S1) Epithelial Transport

Question 1

The movement of monosaccharides, ions and amino acids from ultra-filtrate in the nephron is a two-stage transcellular process.

Describe the import stage

Answer 1

• Import of substances from the nephron lumen into the epithelial cells
• Carried out by membrane transport proteins on the apical surface of nephron cells

Question 2

The movement of monosaccharides, ions and amino acids from ultra-filtrate in the nephron is a two-stage transcellular process.

Describe the export stage

Answer 2

• Export of substances from the cells into the interstitial fluid surrounding the basolateral surface
• Carried out by transport proteins on the basolateral plasma membrane

Question 3

What are the 2 requirements for trans-epithelial transport to occur?

Answer 3

• Epithelial cells must be polarised
• Different sets of transport proteins localised in the basolateral and apical surfaces

Question 4

Explain how tight junctions allow for polarised epithelial cells to function

Answer 4

Tight junctions are specialised regions of plasma membrane that connect adjacent epithelial cells and help to form a barrier to separate surrounding ECF

Question 5

Which preventative function do tight junctions have?

Answer 5

Prevent diffusion of the membrane proteins and glycolipids between the apical and basolateral regions of the plasma membrane

Question 6

Why does epithelial paracellular permeability decrease from the PCT to collecting duct in the mammalian kidney?

Answer 6

There are changes in the permeability of tight junctions expressed along its length

Question 7

What is the type of transport mechanism used through the tight junction?

Answer 7

Passive paracellular transport driven by electro-osmotic gradients produced by transcellular transport

Question 8

How is epithelial transport performed through transport and energy coupling?

Identify three mechanisms

Answer 8

Question 9

How is epithelial transport performed by facilitated diffusion?

Answer 9

Question 10

How does epithelial transport vary in number of solutes and direction?

Answer 10

Question 11

Explain what adaptations enable the proximal tubule to reabsorb 1/3 of total fluid through the paracellular pathway

Answer 11

• A leaky epithelium
• Low transepithelial resistance (TER)
• High paracellular transport

Question 12

Identify some important determinants of paracellular transport

Answer 12

• Peritubular protein concentration
• Renal hydrostatic pressure

Question 13

Which 3 substances also increase paracellular permeability?

Answer 13

• Nitric oxide
• Prostaglandins
• cAMP

Question 14

What drives paracellular reabsorption of Mg²⁺ and Ca²⁺ in the thick ascending limb?

Answer 14

• Driven by a lumen-positive transepithelial voltage
• Voltage is generated by Na⁺-K⁺-2Cl (NaKCC) cotransport and luminal K⁺ recycling

Question 15

In terms of epithelial transport, describe the properties of the distal nephron

Answer 15

• Tight epithelium
• Low passive permeability to cations
• High transepithelial resistance

Question 16

In terms of paracellular transport, what is the role of tight junctions in the distal nephron?

Answer 16

Tight junctions function as cation barriers to prevent the dissipation of transtubular Na⁺, K⁺, and H⁺ gradients

Question 17

In terms of paracellular transport, what is the role of aldosterone in the distal nephron?

Answer 17

Aldosterone regulates the transport of Na⁺ and K⁺ in this segment

Question 18

In terms of paracellular permeability, describe the role of tight junctions in the distal nephron

Answer 18

Tight junctions of the cortical and distal collecting duct epithelia lead to increased paracellular chloride permeability

Question 19

In terms of paracellular permeability, describe the role of aldosterone in the distal nephron

Answer 19

An aldosterone-sensitive interaction regulates Na⁺ channel activity to increase paracellular permeability