Session 2 Flashcards
Which molecules cannot diffuse?
How are they transported?
Large, uncharged polar molecules e.g. glucose, sucrose etc
Ions e.g. H+, Na+, K+ etc.
Need to be transported actively
What type of molecules can diffuse?
Small hydrophobic molecules e.g. oxygen, CO2, N2 etc
Small uncharged polar molecules e.g. water, urea, glycerol etc
Why are transport systems needed?
Maintenance of pH intracellularly
Maintenance of ionic composition
Regulation of cell volume
Concentration of metabolic fuels and building blocks
Extrusion of waste products of metabolism and toxic substances
Generation of ionic gradients (needed for nerves)
How are Cl- ions transported?
By facilitated diffusion - band 3 protein forms a Cl- selective pore and carries out exchange of Cl- for HCO3-
What models are there for facilitated transport?
Protein pores (channels), carrier molecules (ping pong) and protein flip-flop (unlikely thermodynamically)
Give examples of gated pores
Ligand-gated ion channels - open or close in response to ligand binding to a receptor site Voltage-gated ion channels - open and close in response to pd across membrane Gap junction (connexin) - closed when cellular calcium concentration rises about 10 micrometers or the cell becomes acidic
How is passive or active passage decided?
By the free energy change of the transported species.
The free energy change is determined by the concentration gradient and the electrical potential across the membrane bilayer.
What is secondary active transport?
When the transport of a substance is linked to the concentration gradient for another substance via a cotransporter
Define uniport
One solute molecule species is transferred from one side of the membrane to the other
Define co-transporters
Transfer of one solute molecule depends on simultaneous or sequential transfer of a second solute
Define symport
Two solute molecules are moved in the same direction (requires a cotransporter)
Define antiport
Two solute molecules are moved in opposite directions (requires a contransporter)
Describe Na+/K+ ATPase pumps
Form Na+ and K+ gradients. Drive many secondary active transport processes
How are resting intracellular calcium levels controlled?
Primary active transport - PMCA expels Ca2+ out of the cell (high affinity, low capacity), removes residual Ca2+
SERCA - accumulates Ca2+ into SR/ER (high affinity, low capacity), removes residual Ca2+
NCX - removes most Ca2+ (low affinity, high capacity), mitochondrial Ca2+ uniports operate at high [Ca2+]i as a buffer system
Describe the NCX
NCX = sodium calcium exchanger. Exchanges 3Na+ for 1Ca2+ therefore electrogenic with current flowing in direction of Na+ gradient. In depolarised cells it’s reversed therefore Ca2+ moves into the cell, allowing for influxes of Ca2+
Why is calcium important in ischaemia?
Ca2+ toxicity –> depleted levels of ATP therefore inhibition of Na+ pump. [Na+]i accumulates, depolarisation, NCX reversal
How is acidification countered?
Expulsion of H+ ions or inward movement of bicarbonate ions
How is alkalisation opposed?
Expulsion of bicarbonate ions via the anion exchanger
Give examples of acid extruders
Na+/H+ exchangers (NHE)
Na+ dependent Cl-/HCO3- exchanger - NBC (sodium bicarbonate cotransporter)`
Give an example of a base extruder
Cl-/HCO3- exchanger - AE (anion exchanger)
What is the function of electroneutral transport?
Allows osmotic strength of cytoplasm to be varied without effect on the membrane potential.
How do cells prevent swelling?
Pump ions out e.g. K+ pump & Cl- pump work conductively
K+ pumps & Cl-/HCO3- pumps (cotransporting)
K+/Cl- pump and amino acid pumps (cotransporting)
What mechanisms resist cell shrinking?
Bringing ions into the cell e.g. Na+ and Ca2+ entering cell
Na+/H+ pump and HCO3-/Cl- pump
How can we treat oedema?
By blocking one or more of the Na+ reabsorption mechanisms with diuretic drugs e.g. loop diuretics, aldosterone, thiazides, spironolacetone, amiloride etc
How does amiloride work?
Amiloride blocks the epithelial Na+ channel (ENaC) that is present in the cortical collecting duct.
How is glucose transported from the intestinal lumen to the blood by intestinal epithelium?
There is a high concentration of dietary glucose and high dietary Na+ in the intestinal lumen. The Na+/glucose symporter brings 2Na+ and a glucose into the cell. The Na+/K+ ATPase pump can then generate ATP using the Na+ and the glucose transporter can pump the glucose into the blood.
