Cell membranes, Transport and Gradients Flashcards
What are the 3 types of membrane proteins?
- Integral
- Peripheral
- Lipid anchored
What are the permeability properties of the lipid bilayer?
- 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).
What is passive transport?
Transport down the concentration gradient which does not require energy.
What is active transport?
Transport up the concentration gradient requiring energy in the form of ATP.
Describe primary active transport
Active transport which uses the energy source directly
Describe secondary active transport
Active transport which uses the energy source indirectly
Illustrate symport
Subtype of cotransport
Illustrate antiport
A type of cotransport
Explain this equation
- 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
What is the partition coefficient?
Partition coefficient gives a measure of how well a substance dissolves in lipid or aqueous phase.
How do you calculate the partition coefficient?
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.
What is the relationship between rate of transport through the lipid phase of the membrane and polarity of the molecules attempting to diffuse?
The less polar the molecule trying to difuse, the faster the rate of movement through the membrane.
Describe passive transport of a substance which stays in an aqueous solution?
It passes through hydrophilic channels which are pores in transmembrane proteins.
What are the channels for diffusion of water?
Aquaporins
- At least 13 different types in mammals
- Common in certain areas, for example RBCs and Kidney
Describe the rate of influx through aqueous membrane channels for molecules other than water.
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.
What happens at a voltage-gated channel?
The potential difference inside / outside the cell causes a conformational change.
What happens at a ligand-gated channel?
Bindind of a chemical ligand (eg. ACh) causes a conformational change.
What are the 2 types of carrier mediated transport?
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.
What can happen in carrier mediated transport?
The transport can saturate. There are limited binding sites and it takes time for transport to occur.
Describe the properties of active transport.
- 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.
What is primary active transport?
- Active transport which uses ATP directly as an energy source.
- Example - sodium potassium pump.
What is secondary active transport?
- Active transport indirectly using an energy source other than ATP.
- The movement of 2 or 3 substances is linked.
What is the resting membrane potential?
The potential difference across the cell membrane.
Explain the Donnan effect
- 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.
Describe the net transfer of ions in the sodium-potassium pump?
- 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.
What are the consequences of the Donnan effect and the sodium-potassium pump?
- 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.
What is an action potential?
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.
How does the opening and closing of ion channels lead to an action potential?
- 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.
What would happen if the concentration gradients were altered artificially?
An increased proportion of responses will reach threshold. There will be increased frequency and inappropriate action potentials. It can be fatal.
