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ICPP > 2 - Membrane Permeability (and Ion Transporters) > Flashcards

2 - Membrane Permeability (and Ion Transporters) Flashcards

1
Q

What is a semi-permeable membrane

A

a layer through which only allowed substances can pass
- Pores, channels and transporters contribute to this
- Hole needs to be big enough for molecule to pass through
- Charge / potential difference needs to allow for molecule to pass
- Occurs until solute equilibrium has been reached

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2
Q

Black film + membranes

not on ILOs

A
  • Phosphatidylserine (lipids) mixed + spread onto glass or plastic plates with pinhole
  • Submerged into hydrophilic solution
  • Phospholipid bilayer formed in pin hole
  • Appears black because light is reflected back
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3
Q

Relative permeability of a synthetic lipid bilayer to different classes of molecules

A
  • small gases / small uncharged eg O2, CO2, N2, H2O + urea. These pass straight through the membrane, and then back out again
  • large uncharged polar molecules / ions eg glucose, sucrose, K+. Cannot diffuse through the membrane easily and therefore need a transport mechanism.
  • Therefore can estimate permeability of a membrane
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4
Q

What is passive transport dependent on (Fick’s Law – equation will be given in exam)

A

J = P (C1 – C2)
Where J = net rate of transport (ie cm/s)
P = permeability coefficient (dependent on distance travelled + viscosity of medium)
C1 and C2 = concentration gradients on side 1 and 2

passive transport is dependent on permeabilty and concentration gradient
☞ rate of passive transport increases linearly with increasing concentration gradient

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5
Q

How is water transported through the phospholipid bilayer

A

passive diffusion aka osmosis
- Over entire surface of cell membrane
- Bi-directional flow
- Water crosses to reach solute equilibrium
- This is the main method of transporting water in and out of erythrocytes

aquaporins
- Integral membrane proteins
- Bi-directional flow
- Reaches equilibrium quickly

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6
Q

Passive vs active transport

A

passive = no energy required as long as there is a concentration gradient. Solute moves by diffusion down concentration / electrical gradients
active = cellular energy required. Solute moves across membrane against it’s concentration gradient.

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7
Q

Non-gated and gated pores

A

non gated eg connexins. Pores… they are always open.
gated are conditionally open. These are channels. Movement of molecule is due to its interaction with the protein → conformational change

Both have hydrophilic centres.

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8
Q

The CFTR transporter in cystic fibrosis vs diarohea

A

☞ both involve the CFTR (cystic fibrosis transmembrane conductance regulator transporter)
☞ in cystic fibrosis, CFTR not working → no Cl- outside cell → no water moving out with Cl- → produces sticky, thick mucus
☞ in diarrhoea, protein kinase A phosphorylates CFTR → loads of Cl- being pumped out of cell → loads of water moving with it → diarrhoea

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9
Q

Facilitated diffusion via gated channels (and examples)

A
  • Can be ligand gated where a molecule like ACh binds → conformational change → allowing Na+ to flow through (nicotinic acetylcholine receptor in neuromuscular junction)
  • another ligand gated ion channel is when ATP binds (not hydrolysed, just binds) → conformational change → closes channel → K+ cannot flow out of cell (ATP-sensitive K+ channel in insulin-secreting beta cell)
  • Can be voltage gated where a membrane depolarisation occurs → sensed by voltage sensor → conformational change → opens channel → Ca2+ can flow into cell (Ca2+ channel)
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10
Q

Facilitated diffusion via carrier proteins

A
  • Sequential binding and conformational change
  • Allows different control mechanisms
  • Limited number of substances can be moved (reaches saturation point)
  • Facilitated diffusion is usually much slower, depending on number of carrier proteins at membrane
  • Central pore is usually aqueous
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11
Q

Types of carrier (transport proteins)

A

one substance being transported
☞ uniport

coupled transport (co-transport)
☞ symport = two or more substances moving in the same direction
☞ antiport = two or more substances moving in opposite directions

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12
Q

Uniport vs co-transport

A

uniport
- Only one molecule transported at a time per cycle down the concentration gradient
- Can be a channel or a carrier protein
- Sometimes no energy needed (just kinetic energy of particles) eg GLUT3
- Sometimes ATP is needed to transport substance against it’s concentration gradient (eg proton pump)

co-transport
- Usually two or more substances transported on a membrane transporter per reaction cycle
- One substance could be moving down its concentration gradient, whereas the other might be carried against its concentration gradient

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13
Q

Symporter vs antiporter

A

symporter = co-transport, where two or more substances are being carried in the same direction
antiporter = co-transport, where two or more substances are being carried in different directions

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14
Q

What are some of the general physiological roles of transport processes

A
  • Maintenance of ionic composition
  • Maintenance of cellular pH
  • Regulation of cell volume
  • Concentration of metabolic fuels + building blocks
  • Expulsion of metabolic waste products and toxic substances
  • Generation of ion gradients necessary for the electrical excitability of nerve and muscle
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15
Q

What is active transport

A
  • the transport of ions or molecules against a concentration gradient and/or electrical gradient
  • Energy directly (or indirectly) comes from ATP hydrolysis
  • Can be primary (energy derived directly from breakdown of ATP) or secondary (co-transporters aka symporters)
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16
Q

What is the difference between primary and secondary active transport

A

primary directly uses a source of chemical energy (eg ATP) to move molecules against their concentration gradient
secondary (aka co-transport) uses an electrochemical gradient (generated by primary active transport) to move molecules against their gradient, and therefore doesn’t directly require a chemical source of energy such as ATP

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17
Q

What are the primary active transporters

A
  • Sodium potassium pump
  • Calcium ATPases (PMCA and SERCA)
  • Potassium/proton ATPase, aka proton pump
  • F1F0 ATPase (aka ATP synthase)
    these can also be used to set up an electrochemical gradient for secondary active transport
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18
Q

Sodium-potassium pump

A

3Na+ bind → conformational change → allows ATP to bind → CC → Na+ released out of cell → CC → allows 2K+ to bind → K+ released into cell

  • Key concept: 3Na+ out for every 2K+ that enter
  • Very important for generating action potential
  • Driven by conformational change
  • Antiporter
  • Consists of an α and β subunit (more detail on next card)
  • Every cell has one of these
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19
Q

What do the α and β subunits of the sodium-potassium pump do

A

α has K+, Na+ and ATP binding sites
β is responsible for directing pump to cell surface membrane

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20
Q

What are the main functions of the sodium-potassium pump

A

forming Na+ and K+ gradients which are necessary for electrical excitability

driving secondary active transport
- pH control
- cell volume regulation
- absorption of Na+
- nutrient uptake eg glucose

21
Q

Free ion distribution across the cell membrane

A
  • there are a number of cations and anions present in cells + outside the cell membrane
  • this means that there is a concentration gradient present
  • these concentrations need to be carefully controlled in order to ensure proper function of the membrane
  • K+ there is a much greater concentration of this inside the cell than out. Too low = coma. Too high = cardiac arrest.
  • Ca2+, Cl- and Na+ all have a greater concentration outside the cell than in.
  • Ca2+ is toxic to cells in high quantites, due to the high phosphate concentration inside cells → form calcium phosphate → calcified cells
22
Q

Ca2+ - ATPase (aka PMCA)

A

excludes Ca2+ ions from cell
- Does this by ATP → ADP + Pi (inside cell)
- Helps control the [Ca2+] with other transporters
- Uniporter

23
Q

Control of resting Ca2+ concentration (in general)

A

primary active transport
- PMCA (expels Ca2+ out of cell, removing residual Ca2+ as high affinity + low capacity)
- SERCA (accumulates Ca2+ into the SR/ER as high affinity + low capacity)
secondary active transport
NCX (low affinity + high capacity so removes most Ca2+)

facilitated diffusion
Mitochondrial Ca2+ uniports (operate at a high [Ca2+] to buffer potentially damaging high concentrations)

24
Q

Sodium-calcium exchanger (NCX)

A

one Ca2+ out for every 3Na+ in
- Works with PMCA primarily to control [Ca2+]
- Antiporter
- Activity is membrane potential dependent (depolarised membrane potential reverses mode of operation)
- Ie when inside cell is more positive, 3Na+ out and one Ca2+ in

25
Q

What is the main difference between NCX and PMCA

A

both control [Ca2+] in the cell
NCX is low affinity, high capacity (doesn’t bind to calcium well but can transport it very fast). Antiporter
PMCA is high affinity, low capacity (binds a small amount of calcium well). Uniport

26
Q

F1F0 – ATPase (ATP synthase)

A
  • Sits between the inner + outer membrane (intermembranous space) in mitochondria
  • Active transport in reverse
  • For every 3H+ pumped from intermembranous space → mitochondrial matrix, one molecule of ATP is formed
  • ADP + Pi → ATP (in matrix)
  • The H+ are from the electron transport chain
  • Cause conformational change
27
Q

Secondary active transporters

A
  • Co-transporters
  • Usually using an electrochemical gradient already set up by primary active transport
  • symporters in same direction, antiporters move two or more substances in opposite directions
  • Usually electroneutral (no change in electrical charge across the plasma membrane)
28
Q

Definition: electroneutral

A

No change in electrical charge across the plasma membrane

29
Q

How many Na+ does SGLT1 and SGLT2 move?

A

SGLT1 moves 2
SGLT2 moves 1
(from extracellular space → cytosol)

30
Q

Mechanism of ischaemia (involving NCX and Na+-K+-ATPase)

A

sodium-potassium ATPase
ATP is depleted → sodium pump inhibited → less Na+ being pumped out of cell → more Na+ accumulates inside cell → cell is depolarised (ie becomes more positive inside cell than out)

NCX
Depolarised cell is energetically unfavourable → NCX reverses → now 3Na+ out and one Ca2+ in → this means more Ca2+ is entering the cell → high Ca2+ is toxic

31
Q

Na+/H+ exchanger (NHE)

A

one Na+ into cell, while one H+ out
- Works using Na+ gradient set up by sodium-potassium pump
- Electroneutral due to 1:1 ratio
- Antiporter
- Regulates pH and cell volume
- Activated by growth factors
- Can be inhibited by amiloride which is a potassium-sparing diuretic

32
Q

Control of cell pH

A
  • Na+-K+-ATPase is in all cells, creates an Na+ gradient for other transporters
  • NHE involved in Na+ and H+ exchange (acid extrusion)
  • NBC Exchanger is also acid extrusion, and alkali influx
  • **sodium-bicarbonate co-transport* is involved in alkali influx
  • anion exchanger (aka band 3 in erythrocytes) are involved in alkali extrusion
33
Q

Bicarbonate transporters (NBC + AE) – details on separate cards

A
  • NBC aka Na+-bicarbonate-chloride co-transporter alkalinises cell
  • AE aka band 3 in erythrocytes acidifies cell
  • Both are involved in cell volume regulation
34
Q

NBC (sodium-bicarbonate-chloride exchanger)

A
  • One Na+ and one HCO3 in
  • One Cl- and one H+ out of cell
  • Alkalinises cell
  • Involved in both acid extrusion + alkali influx
  • Also involved in cell volume regulation
35
Q

Anion exchanger (AE)

A
  • Aka band 3 in erythrocytes
  • One HCO3- out and one Cl- in
  • Acidifies cell by alkali extrusion
  • Antiporter
  • Electrogenic due to 1:1 ratio
36
Q

Sodium-bicarbonate co-transport

A
  • 3HCO3- and one Na+ into cell
  • Involved in cellular pH regulation
  • Alkali influx into cell
37
Q

Sarco(endo)plasmic reticulum ATPase (SERCA)

A
  • One H+ from inside SR/ER → intracellular fluid
  • One Ca2+ from intracellular fluid → SR/ER
  • One ATP → ADP + Pi (intracellular fluid)
  • This is the equivalent of the PMCA but on the sarco(endo)plasmic reticulum
  • Responsible for replenishing Ca2+ stores in the SR/ER
  • Antiporter
38
Q

Hydrogen-potassium ATPase

A
  • One H+ into gastric lumen, where one K+ into cytoplasm of parietal cell
  • Antiporter
  • Responsible for acidification of stomach contents
  • Found in parietal cells (highly specialised epithelial cells located in the inner stomach lining)
  • electroneutral due to 1:1 ratio, so no effect on membrane potential
39
Q

Mitochondrial Ca2+ uniporters

A
  • Pump Ca2+ into mitochondrial matrix from the intracellular fluid
  • Form of facilitated transport
  • Used at high [Ca2+] to buffer damaging Ca2+ concentrations
40
Q

Why is high [Ca2+] damaging to cells

A
  • There is a high phosphate concentration inside cells
  • Calcium phosphate formed
  • Leads to calcified cells
  • May end up interfering with organ function or cause other health issues eg cancer
41
Q

Km is half of Vmax – how do we make this estimate more accurate

A

Use a Lineweaver-burk plot
(it’s a graphical representation of enzyme kinetics)

42
Q

Glucose transport mechanisms – uptake in the gut

A

uses SGLT-1 and GLUT2
- Microvilli express SGLT1 (co-transporter of Na+ and small-no polar molecules)
- Therefore Na+ drawn into enterocyte with glucose from the ileum lumen
- Na+ recycled using sodium-potassium ATPase
- Some of the glucose remains in the enterocyte to be used
- 60% of the glucose is transported immediately to interstital space, and then into blood by GLUT2
- A small amount of the glucose slowly leaves by passive diffusion (glucose can cross membranes, but extremely slowly)

43
Q

Glucose absorption using H+

A
  • Hydrogen-potassium ATPase (found in parietal cells of the inner stomach lining) are responsible for acidification of the stomach contents
  • H+-K+-ATPase pump K+ into the cytoplasm of parietal cell, and H+ into the gastric lumen (antiporter)
  • This creates a proton differential
  • Can now use sucrose-H+ symporter
  • Proton and glucose transported into enterocyte simultaneously
  • Sucrose → glucose + fructose
  • Glucose transported into blood using GLUT2
44
Q

Glucose re-absorbtion in the kidney

A

the proximal convoluted tubule has 3 segments
☞ to get glucose from PCT → epithelial cell
in segment 1, SGLT2 + 1Na+ used
in segment 2+3, SGLT1 + 2Na+ used

☞ to get glucose from epithelial cell → bloodstream
In segment 1, GLUT2 used
In segment 2+3, GLUT1 used

Na+ ions are recycled using Na+-K+-ATPase

45
Q

What are the GLUTs used to transport glucose in liver, muscle and adipose

A

GLUT2 + GLUT4
Insulin and its receptor regulate GLUT synthesis and intracellular transport

46
Q

What are MCT proteins

A

Monocarboxylate transporters
Symporters that transport H+ ion and lactate ion (-) from astrocyte to neurone in CNS

47
Q

Glucose transport in the CNS

A

☞ Pathway 1 key molecules = GLUT1, MCT1, MCT2 and MCT4
Glucose into cell by GLUT 1 → glucose converted to lactate (via pyruvate) by astrocyte → lactate transported to neurone by MCT

☞ Pathway 2 key molecules = GLUT1, GLUT3, MCT1 and MCT4
Glucose transported by GLUT1 (in apical + basal surface of epithelial cell) to interstitial space → glucose transported from interstitial space to neurone by GLUT3 in neurolemma

(in both pathways, glucose → G6P so it’s trapped inside cell)

48
Q

In general, what is the mechanism of GLUT

A

facilitated diffusion
Insulin promotes GLUT containing vesicles from cytoplasm → plasma membrane
This is an example of regulated secretion
☞ glucose transport

49
Q

In general, what is the mechanism of SGLTs

A

secondary active transport
Na+-K+-ATPase generates the sodium electrochemical gradient that drives the transport of glucose

ICPP (5 decks)

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