Membrane Permiability

Question 1

What is a semi-permeable membrane!?

Answer 1

A layer through which only allowed substances can pass.

Water is a component of plasma membranes as it allows folding. Without water, tissue will shrink.

Question 2

Describe the black film technique used when studying cell membranes.

Answer 2

• phospatidylserine is mixed and spread into a glass a plastic plate that has a pinhole in it.
• It is then submerged into a hydrophilic solution
• It (the hole) appears black because a phospholipid bilayer has been creates and therefore light is reflected back.

Question 3

Is the phospholipids bilayer permeable to hydrophobic molecules?

Answer 3

Yes. Hydrophobic molecules such as oxygen, carbon dioxide, nitrogen and benene easily pass through the membrane.

Question 4

Is the phospholipids bilayer permeable to small, uncharged, polar molecules?

Answer 4

Yes. The membrane is permeable to small, uncharged polar molecules such as water, Urea and glycerol because there are small gaps between phospholipid molecules which are big enough to let polar molecules through.

Question 5

Is the phospholipids bilayer permeable to large, uncharged, polar molecules?

Answer 5

No. These molecules can go through the membrane but, they travel veryyyyyyy slowly so they cannot rely on diffusion through the membrane as a method of transport.

Question 6

Is the phospholipids bilayer permeable to ions?

Answer 6

No.

Question 7

What are the two things that passive transport is dependant on?

Answer 7

Permissibly and concentration. The rate of passive transport increases linearly with increasing concentration gradient.

Question 8

What are permeability coefficients?

Answer 8

These are numbers given to different molecules on a log scale to express how permeable the membrane is to them. Eg water has a high coefficient (10^-2) as it easily passes through the membrane and sodium ions have a low coefficient (10^-14) as they do not. The changes in these coefficients can be used experimentally to work out the rough composition of a membrane.

Question 9

What important roles to transport processes play?

Answer 9

• Maintain the ionic composition
• Maintain the intracellular pH
• Regulate cell volume
• Concentrate metabolic field and building blocks
• Expel metabolic waste products and toxic substances (eg drugs)
• Generate an ion gradient that is necessary for the electrical excitability of nerves and muscles.

Question 10

What are some models of membrane transport proteins that are unlikely?

Answer 10

-Reciprocating carrier (flip flop), Rotating carrier (although both of these require a lot of energy so are thermo dynamically unlikely. Basically not happen!)

Question 11

What are some models of membrane transport that are more likely?

Answer 11

Gated pore - substances comes along and binds to middle of protein. This causes a conformational change so substance is released on the outer side (which causes another conformational changed) This is called also called ping pong transport.
Facilitated diffusion via channels - this is when the channel opens so that many molecules can go in all in one go. Eg in muscle contraction, lots of Ca go through in one go.

Question 12

What is the problem with ping pong transport?

Answer 12

Is requires a lot of time

Question 13

What are the two types if ion channels and how are they different form each other?

Answer 13

Ligand gated ion channels - a ligand (eg Ach) bind to a receptor on the channel (eg Nicotinic acetylcholine receptor) which causes a conformational change and ions (eg Na) to flow in. The conformational change created can also have the opposite effect where is closes the ion channel and prevents molecules from leaving. But, they are bidirectional so ions will flow down concentration gradient.
Voltage gated ion channels - positive charged amino acid in channel. In rest state, this amino acid will be associated with the inner, hydrophobic part of the membrane. When depolarisation occurs, channel opens because amino acids repel. This is called voltage sensor.

Question 14

What is delta-G? What value of it shows transport is active?

Answer 14

Delta-G is Gibbs free energy. If transport is active, then delta-G will be positive because the energy change is not favourable and so requires energy.
There are a few different mathematical equations that can be used to work out the value of delta-G.

Question 15

What is the relationship between delta-G and membrane potential or concentration ratio?

Answer 15

Both the concentration ratio and membrane potential have a positive, linear relationship with delta-G.

Question 16

What is active transport?

Answer 16

-It is a form of membrane transport that allows the transport of ions or molecules against an unfavourable concentration and/or electrical gradient.
-The energy for this process comes directly or indirectly from ATP hydrolysis.
Some cells eg nerves cells, muscle cells, erythrocytes, enterocytes (use Na pump), hepatocytes spend up to 30-50% of their energy on active transport.

Question 17

What are the three types of passive transport?

Answer 17

Simple diffusion.
Facilitated diffusion through carriers.
Facilitated diffusion through channels.

Question 18

What are the concentrations on ions (Na, K, Ca, Cl) inside compared to outside a cell?

Answer 18

Na, Ca has Cl all have a higher concentration extracellularly than intracellularly. Calcium’s concentration extracellularly is 10,000 times greater than that found intracellularly. Whereas, K has a higher concentration extracellularly compared to inside the cell.

Question 19

Are electrolyte levels those found intracellularly or extracellularly?

Answer 19

Electrolyte levels are those found extracellularly.
K: 4mM (3-5 so tight range)
Na: about 145mM
Cl: 123mM
Ca: 1-1.5mM (need small amount to her changes inside the cell)

Question 20

How is the level of calcium inside a cell controlled?

Answer 20

Using CaATPase. This is a primary active transporter and it’s role is to exclude calcium ions. (PMCA)

Question 21

What is ATP synthase?

Answer 21

ATP synthase is also known as F1F0-ATPase and it is active transport in reverse. It is located in the membrane of the mitochondria and it used H+ to generate ATP. It is NOT a pump because it’s not on the plasma membrane.

Question 22

What is the difference between uni-transport and co-transport?

Answer 22

In uni-transport, only one molecule is transported at a time eg a proton pump. Whereas, in co-transport, more than one type of ion or molecule may be transported on a membrane transporter per reaction cycle.

Question 23

What are the two types of co-transporter?

Answer 23

Symport where both molecules are transported in the same direction
Antiport where both molecules are transported in different directions

Question 24

Describe the action of NaKATPase.

Answer 24

3 molecules of Na are pumped out for every two molecules of K pumped in. This is very important for generating thr ion gradients that are used to allow an action potential.
It uses ATP so it’s active transport.
It is located in the plasma membrane so is a pump
25% of the basal metabolic rate is used to keep this pump working
It is made of two subunits alpha and beta but, all binds to alpha.
Alpha subunit also contains Quabain which controls heart rate. The beta unit directs the pump to the cell surface and has oligosaccharides attached it it.
It is called a P-type ATPase because ATP phosphorylates aspartate (on alpha subunit) and produces a phosphoenzyme intermediate.

Question 25

Does NaKATPase control the action potential?

Answer 25

No! It creates high intracellular calcium but, it only generates 5-10mV. K+ diffusion through channels is mainly responsible for the resting membrane potential.

Question 26

What are the two types of Ca transporter?

Answer 26

• CaATPase which is a uniporter and has high affinity, low capacity.
• NaCa exchanger which is an antiporter co-transporter and NOT a pump (as no ATP). This is low affinity, high capacity.

Question 27

How does the Na+ H+ exchanger Work?

Answer 27

It uses the Na generated by NaKATPase to drive the antiport of Na back into the cell and H+ out.

Question 28

How does Floxetine (Prozac) work?

Answer 28

• The outwards flow of K+ down its concentration gradient leads to the uptake of serotonin in the presynaptic cleft and by platelets with Na+ ions. (Symport)
• Fluoxetine inhibits SERT and increases 5-HT (serotonin) action, elation and “sticky blood”. This means that it blocks the reuptake of serotonin and therefore allows it to stimulate the post synaptic neurone for longer.

Question 29

How do mutations in transporters cause cystic fibrosis?

Answer 29

A mutation in the CFTR transporter means that chloride ions cannot be moved out of the cell. This means that water is not moved out with them so mucus becomes thick and sticky.

Question 30

How do faults in transporters cause diarrhoea (cholera toxin)?

Answer 30

CFTR protein is also in the gut. The cholera toxin causes phosphorylation of this transporter. Therefore, more chloride ions are moved out of the cell and into the lumen. This moves more water with it so watery stools occur.