Membrane Permeability

Question 1

What is a semi-permeable membrane?

Answer 1

A layer through which only allowed substances can pass.

Question 2

What are some examples of hydrophobic substances?

Answer 2

O2, CO2, N2.

Question 3

Why can hydrophobic molecules pass through a semi-permeable membrane?

Answer 3

Because they require no energy to pass through.

Question 4

What are some examples of small, uncharged polar molecules?

Answer 4

H20, urea, glycerol.

Question 5

Why can small, uncharged molecules pass through?

Answer 5

Because they are so small - can pass through membrane.

Question 6

What is an example of large uncharged polar molecule and why can it not pass through a semi-permeable membrane?

Answer 6

Glucose, too large.

Question 7

Name some examples of ions.

Answer 7

H+, Na+, K+, Ca2+, Cl-

Question 8

What are the roles of transport processes?

Answer 8

Maintenance of ionic composition
Maintenance of intracellular ph
Regulation of cell volume
Expulsion of waste products

Question 9

Describe a reciprocating carrier.

Answer 9

‘Flip flop’ method. Molecule attaches to receptor on plasma membrane and ‘flips’ from outside to inside.

Question 10

What is a rotating carrier?

Answer 10

Where the receptor rotates to allow the molecule to be released inside the cell.

Question 11

What are gated carriers?

Answer 11

Where the ‘gate’ (receptor) undergoes a conformational change, allowing the molecule to pass through.

Question 12

What is facilitated diffusion?

Answer 12

A term to describe the use of channels where molecules can move BOTH WAYS (in and out).

Question 13

What is a ligand gated channel?

Answer 13

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+.

Question 14

What is a voltage gated channel?

Answer 14

Voltage sensors detect change in voltage and open up accordingly. It uses the electrical gradient to initiate membrane depolarisation.

Question 15

What is more effective - facilitated diffusion or simple diffusion and why?

Answer 15

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.

Question 16

For a molecule top as against its electrical gradient, what does it require?

Answer 16

ENERGY

Question 17

What is active transport? The difference with passive transport?

Answer 17

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.

Question 18

What can be calculated to see whether passive or active transport is being used?

Answer 18

Gibbs free energy - if the answer is negative then it is using PASSIVE transport, if it is positive then it is using ACTIVE transport.

Question 19

Through the four molecules Na+, Cl-, Ca2+ and K+, are they more present intracellular or extracellular?

Answer 19

Intracellular - K+

Extracellular - Na+, Cl-, Ca2+

Question 20

What is uni-transport?

Answer 20

When one molecule is transported at a time.

Question 21

What is co-transport? What are the two types?

Answer 21

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.

Question 22

The is the Na+ pump’s principle function?

Answer 22

To set up the gradient concentration for Na+ and K+.

Question 23

How does the Na+ pump work?

Answer 23

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.

Question 24

What are the two main subunits of the Na+ pump?

Answer 24

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.

Question 25

What is the Ca2+ - Mg2+ - ATPase pump (PMCA)?

Answer 25

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.

Question 26

What is the Na+ - Ca2+ (NCX) exchanger?

Answer 26

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.

Question 27

What is affinity?

Answer 27

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.

Question 28

What is capacity?

Answer 28

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.

Question 29

What is the Na+ - H+ exchanger?

Answer 29

Exchanges one Na+ INTO the cell for one H+ OUT of the cell. Is used in maintaining a balanced PH.