Session 2 Flashcards by Ben James

How can a lipid bilayer be observed?

Use of a black film

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Which molecules can cross the plasma membrane freely?

Hydrophobic molecules, and small uncharged polar molecules

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What is the name of channels that facilitate the flow of water across the plasma membrane?

Aquaporins

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Which molecules cannot freely cross the plasma membrane and hence require transport proteins?

Large uncharged molecules and ions

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What is a permeability coefficient?

The ease with which a molecule can dissolve across a phospholipid bilayer

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What to key things does the rate of passive transport depend on?

Permeability and concentration gradient

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How does the rate of passive transport vary with increasing concentration gradient?

Increases linearly

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List three mechanisms by which membrane transport proteins work

Protein pores (channels), carrier molecule model (ping-pong), flip-flop and rotating carrier model (unlikely)

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Describe the basis of the functioning of facilitated diffusion by ion channels

The ion channel is closed/open at rest, a stimulus arrives and this causes the opening/closing of the channel

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Name three mechanism of ion channels involved in facilitated diffusion

Ligand-gated, voltage-gated, gap junction

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What are ‘saturable’ transport processes?

Those with a maximum rate; they can be modelled in michaelis-menten style plots

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Give approximate values for the extracellular and intracellular concentrations of sodium ions

E: 145mM
I: ~10mM

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Give approximate values for the extracellular and intracellular concentrations of potassium ions

E: 4.5mM
I: 160mM

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Give approximate values for the extracellular and intracellular concentrations of chloride ions

E: 110-120mM
I: 3-4mM

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Give approximate values for the extracellular and intracellular concentrations of calcium ions

E: 1-2mM
I: ~100nM

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What is co-transport? What are the two types?

More than one ion or molecule transported on a membrane transporter per reaction cycle; symport and antiport (uniport is for only one molecule/ion)

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What type of co-transport if the Na+ pump? In what ratio do the ions move?

Antiport; 3 Na+ out, for 2 K+ in

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Why is the Na+ pump described as a ‘P-type ATPase’?

Transport requires the hydrolysis of ATP; it self-phosphorylates an aspartate residue on its structure

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What does the molecule Ouabin do to the sodium pump?

Inhibits it

How much charge does the sodium pump generate through electrogenic pump activity?

About -5 to -10 mV

What does the PMCA do?

Exports calcium from the cell, with hydrolysis of ATP (needs Mg2+)

Describe the affinity and capacity of the PMCA and NCX

PMCA: high affinity, low capacity, good at low concentrations
NCX: low affinity, high capacity, good at high concentrations

What is the NCX?

Sodium calcium exchanger; uses gradient from Na+ to export 1 Ca2+ for every 3 Na+ in (antiport)

Why is the NCX described as a secondary active transport?

Needs the action of the sodium pump to generate the sodium gradient

What direction does the NCX work in when the cell is polarised?

NCX acts to export calcium and import sodium

What direction does the NCX work in when the cell is depolarised?

NCX is reversed; calcium in, sodium out

Which direction does the NCX work in during ischemia?

Lack of ATP results in no sodium pump activity; hence sodium builds up inside the cell, and the NCX acts to removing, bringing calcium into the cell; the high concentration of calcium is toxic

What effect cystic fibrosis have on the sodium pump?

The faulty CFTR results in a lack of movement out of the cell; in order to keep electroneutrality, the cell retains sodium by decreasing the activity of the sodium pump

How is the CFTR transporter affected by the cholera toxin?

Protein kinase A stimulates an increase in the activity of the CFTR transport protein, resulting in chloride ions flowing out of the cell; other ions follow to retain electroneutrality, and water follows

Why must the intracellular calcium concentration be tightly regulated?

Used in cell signalling

What is the SERCA? In what conditions does it act?

Sarco(endo)plasmic reticulum Ca2+-ATPase; accumulates calcium into the SR/ER using ATP and the expulsion of H+; it is high affinity, low capacity (removes residual Ca2+)

When do mitochondrial Ca2+ uniports operate?

High calcium concentrations, to buffer potential damage

Name the two acid extruders of the cell membrane

Na+/H+ exchanger (NHE), and Na+ dependent Cl-/HCO3- exchanger

Why is the NHE described as electroneutral?

It exchanges an extracellular Na+ for an intracellular H+, in a 1:1 ratio, which hence has no effect on membrane potential

What does the NHE regulate?

Intracellular pH and cell volume

What inhibits the NHE?

Amiloride

Describe the sodium dependent Cl-/HCO3- exchanger

Exchanges H+ and Cl- out for Na+ and HCO3- in; it is electroneutral

Which transporter regulates alkali influx?

Na+-HCO3- cotransporter; sodium and bicarbonate travel into the cell together, and this can be 1,2, or even carbonate molecules (only found in some cells)

Which membrane protein is responsible for alkali extrusion?

Anion exchanger (band 3); removes HCO3-, whilst taking in Cl-; this acidifies the cell

What are conductive system mechanisms to resist cell swelling?

K+ and Cl- ion channels (voltage gated)

What are cotransport system mechanisms to resist cell swelling?

Cotransport systems: bicarbonate intake for Cl- release, H+ intake for K+ release (loss of bicarbonate as CO2), amino acid efflux and K+ and Cl- efflux via carrier proteins

What are conductive system mechanisms to resist cell shrinkage?

Sodium and calcium ion channels open, allowing their influx into the cell (chloride actively trasnsported in, to keep electroneutrality)

What are the cotransport system mechanisms to resist cell shrinkage?

Bicarbonate efflux, resulting in Na+ and Cl- influx, in place of K+; cotransport influx of: (Na+, K+ and 2Cl-; NKCC2), (Na+ and organic osmolytes), and (Na+ and Cl-)

Which transporters are present in the thick ascending limb of the kidney, between the lumen of the ascending limb and epithelial cells?

NKCC2 (takes up K+, Na+, and 2Cl-), and K+ leaves by ROMK channel (into the lumen)

Which transporters are present in the thick ascending limb of the kidney, between the lumen capillaries and epithelial cells?

K+ and Cl- enter the capillary via the symport KCICT, Cl- enter the capillary by the CIC-Kb channel, and the sodium pump also operates

Which ions flow between cells from the lumen of the thick ascending limb of the kidney to the lumen of the capillary?

Ca2+, Mg2+

How do loop diuretics act to treat hypertension?

Inhibit NKCC2; prevents sodium reuptake, reducing the amount of water reuptake