Membrane Potentials And Action Potentials

Question 1

Explain membrane potentials.

Answer 1

• If the cell has a resting membrane potential of 0 the charge on the inside and the outside is the same. If the cell doesn’t have a resting membrane potential of 0 the charge on the inside and the outside is not the same. If the cell has a resting membrane potential of -60mV there are more positive ions outside the cell than inside/excess negative inside.
• Usually imbalance in charge/numbers is very small (usually ion concertation on either side won’t change)
○ Charge is produced by changed particles (eg: Na+, K+, Ca2+ and Cl-)

Question 2

How do ions move across the cell membrane and through ion channels and why usually can a max of +20mV can be reached only?

Answer 2

• Ions move through specific channel proteins on the cell surface membrane eg (Na+ channel proteins)
○ They allow passive diffusion of ions and are open at all times
○ Selectively permeable
○ Known as non-gated or leak channels
• other channel proteins can be gated for specific ions
○ Eg voltage gated which open at certain membrane potentials.
○ The Na+ voltage gated channel only opens for a short period of time there for the membrane potential will usually only reach around +20mV during depolarisation (influx of Na+). Na+ will enter cell though voltage gated ion channels which only open for a moment. This means that an overall depolarisation of +20mV is usually reached. (to produce action potential)

Question 3

What are chemical and electrical force used in ion movement?

Answer 3

• Chemical force: relative extracellular and intracellular concentrations of the ion
○ Concertation of ions on each side of the cell
○ Movement due to a chemical force would also be known as movement down a concertation gradient
• Electrical force: the relative extracellular and intracellular charge
○ Eg which side of the cell has a lower or higher charge
○ The changed particles/ions will move todays the side with the opposite charge to itself ( eg Na+ will move to the more negatively charged side)

Question 4

How are ion gradients produced?

Answer 4

• Actives of using ATP to move a particle against the concertation gradient
• This creates an unequal concentration of ions inside and outside the cell
○ Essentially ‘resets’ the ion gradient
○ If active transport wasn’t used equilibrium would be reached and no net movement would take place.
○ The ion pumps are not sensitive to concertation so not used to control the ion gradient.

Question 5

What are the forces controlling the ion movement and how do they influence the
membrane potential?

Answer 5

• Ions will initially move from a high to low concertation ( all membranes are more permeable to K+ so K+ will be used in the example)
• k+ will move out of cell due to higher concentration inside
○ This is chemical force
•
• If a K+ ion channel is used more K+ will diffuse out of the cell as there is a higher concentration inside the cell ( move down concertation gradient)
• This results in a more negative charge inside the cell (as the decreased K+ causes an increased concentration of A-). Cell has acquired a negative charge
• Therefore K+ moves by an electrical force into the cell
• There is a greater chemical force than electrical force so K+ continues to move out.
•
• The diagram bellow shows the chemical and electrical forces are equal so no net movement
• The membrane potential that must be achieved to cause this is called the equilibrium potential.
•

Question 6

What are the approximate equilibrium potentials for K+, Na+, Ca2+ and Cl-?
•

Answer 6

EK = - 80 mV
E Na = + 62 mV
• ECa = + 123 mV
• ECl = - 65 mV

Question 7

Explain how K+ and Na+ have an effect on the membrane potential.

Answer 7

1. Na+ channels are present so Na+ will diffuse into cell ( as there is a higher concentration of Na+ outside) due to a chemical force
   ○ The charge inside is higher than outside so electrical for of Na+ inwards
   ○ Electrical and chemical forces don’t cancel out so Na+ is under the influence of a driving force
   
   2. Na+ enters the cell causing an and depolarises membrane. The membrane is depolarised (more positive ions inside than outside)
      ○ K+ moves out of cells due to the change in electrical force positive K+ move towards more negative outside cell
   3. Na+ is entering and K+ is leaving cell. But more Na+ enter so continued despoliation
   4. The continued depolarization causes an increased electrical fore of K+ inwards
      ○ More movement of K+ and less movement of Na+
      ○ Eventually the number of both ions entering and exiting the cell will stop changing
      ○ Eventually the number of K+ exiting will equal the number of Na+ entering the cell
   5. This is called an equilibrium situation - resting membrane potential will be around -65mV
   6. Actual membrane potential value will depend on the permeability of the membrane to Na+ and K+
      ○ If the membrane is more permeable to K+ than Na + then the potential at which the equilibrium situation is achieved will be closer to EK then E Na - neuronal resting membrane potential is actually closer to EK than E Na