lecture 1+2

Question 1

What are the 4 junctional complexes which make up the epithelium?

Answer 1

Tight junction
Adherens junction
Desmosomes
Gap junction

Question 2

What are the two transport routes in epithelia?

Answer 2

• Paracellular- parallel to the cell, not through it

- Transcellular- through the cell

Question 3

What types of epithelia do not have a paracellular route?

Answer 3

Tight epithelia e.g. stomach, colon

Leaky epithelia e.g. small intestine, kidney proximal tubule

Question 4

What are the two main junctional proteins?

Answer 4

occludins and claudins

Question 5

How do you study different types of claudins?

Answer 5

Study by removing certain claudins and seeing how it affects permeability.

Question 6

What are the 3 functions of junctional complexes?

Answer 6

1. Barrier- mechanical integrity of epithelium
2. Permeability- paracellular pathway
3. “Fence”- maintaining polarity by separating apical and basolateral membranes.

Question 7

What are the products of digestion of; proteins, polysaccharides, fats/ triglycerides?

Answer 7

Proteins->amino acids, di- &tri- peptides
Polysaccharides-> monosaccharides
Fats/ triglycerides-> free fatty acids, monoglycerides.

Question 8

What are the 3 ways of absorption and give an example of each.

Answer 8

Simple diffusion- lipids
Carrier-mediated- facilitated diffusion, secondary active
Endocytosis- vit B12+ intrinsic factor, cholesterol

Question 9

What are the 5 sites of absorption and how much absorption happens there?

Answer 9

1. Mouth, oesophagus, stomach- limited diffusion
2. Duodenum and jejunum- MAJOR site
3. Ileum- Vit B12 & bile acids
4. Colon- some Na and H2O
5. Rectum- limited diffusion

Question 10

Why is small intestine such a good site for absorption?

Answer 10

1. Large surface area: foldings in wall-> vili-> microvilli
2. Rich supply of blood vessels and lacteals
3. Different transporters in apical and basolateral membrane.

Question 11

How is glucose transported across membranes?

Answer 11

Na+ dependent co-transport of glucose at apical membrane.
Glucose accumulates inside which generates a gradient that allows glucose to leave the cell via Na+ independent transporter GLUT2- facilitated diffusion at basolateral membrane.

Question 12

Features of glucose-galactose malabsorption syndrome.

Answer 12

• Genetic disease
• SGLT1 gene mutated= no absorption of glucose or galactose
• Severe and chronic diarrhoea in infants

Question 13

What are the different specialised apical membrane transporters for different amino acids?

Answer 13

• B0: neutral aa (cotransport)
• B0+: cationic and cysteine (exchange)
• PAT1: proline (cotransport)
• Xag-: anionic- aspartate, glutamate (cotransport)
• PepT1: di and tri peptides (cotransport)

Question 14

What are peptidomimetic drugs?

Answer 14

Drugs that use PepT1 transporter for absorption

Question 15

What are labile drugs and why are they good?

Answer 15

Sublingual or rectal delivery. Avoid hepatic portal vein and so are not subject to ‘first-pass’ metabolism (deactivation) by the liver.

Question 16

Process of fat digestion and absorption.

Answer 16

1. Emulsification of large fat droplets (inc surface area for action of lipase)
2. Formation of mixed micelles (stabilises products of triglyceride hydrolysis= monoglyceride+ fatty acids)
3. Monoglyceride and fatty acids are transported with bile salts as micelles to apical membrane.
4. Simple diffusion of MG and FA.
5. Fatty acids transporters= FAT+CD36
6. Recycling of bile salts

Question 17

What happens to fats at the basolateral membrane?

Answer 17

1. Triglycerides re-synthesized in ER, packaged in chylomicrons.
2. Exocytosis of chylomicrons
3. Chylomicrons transported in the lymphatic system to the liver.