Membrane Permeability Flashcards
What closes a gap junction?
When cellular concentration of calcium rises above 10μm
Cell becomes acidic
What models are there for facilitated diffusion?
Protein pores (channels) Carrier molecules (ping pong) Protein flip flop
What is the carrier model for facilitated diffusion?
A substrate binds which causes a conformational change
List the different types of gates a pore can have
Ligand gated
Voltage gated
Gap junction
Where does energy come from for active transport?
Hydrolysis of ATP
Electron transport
Light
Why does a membrane act as a permeability barrier to hydrophilic molecules/ions
Because movement of these things across a hydrophobic core wold require a large, free energy change.
What important roles do transport process have across the cell membrane?
Maintenance of intracellular pH
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 electrical excitability of nerve and muscle
What is uniport?
When one solute species is transported from one side of the membrane to the other
What is co-transport?
When transport of one molecule depends on the simultaneous or sequential transport of another
What is symport
Co-transport when the two molecules are going in the same direction
What is antiport?
When two molecules are being transported in opposite directions
What does the Na-K-ATPase (Na pump) transport?
3 sodium out
2 potassium in
Requires ATP
What secondary active transport processes does the Na pump drive?
Control of pH Regulation of calcium conc Absorption of sodium in epithelia Nutrient uptake such as glucose from small intestine Regulation of cell volume
What does the Ca-ATPase (PMCA) pump transport? And function?
Calcium out of the cells.
Therefore controls calcium conc in cells
What is the affinity and capacity of the PMCA pump? (Calcium pump)
High affinity and low capacity.
Means it can transport few ions at any one time but works well when there is a low concentration of ions.
What does the SERCA pump do?
Accumulates calcium into the ER while transporting H+ out at the same time.
Affinity and capacity of SERCA?
High affinity, low capacity
What does the NCX (sodium-calcium exchanger) transport?
3 sodium into the cell for 1 calcium out
When is the NCX pump reversed? What effect does this have?
When cells are depolarised
Contribute to calcium toxicity during ischaemia
Contributes to calcium influx during a cardiac action potential
What does sodium-hydrogen exchange (NHE) transport?
1 sodium in, 1 hydrogen out - electroneutral
What is the function of the NHE?
Regulate cell pH and volume
What is the NHE activated and inhibited by?
Activated by growth factors
Inhibited by omiloride
What is the function of the AE
An acidifier
What does the AE (anion exchange) transport?
Chloride in
HCO3 out
What is the relative concentration of potassium inside the cell compared to outside?
Potassium is higher inside the cell than outside
Relative concs of sodium inside the cell compared to outside?
Na is higher outside the cell
Relative conc of chloride inside compared to outside?
Higher outside
Relative conc of calcium inside compared to outside?
Lower inside
List the ion channels which control cell pH
Na-K-ATPase Na-H exchange Na-H and anion exchange coupled together Na-HCO3 co-transport Anion exchange (band 3)
What does the glomerulus do?
Filters substances out of the blood into Bowman’s capsule
How do loop diuretics work?
Block the NKCC2 transporter in the loop of Henle. This blocks the re-uptake of sodium into the blood, so less water follows. Reduces BP.
What metabolites rely on the sodium gradient for their uptake into cells against their conc gradient?
Glucose and sodium
Which transporters are involved in the uptake of glucose from the gut and where?
Na-dependent glucose symport - glucose and Na from lumen into epithelial cell
GLUT - glucose out of cell into bloodstream
Na-K-ATPase - moves sodium into blood to maintain sodium conc gradient across the cell.
How does insulin stimulate the rate of uptake of glucose into adipose tissue and skeletal muscle?
Recruits GLUT-4 transporters from internal vesicular membranes to the plasma membrane to increase the transport capacity of the membrane.
What prevents the efflux of glucose back out of the cell when circulating glucose falls a few hours after a meal?
On entering the cell, glucose is converted to glucose-6P. Catalysed by hexokinase/glucokinase
