Digestion and Absorption Flashcards
What is paracellular transport?
Substances diffuse between adjacent cells of the epithelium
Quite restrictive - tight junctions
What is transcellular transport?
Substances move into an epithelial cell across either the luminal or basolateral surface, diffuse through the cytosol and exit across opposite membrane
What is glucose?
Aldose containing 5 carbon sugar
How do carbohydrates form chains?
Via alpha or beta linked OH groups
What is the type of linkage determined by?
Presence of an alpha or beta carbon on one monosaccharide and position of disaccharide bond on the adjacent monosaccharide
How is starch broken down?
By salivary and pancreatic enzymes
1:4 alpha linkages broken down by glucoamalyse
1:6 alpha linkages broken down by alpha limit dextrinase
Why must carbohydrates be digested in their simplest form?
Only monosaccharides can cross epithelial barrier
What are some carbohydrate digestion disorders?
Lactose malabsorption syndrome
Congenital lactose intolerance
Isomaltose deficiency
galactose malabsorption syndrome
What are sodium gradients crucial for?
Absorption of glucose
Generated by Na+/K+-ATPase
Where is the Na+/K+ ATPase pumps located?
Basolateral membrane of intestinal epithelial cells
What does the sodium bind to on the apical membrane?
SLGT1 transporter
Only works if glucose and sodium are both present
Glucose coupled to sodium gradient (secondary active transport)
What transporter does glucose use on the basolateral membrane?
GLUT2 - facilitated diffusion
What sugar has a separate transporter?
Fructose - on proximal tubule S3 segment
How does the sodium gradient affect water?
Causes water to pass from lumen of the gut by osmosis
What substances digest peptides?
Pancreatic, brush border and intracellular proteases
What are large polypeptides absorbed by?
They can’t except in neonatal receptor mediated endocytosis
How are dipeptides, tripeptides and amino acids absorbed?
Across brush border
stereospecific -high affinity for L-type amino acids
How do amino acids and small peptides cross the epithelia?
Sodium gradient
Neutral AA coupled to sodium channel SNAT
How do AA cross the basolateral membrane?
By facilitated absorption
What are the 3 lipases involved in lipid digestion?
Triacylglycerol hydrolase
Cholesterol ester hydrolase
Phospholipase
What do bile salts facilitate?
Micelle formation at the epithelial surface
What are the 6 steps of fat emulsification?
- Large fat globule
- Smaller emulsion droplets
- Micelle formation
- Passive Diffusion across epithelial
- Re-synthesis of lipids in ER
- Chylomicron formation and transport
What must be added to bile to emulsify lipids?
lecithin
2- monoglycerides
What are bile salts?
Flat molecules with a polar face and a non-polar face
Where are hydrophobic molecules digested?
Interior of the micelle of the bile salt
What is the significance of the critical micelle concentration?
Any concentration below critical micelle formation, micelle formation will not occur
Where are lipids reformed?
Smooth Endoplasmic Reticulum
What happens to reformed lipids?
Packaged into chylomicrons coated with beta-lipoproteins and exocytosed into lacteal system
Where are short free fatty acids absorbed?
Directly into portal vein
What do micelles transport?
Monoglycerides, cholesterol and fat soluble vitmains
Where are bile salts recycled?
Terminal ileum by enterohepatic circulation
What is ASBT?
Apical sodium dependent bile salt transporter
What is OSTa/b?
Basolateral sodium independent bile acid transporter
Do dietary lipids enter the faeces?
no
What can lead to increased fat in the faeces?
Removal of the gall bladder
Where are remanent chylomicrons processed/
In the liver where new liposomes are formed
How are triglycerides processed?
Delivered to adipose tissue via chlyomicrons from the GIT
