Membrane Transport and Potentials

Question 1

Why do ion gradients exist?

Answer 1

Cells generally contain proteins which are usually negative charge and can’t cross the membrane

Question 2

Why is intracellular [K+] high?

Answer 2

K+ accumulates where impermeable A- located

Question 3

How does Na+ enter and leave cells as it’s impermeable?

Answer 3

It leaks into cells slowly.
Then pumped out against a concn gradient by a Na pump
This requires ATP
Primary Active Transport

Question 4

Secondary Active Transport

Answer 4

E.G. the GLUT protein can be used as a diffusion pathway for Na+ which drags Glucose with it.

Question 5

What happens if the intracellular [K+] is greater than the extracellular?

Answer 5

K+ diffuses out of the cell
This loss leaves a slight negative charge on the inside face of the membrane
This develops a membrane potential

Question 6

What if the extracellular [K+] is higher than the intracellular?

Answer 6

The membrane potential will be less negative causing the membrane to become depolarised

Question 7

What is an Action Potential?

Answer 7

Transient Depolarisation of a cell

Question 8

What type of cells generate action potentials?

Answer 8

Excitable cells

Question 9

All or nothing law

Answer 9

Once an action potential has been initiated, varying the stimulus strength does not alter the configuration of the action potential.

Question 10

Threshold

Answer 10

To initiate an action potential it is necessary to depolarise the cell to a threshold potential

Question 11

Refractory period

Answer 11

When a time must elapse after one action potential before another can occur

Question 12

Stimulus strength…

Answer 12

increases the frequency of action potentials

Question 13

Depolarisation is caused by:

Answer 13

A transient increase of membrane Na+ permeability

Question 14

Repolarisation is aided by:

Answer 14

A greater than normal increase of K+ into the cell

Question 15

How do ions enter and leave cells?

Answer 15

ion channels found in lipid bilayer of membrane

Question 16

Voltage gated channels:

Answer 16

Activated by small change in membrane potential e.g. electrical stimulus
Support action potentials

Question 17

Ligand gated channels:

Answer 17

An extracellular chemical binds to receptor on a membrane
Receptor is an ion channel
Cause depolarisation if cation selective
Cause hyperpolarisation if anion selective

Question 18

Threshold potential

Answer 18

-60 mV

Question 19

Local circuit hypothesis

Answer 19

A region of a cell generates an Action Potential; the active region.
This is represented by a reversal of the membrane potential polarity.
A local current is carried down the cytoplasm
Some current leaks across the membrane, depolarising it.

Question 20

What factors affect AP conduction velocity?

Answer 20

Cell diameter: CV increases as diameter increases
Temperature: CV increases as temp increases
Myelination: CV increases with myelination as AP jumps from node to node