Membranes

Question 1

Membranes acts as barriers to…

Answer 1

diffusion of substances and regulate their movement

Question 2

What are the three major routes of diffusion?

Answer 2

i) diffusion through aqueous pores
ii) diffusion in the lipid portion
iii)Carrier-mediated diffusion (facilitated).
All three are passive processes and require no energy, they happen continually until equilibrium is reached.

Question 3

What is diffusion through aqueous pores?

Answer 3

Simple diffusion where polar molecules pass through channels in lipid layer down a concentration gradient. These channels are 10 Angstroms wide. Rate is proportional to concentration gradient.

Question 4

What is diffusion in the lipid portion?

Answer 4

When a substance can dissolve into and out of the layer of phospholipids down a concentration gradient. Substances include gases such as O2 and CO2, steroids, fatty acids. Rate is proportional to concentration gradient.

Question 5

What is carrier-mediated diffusion?

Answer 5

Diffusion whereby carrier proteins embedded in the phospholipid layer help to carry substances from one side to another. Rate is dependent on the concentration gradient only in so far as the carrier-proteins do not become saturated.

Question 6

Despite the three passive processes of diffusion, substance concentrations in cells are not usually in equilibrium - how?

Answer 6

Due to the active processes across the membrane that use ATP.

Question 7

What are these active processes called?

Answer 7

Active Transport

Question 8

What are Active Transport systems called and name an example?

Answer 8

Membrane pumps. (example is the sodium-potassium pump)

Question 9

What does the Sodium-Potassium pump do?

Answer 9

It pumps sodium ions (Na+) out of the cell and potassium ions (K+) into the cell at a rate of three sodium for every two potassium.

Question 10

Describe the normal sodium and potassium concentrations inside and outside of a cell.

Answer 10

Inside - Sodium low, potassium high

Outside - sodium high, potassium low

Question 11

Potassium ions (K+) by diffusion leak out of the cell meaning the cell loses…

Answer 11

positive charge. Nearly every cell sits at around -70mV

Question 12

What is the normal -70mV called?

Answer 12

The resting membrane potential.

Question 13

Where are the excitable cells?

Answer 13

Nerve and muscle cells

Question 14

Excitable cells can alter their membrane potential using…

Answer 14

gated ion channels.

Question 15

Describe the process that occurs when an excitable cell is activated. (see notes for graph)

Answer 15

Na+ gates open up
Sodium ions rush into cell down concentration gradient adding positive charge to cell.
Cell becomes positively charged to around 30mV while the outside of the cell becomes negatively charged.
This is called depolarisation
Na+ gates begin to close and K+ gates begin to open
K+ ions rush out of cell
The cell repolarises
Gates close.

Question 16

What is this change in the membrane potential called.

Answer 16

Action potential

Question 17

What do action potentials do?

Answer 17

Code information in nerve and muscle cells.

Question 18

What is the threshold and the threshold potential?

Answer 18

The minimum amount of stimulus required to initiate and AP

Question 19

What is the action potential absolute refractory period?

Answer 19

The time between an initial action potential and the relative refractory period where it is impossible regardless of stimulus size to generate a secondary AP.

Question 20

What is the relative refractory period?

Answer 20

The time between the absolute refractory period and the point where the threshold potential returns to normal. During this period a secondary AP is possible id the stimulus of a significant size.