Membrane Permeability Flashcards
What is a semi-permeable membrane?
A layer through which only allowed substances can pass.
What are some examples of hydrophobic substances?
O2, CO2, N2.
Why can hydrophobic molecules pass through a semi-permeable membrane?
Because they require no energy to pass through.
What are some examples of small, uncharged polar molecules?
H20, urea, glycerol.
Why can small, uncharged molecules pass through?
Because they are so small - can pass through membrane.
What is an example of large uncharged polar molecule and why can it not pass through a semi-permeable membrane?
Glucose, too large.
Name some examples of ions.
H+, Na+, K+, Ca2+, Cl-
What are the roles of transport processes?
Maintenance of ionic composition
Maintenance of intracellular ph
Regulation of cell volume
Expulsion of waste products
Describe a reciprocating carrier.
‘Flip flop’ method. Molecule attaches to receptor on plasma membrane and ‘flips’ from outside to inside.
What is a rotating carrier?
Where the receptor rotates to allow the molecule to be released inside the cell.
What are gated carriers?
Where the ‘gate’ (receptor) undergoes a conformational change, allowing the molecule to pass through.
What is facilitated diffusion?
A term to describe the use of channels where molecules can move BOTH WAYS (in and out).
What is a ligand gated channel?
Where a ligand (small molecule) is uses to facilitate movement through the channel e.g. The binding of ACh to a receptor causes the channel to open, letting in Na+.
What is a voltage gated channel?
Voltage sensors detect change in voltage and open up accordingly. It uses the electrical gradient to initiate membrane depolarisation.
What is more effective - facilitated diffusion or simple diffusion and why?
Facilitated diffusion - reaches vmax far quicker. Can work molecules across the plasma membrane far more effectively as it is ASSISTED whilst simple diffusion simply moves across the chemical / electrical gradient naturally.
For a molecule top as against its electrical gradient, what does it require?
ENERGY
What is active transport? The difference with passive transport?
Allows the movement of ions AGAINST unfavourable concentration and/or electrical gradients.
Energy is sourced from ATP HYDROLYSIS.
In passive transport the molecules move across the conc/electrical gradient - this can be simple OR facilitated diffusion.
What can be calculated to see whether passive or active transport is being used?
Gibbs free energy - if the answer is negative then it is using PASSIVE transport, if it is positive then it is using ACTIVE transport.
Through the four molecules Na+, Cl-, Ca2+ and K+, are they more present intracellular or extracellular?
Intracellular - K+
Extracellular - Na+, Cl-, Ca2+
What is uni-transport?
When one molecule is transported at a time.
What is co-transport? What are the two types?
Where two molecules can pass through a channel at the same time.
In symport, two molecules can pass at once in ONE direction.
In antiport, two molecules can pass at once in OPPOSITE directions.
The is the Na+ pump’s principle function?
To set up the gradient concentration for Na+ and K+.
How does the Na+ pump work?
It lets 2 K+ molecules INTO the cell is exchange for 3 Na+ molecules OUT. During this, energy is used - ATP is broken down into ADP plus a phosphate molecule. ACTIVE TRANSPORT.
What are the two main subunits of the Na+ pump?
An alpha subunit - this is the binding site for Na+, K+ and ATP.
A beta subunit - this is a glycoprotein which directs the pump to the cell surface.
What is the Ca2+ - Mg2+ - ATPase pump (PMCA)?
This a form of ACTIVE TRANSPORT.
Ca2+ is moved out of the cell AGAINST the concentration gradient is order to maintain low levels of Ca2+ in the cell. This pump uses energy in the form of ATP to ADP and P.
This pump has a high affinity and a low capacity.
What is the Na+ - Ca2+ (NCX) exchanger?
This pump exchanges 1 Ca2+ out the cell in exchange for 3 Na+ into the cell. This is a secondary active transporter driven by sodium concentration.
Used to remove calcium from the cell.
This pump has a low affinity and a high capacity.
What is affinity?
Affinity is the degree to which a substance tends to bind to another, the likeliness of two substances binding e.g. The PMCA has a high affinity for Ca2+ and therefore is likely to pick it up from the cell and transport it out.
What is capacity?
The maximum amount that something can contain e.g. The PMCA has a low capacity. High affinity means it is likely to pick up any excess Ca2+ but low capacity as it can only pass through one molecule at a time.
What is the Na+ - H+ exchanger?
Exchanges one Na+ INTO the cell for one H+ OUT of the cell. Is used in maintaining a balanced PH.
