Membrane As Permeability Barrier

Question 1

How do we find out whether a molecule can cross a lipid bilayer or wether it uses transport proteins?

Answer 1

We put a lipid bilayer (black film) across a pinhole between two separate chambers and see which molecules cross alone. Those who cross can permeate the membrane, those who cant need a transport protein.

Question 2

Why is the membrane referred to as semi permeable?

Answer 2

As some molecules can cross easily, but others require transport proteins

Question 3

What molecules is the lipid bilayer permeable to? (They don’t need transport proteins)

Answer 3

Hydrophobic molecules - O2, CO2, N2, Benzene (mostly gases)

Small, Uncharged polar molecules- H2O, Urea, Glycerol (small 2 C’s_

Question 4

What is the lipid bilayer not permeable to? (Molecules that require a transport protein)

Answer 4

Large, Uncharged Polar Molecules - Glucose (6C’s), Sucrose (disaccharide)

Ions - H+, Na+, K+, Ca2+, Mg2+, Cl-, HCO3- (charged)

Question 5

How are permeability co-efficient written?

Answer 5

In a logarithmic scale as they vary so widely.

Question 6

What is the link between passive transport and concentration gradient?

Answer 6

Rate of passive transport increases linearly with increasing concentration gradient.

It also depends on permeability.

Question 7

What key roles do transport processes have?

Answer 7

Maintaining ionic composition
Maintaining intracellular pH (work with buffers)
Regulate cell volume
Concentration of metabolic fuels and building blocs
Extrusion of waste metabolites and toxic substances
Generation of ion grads for electrical excitability of nerve/muscle

Question 8

Why do protein carries not flip flop or rotate with the molecule?

Answer 8

As this it very thermodynamically unlikely, it requires to much energy for each molecule to be moved.

Question 9

How do gated pores work? (Ping pong protein transporters)

Answer 9

They have a recognition site, when a molecule binds to this a conformational change takes place allowing the molecule through. It has a maximum rate when all pores are in use.
It’s thermodynamically more likely.

Question 10

Describe facilitated diffusion via an ion channel.

Answer 10

Passive process (down conc/electric grad)
Gated at rest
Opens to a specific stimulus
Is highly selective to a single molecule
Is rapid-used for electrical signalling

Question 11

Describe a ligand gated ion channel completing facilitated diffusion. And give 2 examples

Answer 11

Has a seperate binding site for ligand (ACh)
Opens or closes to a diff molecule (Na+) when ligand is bound

Eg-Nicotinic Acetylcholine receptor
(Ligand=nicotine or ACh. Molecule transported=Na+ moves in)

Eg-ATP-sensitive K+ channel
(Ligand=ATP, this closes gate. Molecule=K+ moves out)

Question 12

Describe a voltage gated ion channel in facilitated diffusion and give an example.

Answer 12

Opens/closes to membrane depolarisation which causes the protien to change in conformation.

(At rest cell is -ve inside and ve outside, conformational change cures when this is reversed).

Eg-Na+ channel (allows Na+ into cell when its depolarised)

Question 13

How can we tell is transport is active or passive?

Answer 13

If against conc grad its active.
When conc is the same across the membrane we look at membrane potential. If its against the electro-gradient we need energy so its active.

Question 14

What is the link between conc gradient and transport ratio?

Answer 14

It’s a logarithmic ratio.

This means when conc grad increases a little, rate of transport increases a lot.

Question 15

What are the key features of active transport?

Answer 15

Allows transport of molecules/ions against unfavourable concentration and/or electrical gradient.

Energy directly or indirectly form ATP hydrolysis

Cells spend 30-50% of ATP energy on active transport

Question 16

What is the concentration of Na+ outside the cell membrane?

And which way does the gradient go across the membrane?

Answer 16

145 mM

Inward gradient.

Question 17

What is the concentration o Cl- outside the cell membrane? And which way does the concentration gradient go?

Answer 17

123mM

Inward gradient

Question 18

What is the concentration of Ca2+ outside the cell? And which way does the gradient go?

Answer 18

1.5mM

High inward gradient

Question 19

What is the concentration of K+ outside the cell and which way does the gradient go?

Answer 19

4mM

Outward gradient

Question 20

What is a primary active transporter?

Answer 20

It directly takes ATP onto the protein and hydrolyses it. It uses this energy to conformational change the protein and drive a molecule in/out the cell.

Question 21

Give an example of a primary active transporter.

Answer 21

Plasma Membrane Calcium ATPase (PMCA)

It uses ATP energy to drive Ca2+ out of the cell, maintaining a low intracellular Ca2+ conc.

Question 22

What is active transport in reverse mode, give the examples dn explain.

Answer 22

The protein uses transport gradient to drive ATP synthesis.

Eg-ATP synthase discharges the protein gradient in the inter-membrane space to drive ATP synthesis.

Question 23

What is co-transport?

Answer 23

When more than one type of molecule/ion is transported on a membrane transporter per reaction cycle.

Both molecules must be present to low movement

Question 24

What is a uniport transporter?

Answer 24

It transports a single molecule in a single direction

Question 25

What is a symport transporter?

Answer 25

Transports 2 molecules per cycle in the same direction.

Example of co-transport

Question 26

What is an antiport transporter?

Answer 26

Transports 2 molecules but in opposite directions.

Example of co-transport

Question 27

What are the key features of the Sodium Pump?

Answer 27

Plasma membrane associated
Uses ATP - Active transport
Moves 3Na+ outside and 2K+ inside
An Antiport

Called a P-Type ATPase (when it hydrolyses ATP it phosphorylates Aspartate, to produce a phosphoenzye intermediate. This intermediate causes the proteins conformational change.)

Generates ion gradients for secondary active transport and action potentials

Only contributes small amount to membrane rest potential.

2 units- alpha contains al binding sites including inhibitory
Beta directs pump to surface from ER.

Question 28

How is the sodium pump related to membrane potential?

Answer 28

It creates high intracellular K+

K+ diffusion through channels is what causes membrane potential

Th sodium pump only generates about 5-10mV through electrogenic pump activity = only contributes small amount

Question 29

What 2 forms of Ca2+ transport are there and what are the differences?

Answer 29

Ca2+_Mg2+_ATPase = direct active transport, moves Ca2+ out cell, high affinity but low capacity

Na2+_Ca2+_exchanger=2ndry active transport (uses grad established by sodium pump not ATP energy), is an antiport, Ca2+ out and 3Na+ in, low affinity but high capacity

Question 30

Describe the Na+_H+_exchanger.

Answer 30

2ndry active transport

The high Na+ outside the cell (from the sodium-pump) used instead of ATP.

One Na+ in and one H+ out.
Antiport

Question 31

Describe the Na+_glucose transporter.

Answer 31

A symport
2ndry active transport
Entry of Na+ provides energy for entry of glucose against conc gradient.

One Na+ in for every one glucose in

Question 32

Give examples of secondary co-transport system, with their main role and state whether they’re an Antiport or symport.

Answer 32

Na+_K+_ATPase = (maintain cell conc of Na+/K+ antiport)

Na+_Ca2+_exchange = (Na+ inward down conc grad, drive outward flow of Ca2+ up its conc grad, antiport)

Na+_H+_exchange = (inward Na+ down conc grad, leads cell alkalisation by removing H+ from cell, antiport)

Na+glucose co=transport = (entry of Na+ provides energy for entry of glucose against conc grad, symport)

Question 33

What is the role of transporters in Cystic Fibrosis?

Answer 33

The CFTR protein, moves Cl- out of the cell into the lumen,this carries water with it which then thins the mucus in the lumen. This protien doesn’t work in CF so mucus remains thick as Cl- and therefore water isn’t transported into the lumen.

Other transporters work together at the other side of cell to push Cl- into cell form interstitium to allow the CFTR protein to remove it into the lumen.

Question 34

How are transporters relevant in diarrhoea?

Answer 34

A bacteria such as cholera activates protein Kinase. This in turn stimulates the CFTR protein and so much more Cl- is moved into the lumen of the gut. Lots of water follows this leading to diarrhoea and dehydration.