Transport 2: Across the alimentary tract Flashcards
What does the alimentary tract absorb?
Absorb nutrients, salts and water
How is absorption in the alimentary tract achieved?
Absorption is achieved by transport of food components, ions, e.g. Na+
and water follows by osmosis
How much water is absorbed from the colon once it arrives?
Colon absorbs 90% of the water from the contents that arrive at it
What gives the small intestine a large surface area?
epithelial folds, villi, and microvilli (large surface area -
an internal surface area of 200 m2)
Where are all dietary nutrients, water and electrolytes absorbed?
All dietary nutrients, water and electrolytes that enter the upper small
intestine are absorbed
What is transcellular transport across the enterocyte?
Transport through the cell like glucose transport
What is paracellular transport across the enterocyte?
Passive but selective variable and regulated, through a tight junction between epithelial cells
What form can carbohydrates only be absorbed in?
Can only be absorbed in the form of monosaccharides
What are complex carbohydrates reduced to disaccharides by?
Complex carbohydrates are reduced to disaccharides by amylases
What converts disaccharides to monosaccharides?
Specific brush border enzymes convert disaccharides to monosaccharides
Where does digestion of carbohydrates occur?
In the intestinal lumen and at the brush border
What does sucrase, at the brush border, convert sucrose into?
Sucrose –> Glucose and fructose
What does glucoamylase, at the brush border, convert glucose oligomers into?
Glucose oligomers –> Glucose
What does lactase convert lactose into at the brush border?
Lactose –> Glucose and galactose
What is glucose and galactose absorbed by and how fast?
Glucose and galactose are rapidly absorbed by a secondary active transport process
What is fructose absorbed by and how fast?
Fructose is slowly absorbed by passive transport on a GLUT 5 carrier
What are monosaccharides absorbed by?
Monosaccharides are absorbed by specific transporters at apical and basolateral aspects of enterocytes
How is fructose absorbed at the apical aspect of enterocytes?
GLUT5 and GLUT2 function in series to promote fructose absorption
What does GLUT2 at the basolateral aspect of enterocytes promote absorption of?
-Glucose
-Fructose
-Galactose
What action are polypeptides produced by?
Polypeptides produced by action of pepsin
What action are di-, tri peptides produced by?
Polypeptides, di- and tri-peptides by action of pancreatic proteases
What do the di-peptidases in brush border complete?
Di-peptidases in brush border complete digestion to amino acids
What are the 3 sites of protein digestion?
- Lumen
- Brush border peptidases
- Cytoplasmic peptidaces
How are proteins digested in the lumen?
Pepsins, pancreatic proteases
How are proteins digested by brush border peptidases?
Oligopeptides and amino acids –> amino acids + dipeptides and tripeptides
How are proteins digested by cytoplasmic peptidases?
Dipeptides and tripeptides(from brush border) –> amino acids + dipeptides and tripeptides
What is the mechanism in amino acids absorption?
- Amino-acids (AAs) are transported on a sodium-coupled carrier system
similar to that for glucose.
– Separate carriers for different types of AAs.
What is the mechanism in di- and tri- peptides absorption?
Some di- and tri- peptides are transported on a carrier system using
inwardly directed H+ gradient
How are amino acids carried to the liver and via what?
The amino acids are carried in the blood to the liver via the portal vein
what does salivary lipase digest?
salivary lipase digests a small fraction of the TGs
What must happen to triglycerides before they can be digested and why?
-TGs must be dissolved in the aqueous phase before they can be digested
What does lipase action require?
Lipase action requires the emulsification of TGs by bile salts (dissolves TGs in
water)
What does pancreatic lipase bind to?
Pancreatic lipase binds to the surface of the small emulsion particles
What is the 2 processes of Digestion and absorption of lipids facilitated by?
- Emulsification
- Micelle formation
Structure of triglyceride
-1 glycerol
-3 fatty acids
Steps involved in the digestion of fats througout the GI tract
- Lipase of tongue glands
- Emulsification by gastric contractions
- Breakdown in stomach (10-30%)
- Breakdown of micelle formation in duodenum and jejunum (70-90%)
- Absorption in small intestine
- Excretion as Ca2+ soaaps
What do bile salts do to fats?
Bile salts emulsify fats
-Break large fat globules into smaller globules
Why is emulsification of fats beneficial?
The surface area where lipase can act is increased
What are the 3 steps in breakdown and micelle formation
- Oil phase
- Viscous-isotropic phase
- Micellar phase
What happens in the jejunum to micelle?
- In the jejunum, chylomicrons get synthesised
-This is due to triglycerides beint re-synthesised in the jejunum
-Apo-lipo-proteins are added to chylomicrons - Chylomicrons end up in portal circulation via lymphatic system
3.Bile salts dont get transported via jejunum. They enter through mucosal cells in ileum where they enter the portal vein and recycled back to the liver
Why does lymph draining from the small intestine appear milky?
Absorption of large numbers of chylomicrons causes the lymph
draining from the small intestine to appear milky
What is steatorrhoea and what causes it?
reduced fat digestion and absorption causing the presence of
abnormal amounts of fat in the stool
