Lecture 4: Membrane function 1 - Membrane transport Flashcards
Learning objectives
- Understand the nature of membrane impermeability
- Appreciate different classes of membrane transporters
- Understand different modes of membrane transport
- Understand the role energy has to play in membrane transport
Why is the cell membrane largely impermeable?
It allows the cell to maintain a different concentration of solute from external/circulating environment.
Why do molecules need to be moved across the membrane?
- Uptake of essential nutrients
- Excretion of metabolic waste products
- Regulation of intracellular ion concentration
How permeable is the cell membrane to different molecules?
- Hydrophobic molecules (O2, CO2, N2) can pass through the membrane
- Small uncharged polar molecules (H2O, urea, glycerol) can diffuse very slowly through the membrane (too slowly to be of use to the cell)
- Large uncharged polar molecules (glucose, sucrose) can pass very, very, very slowly through the membrane
- Ions (charged molecules, H+, Na+, HCO3-, K+, Ca2+, Cl-, Mg2+) cannot enter the membrane at all and need a mechanism to cross the membrane.
Why can ions not enter the membrane?
Charge and high degree of hydration (ions form many favourable electrostatic interactions with water = hydration shell).
What are the two main classes of membrane transport proteins?
Transporters and channels
What does a channel transport?
Ions
What does a transporter transport?
Import: Larger, bulkier molecules such as amino acids, sugars and cofactors.
Export: toxins, drugs, waster products
Discuss the specificity of membrane transport proteins.
Most membrane transport proteins will only transport one molecular species, e.g. K+, or a particular class of molecules, such as very similar sugars.
Multi-drug resistance efflux proteins cause major health problems through resistance to antibiotics. These proteins export any molecule with a specific characteristic. This has led to resistance to chemotherapies and antimicrobials.
Give two other names for transporters.
Carriers or permeases
e.g. lactose permease transports lactose
Give the generalised mechanism of action of transporters.
The solute binds to the high affinity binding site on one side of the membrane. This causes a dramatic conformational change in the transporter. The binding site for the solute is now exposed on the other side of the membrane and has changed from high to low affinity. This means the solute is released on the other side of the membrane.
There is sometimes energy input and sometimes no energy input required.
Which can transport molecules faster: channels or transporters?
Channels
How is the generalised mechanism of action of channels different to that of transporters?
- No high affinity binding site
- No radical conformational change
- Channels transport molecules much faster than transporters.
Give the generalised mechanism of action of channels.
Channels form aqueous pores across the membrane. The pore opens and allows the movement of the solute from one side of the membrane to the other. This is a much faster method of transport than transporters, because the interactions are much weaker and more temporary.
The biochemical nature of the central region of the channel specifies which ion/molecule can pass through.
What is facilitated diffusion?
A type of passive transport (doesn’t require energy) involving membrane transport proteins (channels/transporters). It is driven be a difference in concentration (concentration gradient) on either side of the membrane. If the solute is charged, facilitated diffusion is driven be an electrochemical gradient.
How does the membrane potential of a membrane affect the transport of ions across that membrane?
If the ions outside the membrane are positive:
- if the outside of the membrane is positive, the positive ions (cations) are repelled, so transport across the membrane to the negative side increases.
- if the outside of the membrane is negative, the positive ions (cations) are attracted, so transport across the membrane to the positive side decreases.
What is the membrane potential of most cell membranes?
Inside negative, outside positive
Why is active transport necessary?
Cell need to transport solutes against a concentration/electrochemical gradient to concentrate metabolites in certain organelles.
Which membrane transport proteins are able to cause active transport?
Only transporters (carriers/pumps), not channels.
What are the sources of energy for active transport?
- Coupled transporters: link uphill transport in one direction with downhill transport in the other
- ATP-driven pumps: utilise energy released in ATP hydrolysis
- Light driven pumps: link uphill transport to energy from light (photons). This is rare - only in certain bacteria.
What the three types of transporter mediated transport?
Uniport - one solute is transported alone
Symport - coupled transport of uphill and downhill from the same side of the membrane (same direction of transport)
Antiport - coupled transport of uphill and downhill from opposite sides of the membrane (opposite directions of transport)
Describe the difference between primary and secondary active transport.
Primary active transport (aka direct active transport) involves ions being pumped by a protein pump which gets the required energy directly from ATP.
Secondary active transport involves molecules being pumped by a protein pump which does not get energy directly from ATP. The energy instead comes from another source: another concentration gradient, which is created using ATP.
Is lactose permease an example of a channel or a transporter?
A transporter. Another name for a transporter is a permease.
