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Flashcards in Digestion and Absorption Deck (44)
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What is digestion?

The breakdown of nutrients into absorbable molecules.


What is absorption?

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


Describe the structure of the intestinal mucosa.

  • 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.


What is the main site of absorption in the intestine?

  • Small intestinal epithelial cells. 


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

What are the minerals (trace metals)?

  • 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


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

  • 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). 


Describe the digestion and absorption which happens in the mouth.

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


Describe the digestion and absorption which happens in the stomach.

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


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

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


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

  • Almost no digestion and absorption (apart from water), some "indigestible" substances used as fuel by gut flora. 


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



Which carbohydrates are absorbed by Na+-dependent cotransport?

Where are they absorbed?

  • Glucose
  • Galactose
  • Absorbed in the small intestine


Which carbohydrates are absorbed by facilitated diffusion?



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

  • 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.


What are the digestible and ingestible CHOs?

  • 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.


What are amylases?

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


Describe the different types of amylases.

  • 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.


Describe the products of amylase.

(Carbohydrate digestion of carbohydrates).

  • 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. 


Describe carbohydrate digestion by disaccharides.

  • 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.


What is the end product of disaccharide digestion of maltose?

  • Glucose - Glucose 
  • α1-4 glycosidic bond digested by MALTASE


What are the end products of disaccharide digestion of sucrose?

  • Glucose - fructose
  • α1-2 glycosidic bond digested by sucrase


What are the end products of disaccharide digestion of lactose?

  • Galactose-Glucose
  • β1-4 glycosidic bond digested by lactase


What are the final products of carbohydrate digestion?

  • Glucose
  • Galactose
  • Fructose


Describe the absorption of carbohydrates.

  • 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.


Summarise protein digestion.

  • 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.


Describe the secretion of proteases.

Proteases are secreted as zymogens before being activated.


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

  • 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).


What are the products of stomach and pancreatic luminal enzymes?

  • Oligopeptides
  • Amino acids


What are the brush border enzymes which continue protein digestion?

  • Oligopeptidase
  • Aminopeptidase


What are the products of protein digestion? 

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

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