus secreting cells (goblet cells).
  • Apical surface of epithelial cells covered by microvilli - brush border.
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4
Q

What is the main site of absorption in the intestine?

A
  • Small intestinal epithelial cells.
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5
Q

What are the fat-soluble and water-soluble vitamins which must be consumed in the diet?

What are the minerals (trace metals)?

A
  • Fat-soluble
    • ​Vitamin A
    • Vitamin D
    • Vitamin E
    • Vitamin K
  • Water-soluble
    • ​B1, B2, B3, B5, B6, B12

​

  • These are not produced by the body (or endogenous amounts are not sufficient).
  • Minerals
    • ​Ca
    • Fe
    • Zn
    • Mn
    • Mg
    • Phosphorus
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6
Q

What are the reasons that we need to consume the vitamins in our diet that we cannot make?

A
  • They are not produced by the body (or endogenous amounts are not sufficient).
  • They may be required as cofactors, antioxidants, ‘hormone’ (Vitamin D - synthesised in the liver in response to sunlight exposure, but required in the diet if not exposed).
  • Deficiencies result in pathologies:
    • Rickets (D)
    • Scurvy (C)
    • Anaemia (B12)
  • May be cytotoxic in increased concentration, therefore should stick to RDA (although water soluble should be okay as excess is excreted in the urine, not stored).
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7
Q

Describe the digestion and absorption which happens in the mouth.

A
  • Very little digestion - small amount of lipid, CHO.
  • Almost no absorption (certain drugs).
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8
Q

Describe the digestion and absorption which happens in the stomach.

A
  • Some digestion - protein but not very important.
  • Almost no absorption (certain drugs).
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9
Q

Describe the digestion and absorption which happens in the small intestine.

A
  • LOTS of absorption - CHO, lipids, proteins.
  • Vital site of digestion and absorption (some regional differences between duodenum, ileum and jejunum).
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10
Q

Describe the digestion and absorption which happens in the large intestine.

A
  • Almost no digestion and absorption (apart from water), some “indigestible” substances used as fuel by gut flora.
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11
Q

Which is the only type of carbohydrate which can be absorbed?

A

Monosaccharides

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12
Q

Which carbohydrates are absorbed by Na+-dependent cotransport?

Where are they absorbed?

A
  • Glucose
  • Galactose
  • Absorbed in the small intestine
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13
Q

Which carbohydrates are absorbed by facilitated diffusion?

A

Fructose

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14
Q

What are the factors which promote digestion of carbohydrates in the mouth, stomach and duodenum respectively?

A
  • Mouth
    • ​Salivary α amylase
  • Stomach
    • ​Amylase continues to act
  • Duodenum
    • ​Pancreatic amylase
    • Brush border enzymes (maltase, sucrase and lactase) - act on disaccharides, producing monosaccharides - fructose, glucose and galactose.
      • Lactose intolerance = no enzyme so bacteria ferment sugar - gas and diarrhoea.
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15
Q

What are the digestible and ingestible CHOs?

A
  • Starch (from plants) and glycogen (from animals):
    • Linear chains of glucose molecules joined by α1-4 glycoside bonds initially digested by amylase.
  • Cellulose (from plants):
    • Linear chains linked by β1-4 glycosidic bonds. No enzymes in humans to digest cellulose.
    • But, there are ways to break through a cell wall that are no enzymic. Largely mechanical - chewing lettuce etc. We just cannot break down this bond chemically.
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16
Q

What are amylases?

A

Free enzymes acting in the GI lumen and digest only internal α1-4 glycosidic bonds.

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17
Q

Describe the different types of amylases.

A
  • Salivary amylases - secreted from the mouth in response to the sight and smell of food - of lesser importance than…
  • Panceatic amylases - secreted from pancreas into the duodenum.
    • Produces maltotriose, maltose and α-limit dextrins.
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18
Q

Describe the products of amylase.

(Carbohydrate digestion of carbohydrates).

A
  • Products of amylase - now digested by oligosaccharides.
    • Attached to the enterocyte mucosal membrane of the brush border of epithelial cells.
  • α-glucosidase - cleaves α1-4 glycosidic bonds to remove single glucose units from the non-reducing end of the polymer.
  • Isomaltase - cleaves α1-6 glycosidic bonds in the α-limit dextrin oligosaccharides.
19
Q

Describe carbohydrate digestion by disaccharides.

A
  • After carbohydrate digestion by carbohydrates:
    • Then, products of amlyase and oligosaccharides are hydrolysed by the disaccharidases which are attached to the brush border membrane.
  • Maltase - produces glucose.
  • Sucrase - produces glucose and fructose.
  • Lactase - produces galactose and glucose.
20
Q

What is the end product of disaccharide digestion of maltose?

A
  • Glucose - Glucose
  • α1-4 glycosidic bond digested by MALTASE
21
Q

What are the end products of disaccharide digestion of sucrose?

A
  • Glucose - fructose
  • α1-2 glycosidic bond digested by sucrase
22
Q

What are the end products of disaccharide digestion of lactose?

A
  • Galactose-Glucose
  • β1-4 glycosidic bond digested by lactase
23
Q

What are the final products of carbohydrate digestion?

A
  • Glucose
  • Galactose
  • Fructose
24
Q

Describe the absorption of carbohydrates.

A
  • Secondary active transport
    • ​SGLT1 (sodium-dependent glucose transporter 1) located on the apical membrane transports glucose and galactose.
  • Facilitated diffusion
    • ​GLUT5 (glucose transporter 5) transports fructose across the apical membrane.
25
Q

Summarise protein digestion.

A
  • Begins in the stomach with pepsin.
  • Completed in the small intestine with pancreatic and brush-border proteases.
    • Endopeptidases hydrolyse the interior peptide bonds of proteins.
    • Exopeptidases hydrolyse one amino acid at a time.
26
Q

Describe the secretion of proteases.

A

Proteases are secreted as zymogens before being activated.

27
Q

What are the peptidases (aka proteolytic enzymes) which are involved in protein digestion?

A
  • Endopeptidases - cleave large polypeptides in the middle of the chain → oligopeptides (2-8AA long).
    • Pepsin (stomach) - hydrolyses links with tyrosine, D alanine and leucine. Hydrolyses long polypeptide chains into shorter ones.
    • Trypsin (SI) - hydrolyses links with arginine and lysine. (Trypsin inhibitor - protection of self - small protein, present in pancreatic juice to inhibit any trypsin found prematurely in the pancreatic cells or duct).
    • Chymotrypsin (SI) - hydrolyses links with tyrosine, tryptophan, D alanine, methionine and leucine.
    • Elastase (SI) - degrades elastin.
  • Exopeptidases - cleave amino acids one at a time from either end of protein.
    • Carboxypeptidases (C-terminal).
    • Aminopeptidases (N-terminal).
28
Q

What are the products of stomach and pancreatic luminal enzymes?

A
  • Oligopeptides
  • Amino acids
29
Q

What are the brush border enzymes which continue protein digestion?

A
  • Oligopeptidase
  • Aminopeptidase
30
Q

What are the products of protein digestion?

What are these products further broken down into and where does this happen?

A
  • Amino acids
  • Dipeptides
  • Tripeptides
  • In enterocyte they are further broken down into amino acids.
31
Q

Describe the digestion and absorption of lipids.

A
  • Dietary lipids are hydrophobic (insoluble in water) so causes problems with digestion and absorption.
  • They must be solubilised before digestion and absorption 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 the pancreatic enzymes.
32
Q

What are the 4 main types of lipids?

A
  • Fats / oils (triacylglycerols (TAG) or triglycerides)
  • Phospholipids
  • Cholesterol and cholesterol esters
  • Fatty acids
33
Q

What are the 3 main types of enzymes involved in lipid digestion?

A
  • Lipases
  • Phospholipases
  • Cholesterol esterases
  • Bile salts also have an inportant role in achieving digestion / absorption, but are not enzymatic
34
Q

Describe the lipid digestion of triacylglycerols (TAG) by lipases.

A
  • In (mouth) and stomach:
    • Salivary / gastric lipases (relatively unimportant ~10% of ingested lipids hydrolysed).
      • Heat and movements in stomach mix food with lipases → lipid emulsion.
      • Hydrolysis initially slow due to largely separate aqueous / lipid interface.
      • As hydrolysis proceeds, rate increases due to fatty acids produced acting as surfactants breaking down lipid globules aiding emulsification.
      • Emulsified fats ejected from the stomach to the duodenum.
  • In duodenum:
    • Pancreatic lipase (important - the main lipid digestive enzyme).
      • Aided by bile salts from the gall bladder.
      • HCO3- neutralises stomach acid → suitable pH.
35
Q

Describe the digestion of lipids in the small intestine.

A
  • 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.
36
Q

What is the role of bile salts in emulsions and micelles?

A
37
Q

What are mixed micelles?

A
  • Products of lipid digestion (cholesterol, monoglycerides, lysolecithin and free fatty acids) are solubilised in mixed micelles (diameter 5nm).
    • Core contains the products of lipid digestion.
    • Surface coating of bile salts which are amphipathic.
38
Q

Describe chylomicrons.

A
  • Chylomicrons (100nm diameter) 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.
39
Q

Bile salts are essential for lipid digestion. But, there is a problem - describe this.

A
  • There are not enough bile salts to deal with the average meal.
  • ~3-4g in the body, but ~3-15g required per meal.
  • After use, most are reabsorbed from the terminal ileum into the liver via enterohepatic circulation (via hepatic portal vein).
40
Q

Describe the enterohepatic circulation of bile salts.

A
41
Q

Describe the mechanism of absorption of fatty acids, monoglycerides and cholesterol.

A
  • Absorbed in the small intestine.
  • Mechanism of absorption:
    • 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.
42
Q

Describe the absorption of water in the small intestine.

A
  • Absorption of H2O (and NaCl).
    • 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.
    • Aldosterone = increased water absorption.
43
Q

Describe the bacterial actions in the large intestine.

A
  • Movements through the large intestine are very slow, therefore 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 “ingestible” matter
44
Q

Summarise the absorption of fluid in the small and large intestine.

A
  • ~9L of fluid enters the 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.
  • “Rough agents” in the large intestine cause problems - e.g. lactose → osmotic diarrhoea.

Gastrointestinal (10 decks)

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