Transport ACROSS the GIT

Question 1

What are the 2 intrinsic neuronal plexuses in the gut wall and what are their involved in?

Answer 1

• Submucosal plexus regulating the digestive glands

Myenteric plexus involved in gut motility

Question 2

Describe the cellular structure of the intestines

What is absorbed in the small intestine?

Answer 2

• There are epithelium with villi, tight junctions, highly vascularised, thin, moist, large surface area - makes them perfect for absorption.
• All nutrients, water and electrolytes

Question 3

What is Transcellular and Paracellular transport?

Answer 3

Transcellular transport - where substances can simply pass through the cell e.g. glucose.

Paracellular transport - passive, but selective and regulated, where they pass between the tight junctions of the cell e.g. Na+, Cl-, H2O.

Question 4

What happens in Carbohydrate absorption and digestion?

Why do those with Lactose Intolerance get “gassy”?

Answer 4

• Can only be absorbed as monosaccharides, so they are broken down by amylases present.
  • During churning, the chyme is moved closer to the brush border, which has enzymes present to convert disaccharides into monosaccharides.
• Insufficient lactase causes lactose intolerance - bacteria ferments unbroken lactose, leading to excess gas formation.

Question 5

What happens during Glucose and Galactose absorption?

LOOK AT DIAGRAM!

Answer 5

Rapidly absorbed by the secondary active transport process:

• Na/K pump actively moves Na+ out into the blood and K+ in from the blood.
• This ↓[Na+] in the cell, so the gradient forces more Na+ from the lumen via SGLT1 symporter - moves glucose/galactose and Na+ into cell.

Question 6

What happens in Protein digestion?

What happens in Protein Absorption?

Answer 6

• Pepsin breaks the proteins down into Polypeptides first. Poly, di, tripeptides are broken down by pancreatic proteases.

Peptidases (enzymes) in brush border continue to break down the peptides into AA’s - easier to transport across into blood.

• AAs are transported using sodium-coupled carrier systems. However, di/tripeptides can be transported into the cell using H+, before being hydrolysed into AAs.

Question 7

What happens in Fat digestion?

How do Bile salts help in fat digestion?

LOOK AT DIAGRAM!

Answer 7

• Fats form most of the dietary lipids, like phospholipids, cholesterol, and TGs. Fats are broken down to make absorption easier - using bile salts.
• Help emulsify the fats into smaller units called MICELLES PARTICLES - they prevent them combining back together by attaching to the molecules’ surface.

Question 8

How are Fats absorbed?

What happens if the fat doesn’t enter at the micelle brush border?

Answer 8

• They don’t enter the blood straight away. Instead, they go into small lymphatic vessels, called LACTEALS (found in villi of epithelium). They then enter the bloodstream later on.

In the epithelial cells, TGs are formed from FAs and Glycerol, and packaged with cholesterol and lipoproteins into CHYLOMICRONS - this then enters the lacteals.

• It will stay in the lumen to be excreted in the faeces

Question 9

What is Steatorrhoea?

What’s it caused by?

Why are problems with the kidney and pancreas causations for fat malabsorption?

Answer 9

• Excess fat in faeces
• Disorders of fat digestion/absorption e.g. Gallstones, Pancreatitis, Crohn’s disease, Liver disease
• They release bile salts and lipase needed for fat (TG) breakdown.