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Flashcards in Session 2 Deck (20):

Outline the relative permeability coefficients of different molecules passing through the membrane (high to low)

Hydrophobic molecules - e.g. O2, CO2, N2, benzene
Small uncharged, polar molecules - e.g. H2O, Urea, glycerol
Large uncharged polar molecules - e.g. Glucose and sucrose cannot pass through
Ions - e.g. H+, Na+, Ca2+ cannot pass through


What water channels facilitate the movement of water in the proximal convoluted tubule?

Aquaporins - it's inclusion in the membrane in stimulated by ADH


What are models for facilitated diffusion?

Carrier molecules (ping-pong) and protein channels. Pores may be gated (eg voltage or ligand)


What is passive transport across a membrane dependent on?

Permeability and concentration gradient


How are unfavourable electrochemical gradients overcome?

By coupling to a thermodynamically favourable reaction. Such as directly or indirectly from the hydrolysis of ATP, electron transport or light.


Describe the free ion distribution inside and outside of a cell

Na+: Inside-12mM. Outside-145mM
K+: Inside-155nM. Outside-4mM
CL-: Inside-4.2mM. Outside-123mM
Ca2+: Inside-10^-7M. Outside-1.5mM


What are co-transporters?

Transporters that transport more than one ion or molecule per reaction cycle.



Uniport - single transported molecule in one direction
Symport - two transported molecules, Same direction
Antiport - two transported molecules, opposite direction


What is secondary active transport?

When the transport on one substance is linked to the concentration gradient of another via a cotransporter


What is the role and structure of Na+/K+ ATPase?

Pumps 3Na+ out and 2K+ per molecule of ATP. Uses 25% of BMR. Alpha subunit provides binding site for ions and ATP, beta subunit directs pump to surface. Ouabain is an inhibitor.
Forms gradients necessary for electrical excitability and drives many secondary active transport processes


What is the role of Na+/H+ exchanger?

Antiport - inward flow of Na+ along concentration gradient leads to cell alkalisation by removal of H+
Is electromeutral and is used to regulate cell pH and volume.
Inhibited by amiloride.


What is the role of Na+/glucose cotransport system?

Symport in small intestine and kidney - entry of Na+ provides energy for entry of glucose


What is the role of the Na+/Ca2+ exchange (NCX)?

Inward flow of Na+ drives outward flow of Ca+. Is electrogenic - current flows in the direction of Na+ gradient, which is dependent on the membrane potential (depolarisation reverses mode of operation).


How does NCX cause toxicity during ischaemia?

ATP is depleted so the sodium pump is inhibited. Na+ accumulates in the cell so NCX reverses. High intracellularly Ca2+ is toxic.


What are the main acid extruders in regulating cell pH?

NHE (main)
Na+-bicarbonate-CL- transporter (NBC): uses Na+ gradient. Acid out, base in


What is the main base extruder in regulating cell pH?

Anion exchanger (AE): HCO3- out, CL- in. Moves base out, acidifying cell.


How is intracellularly pH held at a set point?

Acidification activates NHE and NBC. Alkalinisation activates AE


How is almost all of the Na+ in the glomerular filtrate reabsorbed into the kidney nephron?

A low intracellular Na+ concentration maintained by Na+/K+ ATPase


What is the goal of renal anti-hypertensive therapy?

To reduce the uptake of Na+ so less water is reabsorbed by osmosis. Blood volume and therefore BP falls.


Outline the actions of some drugs that control Na+ re uptake in the kidney

Loop diuretics prevent Na+ reabsorption in the thick ascending limb.
Amiloride acts on both the distal convoluted tubule and the cortical collecting duct to limit Na+ reuptake.
Spironolactone is used if aldosterone is high (limits Na+ reabsorption)