M&R 2.1 Membranes as permeability barriers

Question 1

What method is used to study the permeability of substances through a bilayer? How does it work?

Answer 1

Black Film method
Container with a septum in the middle, which has a pinhole in it. Pain phospholipids over the pinhole so a bilayer forms over it - can then look at permeability of substances from one side of the septum to the other

Question 2

Which types of molecules can passively diffuse through the bilayer?

Answer 2

Gases (O2, CO2, N2 etc) - diffuse through easily because small, uncharged & non-polar  
Hydrophobic molecules (e.g. steroid hormones, benzene) because lipophilic 
Small uncharged polar molecules (e.g. H20, urea, glycerol)

Question 3

Which types of molecules cannot passively diffuse across the bilayer?

Answer 3

Large uncharged polar molecules (e.g. glucose, sucrose) - have 6 carbons so are too large
Ions (e.g. H+, Na+, K+, Ca2+, Cl- etc) because charged

Question 4

How do gases cross the bilayer?

Answer 4

Passive diffusion, because they are small, uncharged and non-polar

Question 5

How do steroid hormones cross the bilayer?

Answer 5

Passive diffusion, because they are lipophilic

Question 6

How do urea, water and glycerol cross the bilayer?

Answer 6

By passive diffusion, because they are small and uncharged

Question 7

Can ions cross the bilayer by passive diffusion?

Answer 7

No, because they are charged

Question 8

Why can’t glucose and sucrose cross the bilayer by passive diffusion?

Answer 8

Because they are too large (6C)

Question 9

How does the rate of passive diffusion relate to the concentration gradient of a substance?

Answer 9

Rate of passive diffusion increases linearly with increasing concentration gradient

Question 10

Membranes act as permeability barriers to what molecules with what properties?

Answer 10

Charged
Large
Hydrophilic

Question 11

What is facilitated diffusion?

Answer 11

Movement of a solute down its concentration gradient by the action of membrane proteins

Question 12

Is facilitated diffusion an example of active transport or passive transport?

Answer 12

Passive transport - because despite involving the action of membrane proteins, the solute is still moving DOWN its concentration gradient (so the process does not directly require chemical energy)

Question 13

Name two main types of membrane proteins that assist with facilitated diffusion

Answer 13

Gated pores (protein binds the substrate and then undergoes a conformational change, releasing the substrate on the other side of the membrane) - AKA ‘ping-pong’ transport

Ion channels (closed channels will respond to a stimulus (ligand or voltage) and open, creating a channel through which the substance can move)

Question 14

Which type of membrane transport protein transports substances at a greater rate, gated pores or ion channels?

Answer 14

Ion channels 
(once the channel is open the substances can be transported through quickly, whereas gated pores have to individually bind each molecule of substrate, which takes longer)

Question 15

Name the two main categories of ion channel

Answer 15

Ligand gated ion channels

Voltage-gated ion channels

Question 16

On which side does the ligand bind on a ligand-gated ion channel?

Answer 16

Can be inside or outside
Outside - e.g. nAChR binding ACh
Inside - e.g. ATP-sensitive K+ channel (normally open, when ATP from inside the cell binds the channel closes)

Question 17

How do voltage-gated ion channels work?

Answer 17

At rest the cell is more negative inside and more positive outside
When depolarised (e.g. during an AP) this changes the other way around 
The channel detects this change and opens

Question 18

How does the rate of facilitated diffusion relate to the concentration gradient of a substance?

Answer 18

As the concentration gradient increases, the rate of diffusion increases, up until a maximum rate of transport (when all the membrane transporters are busy)
Therefore facilitated diffusion is a saturable transport process

Question 19

What is the difference between active and passive transport?

Answer 19

Passive transport - transport of the ion/molecule can occur spontaneously
Active transport - transport of the ion/molecule requires energy

Question 20

What is the purpose of active transport?

Answer 20

It allows transport against an unfavourable concentration gradient and/or electrical gradient

Question 21

What is primary active transport and how does it work?

Answer 21

A molecule is transported against its concentration/electrical gradient using free energy, usually chemical energy from hydrolysis of ATP (but also from electron transport, light etc)

Question 22

What is secondary active transport and how does it work?

Answer 22

When the transport for one molecule is linked to the concentration gradient of another molecule, via a co-transporter. The primary energy source (e.g. ATP hydrolysis) is used indirectly to generate a gradient of something else (often Na+). This gradient is then used to drive the membrane transporters.

Question 23

What is co-transport?

Answer 23

When the transport of one molecule depends on the simultaneous or sequential transport of another molecule, either in the same direction or the opposite direction

Question 24

Co-transport where both molecules move in the same direction is via…

Answer 24

Symporters

Question 25

Co-transport where molecules move in opposite directions is via…

Answer 25

Antiporters

Question 26

What are the concentrations of sodium inside and outside a typical cell?

Answer 26

~145mM Na+ outside cell

~12mM Na+ inside cell

Question 27

Describe the concentration gradient and electrical gradient of Na+ for a typical cell

Answer 27

Concentration gradient INTO the cell
Electrical gradient INTO the cell

So if you opened an Na+ channel, it would rush in

Question 28

What are the concentrations of chloride inside and outside a typical cell?

Answer 28

~123mM Cl- outside

~4.2mM Cl- inside

Question 29

Describe the concentration gradient and electrical gradient for chloride across a typical cell

Answer 29

Concentration gradient INTO the cell

Electrical gradient OUT of the cell (cell at RMP is negative inside and positive outside)

Therefore if open a Cl- channel, Cl- leaves the cell down its conc gradient but not as readily as Na+ because of its electrical gradient

Question 30

What are the concentrations of potassium inside and outside a typical cell?

Answer 30

~4mM K+ outside

~155mM K+ inside

Question 31

Describe the concentration gradient and electrical gradient for K+ across a typical cell

Answer 31

Concentration gradient OUT of cell

Electrical gradient INTO cell (because inside is more negative than outside at RMP)

Question 32

What are the concentrations of calcium inside and outside a typical cell?

Answer 32

~1.5 mM Ca2+ outside cell

~ 10^-4 mM Ca2+ inside cell

Question 33

Describe the concentration gradient and electrical gradients of calcium across a typical cell

Answer 33

Concentration gradient INTO cell (Very large! Cell is going to great lengths to extrude Ca2+ - means small raises of Ca2+ can act as a signal)

Electrical gradient INTO cell

Question 34

What are some examples of primary active transporters?

Answer 34

Plasma membrane Ca2+ ATPase (PMCA)

F1F0 ATPase (ATP synthetase)

Na+/K+ ATPase (also a cotransporter)

(Ca2+ - Mg2+ - ATPase)

Question 35

How does the PMCA transporter work?

Answer 35

= plasma membrane Ca2+ ATPase

Actively transports Ca2+ out of cytosol (either into extracellular fluid or ER, depending which membrane it’s on)

Therefore helps maintain very low Ca2+ inside cytosol

Question 36

What kind of transporter is the F1F0 ATPase transporter?How does it work in the plasma membrane vs the inner mitochondrial membrane?

Answer 36

In the plasma membrane - if H+ is high inside the cell, the transporter uses energy derived from ATP hydrolysis to extrude H+ and therefore maintain intracellular pH

In the inner mitochondrial membrane - high H+ in the intermembrane space drives the transporter in the reverse direction. Therefore it uses the H+ concentration gradient to generate ATP

Question 37

The Na+/K+ ATPase is an example of both a __________ and a __________

Answer 37

Cotransporter (antiporter - moves both Na+ & K+ but in opposite directions)

Primary active transporter (uses ATP)

Question 38

What does the Na+/K+ ATPase do?

Answer 38

Pumps 3Na+ out of the cell and 2K+ into the cell
This maintains the strong concentration gradients of Na+ inwards and K+ outwards

(it DOES NOT set the RMP!!)

Question 39

Name some secondary active transporters, and what they do

Answer 39

Na+ Ca2+ exchanger (NCX) : uses gradient established by Na+/K+ ATPase. Brings in Na+ down its concentration gradient and in exchange extrudes Ca2+ from the cell against its concentration gradient (antiport)

Na+ H+ exchanger - moves Na+ into the cell and sends H+ out against its concentration gradient (antiporter)

Na+ - glucose co-transporter (symporter - moves Na+ into the cell and uses the energy to drive glucose into the cell also) e.g in enterocytes

Question 40

Name 2 transporters involved in Ca2+ transport, and their relative affinity/capacity

Answer 40

Ca2+ Mg 2+ ATPase (actively transports Ca2+ out of the cell using energy from ATP [requires Mg2+ as a cofactor]. Has high affinity but low capacity

Na+ Ca2+ exchanger (Na+ in, Ca2+ out using Na+ gradient) - has low affinity but high capacity

Question 41

What does the CFTR transporter do?

Answer 41

Exports chloride ions so that water will follow

Question 42

How is CTFR affected in cystic fibrosis?

Answer 42

Mutation of CTFR gene leads to a mutated transporter. Cl- not extruded so water does not follow, leading to thickened mucus in the lungs. Also knock on effects for other transporters due to excess intracellular

Question 43

How is CFTR affected in cholera?

Answer 43

CFTR gets phosphorylated which increases its activity. Therefore excess Cl- is secreted and water follows, leading to diarrhoea

Also knock on effects on other transporters because of reduced intracellular Cl-