Cell membranes, Transport and Gradients

Question 1

What are the 3 types of membrane proteins?

Answer 1

• Integral
• Peripheral
• Lipid anchored

Question 2

What are the permeability properties of the lipid bilayer?

Answer 2

• Inside of the bilayer is very hydrophobic so most molecules and ions are impermeable.
• Impermeable barrier is very important to keep substances inside / outside the cell.
• Some substances may pass through the membrane (only under certain conditions).

Question 3

What is passive transport?

Answer 3

Transport down the concentration gradient which does not require energy.

Question 4

What is active transport?

Answer 4

Transport up the concentration gradient requiring energy in the form of ATP.

Question 5

Describe primary active transport

Answer 5

Active transport which uses the energy source directly

Question 6

Describe secondary active transport

Answer 6

Active transport which uses the energy source indirectly

Question 7

Illustrate symport

Answer 7

Subtype of cotransport

Question 8

Illustrate antiport

Answer 8

A type of cotransport

Question 9

Explain this equation

Answer 9

• Vinward = rate of diffusion into cell (moles / second / cm² of membrane)
• ΔS = concentration gradient across the membrane
  
  • So, ΔS = {S}outside - {S}inside
• P = permeability coefficient
• The rate of diffusion into the cell depends on the thickness and viscosity of membrane size, shape, polarity and solubility in membrane of substrate.
• It is constant for a particular system

Question 10

What is the partition coefficient?

Answer 10

Partition coefficient gives a measure of how well a substance dissolves in lipid or aqueous phase.

Question 11

How do you calculate the partition coefficient?

Answer 11

The partition coefficient is essentially how well the substance dissociates in oil and how well it dissociates in water.
A substance may not dissolve completely in water or oil, but may dissociate a little in both.

Question 12

What is the relationship between rate of transport through the lipid phase of the membrane and polarity of the molecules attempting to diffuse?

Answer 12

The less polar the molecule trying to difuse, the faster the rate of movement through the membrane.

Question 13

Describe passive transport of a substance which stays in an aqueous solution?

Answer 13

It passes through hydrophilic channels which are pores in transmembrane proteins.

Question 14

What are the channels for diffusion of water?

Answer 14

Aquaporins

• At least 13 different types in mammals
• Common in certain areas, for example RBCs and Kidney

Question 15

Describe the rate of influx through aqueous membrane channels for molecules other than water.

Answer 15

Passage of other molecules through an aqueou membrane channels is possible though it decreases rapidly with size.

Example - Urea diameter is 20% greater than water, but urea transit is 1000x less than water.

Question 16

What happens at a voltage-gated channel?

Answer 16

The potential difference inside / outside the cell causes a conformational change.

Question 17

What happens at a ligand-gated channel?

Answer 17

Bindind of a chemical ligand (eg. ACh) causes a conformational change.

Question 18

What are the 2 types of carrier mediated transport?

Answer 18

Active transport - up a concentration gradient.

Facilitated diffusion - down a concentration gradient.

In both cases, the substance binds onto a specific carrier, resulting in a conformational change and transport of the substance.

Question 19

What can happen in carrier mediated transport?

Answer 19

The transport can saturate. There are limited binding sites and it takes time for transport to occur.

Question 20

Describe the properties of active transport.

Answer 20

• It can push a substance against a concentration gradient.
• It is key when concentration gradient must be maintaind, eg. between ECF and ICF.
• May involve movement of more than one substance.
• Can be competatively inhibited by substances that combine with the active site.
• It requires energy.

Question 21

What is primary active transport?

Answer 21

• Active transport which uses ATP directly as an energy source.
• Example - sodium potassium pump.

Question 22

What is secondary active transport?

Answer 22

• Active transport indirectly using an energy source other than ATP.
• The movement of 2 or 3 substances is linked.

Question 23

What is the resting membrane potential?

Answer 23

The potential difference across the cell membrane.

Question 24

Explain the Donnan effect

Answer 24

• Fixed intracellular anions affect distribution of relatively mobile anions (chloride) and cations (potassium).
• The plasma membrane is selective with fixed selective anions.
• Anions are stuck inside the cell; they are too big to get out and are negatively charged, so repell the chloride ions
• Anions can attract the positive potassium ions from outside the cell.

Question 25

Describe the net transfer of ions in the sodium-potassium pump?

Answer 25

• The membrane is not very permeable to sodium, but a small amount does leak through - sodium must be pumped out or the cell would eventuallly burst.
• It dilutes cystol, preventing the cell swelling (eventually bursting).
• Maintains potassium gradient which contributes to resting membrane potential.

Question 26

What are the consequences of the Donnan effect and the sodium-potassium pump?

Answer 26

• High concentration of potassium in the cell.
• Lots of negative charge in the cell.
• Chloride ions leave the cell but there is a high concentration of anions trapped inside.
• It will eventually reach a point where electrical gradient balances chemical gradient.

Question 27

What is an action potential?

Answer 27

Brief but large electrical depolarisation of the nerve plasma membrane caused by inward movement of sodium and outward movement of potassium, controlled by opening and closing of ion channels.

Question 28

How does the opening and closing of ion channels lead to an action potential?

Answer 28

• The membrane potential is set by a balance of ion gradients and permeability.
• Depolarisation (caused by a trigger event) upsets the balance.
• If depolarisation <20mV, membrane potential will fall again (no other consequences).
• If depolarisation >20mV, threshold level is reach and AP generated.

Question 29

What would happen if the concentration gradients were altered artificially?

Answer 29

An increased proportion of responses will reach threshold. There will be increased frequency and inappropriate action potentials. It can be fatal.