Bioelectricity

Question 1

How do you find the intracellular membrane potential?

Answer 1

We use two electrodes to this both found in a needle

You have one electrode which impales the membrane of a cell, as it goes into the intracellular fluid and measures the membrane potential

This is in reference to the second electrode in the water bath which has a membrane potential of zero. This water bath is found further back in the needle.

for most cells you have a membrane potential of -70mv

Question 2

What is the difference between the patch clamp technique and the technique to find the intracellular membrane potential of a cell?

Answer 2

1. The patch clamp technique tells you the current going through ion channels whereas the intracellular method tells you the cells membrane potential
2. The tip to measure the intracellular membrane potential is much smaller than the tip used in the patch clamp. The patch tip = 1x 10^-6 whereas the intracellular tip is much smaller
3. The tip for the intracellular membrane is also very sharp and pieces the cell membrane unlike the patch clamp technique. In the tip you need a highly concentrated salt solution.

Question 3

What affects membrane potential?

Answer 3

The unequal distributions of ions

And the different selections of ions like sodium and potassium

Question 4

Remember there is less sodium in the cell than outside
And there is more potassium inside the cell than out

What is the intracellular concentration of sodium and potassium maintained by?

Answer 4

The NA+ / K+ ATPase

Question 5

What are some characteristics of the sodium potassium ATPase and what happens to the net charge of the cell because of it?

Answer 5

The ATPase contributes roughly 20% of resting membrane potential of cell

The transporter is electrogenic - deals with ions

Every time there is an exchange of 3 sodium ions OUT of the cell and two potassium ions IN you get a net loss of one positive charge.

Question 6

What happens when you block a sodium potassium atpase

Answer 6

If you block the ATPase the charge of the cell thus goes up quickly. From -70mV to -56mV.

This causes there to be a slower change in membrane potential overtime

If you block the ATPase you eventually get rid of the membrane potential= it stops the cell from functioning

Why? This is mainly because the driving forces for ion channels are altered so they no longer work how they should do.

Question 7

How do potassium ion channels (this isnt talking about ATPases) maintain the membrane potential?

Answer 7

So remember the concentration of potassium ions in the cell is high and outside they are low

So there is a concentration gradient of potassium ions out of the cell

However as potassium ions move out of the cell it causes the intracellular charge to decrease and become more negative

this is amplified by anions which are negative which stay inside the intracellular, decreasing membrane potential…

this decrease in potential occurs for a small while

Question 8

As potassium ion channels work they make the membrane potential of a cell decrease as positive potassium ions are lost when the potassium ions follow their gradient out of the cell.

What happens after a while of the charge in the cell decreasing?

Answer 8

There becomes a positive potential gradient for potassium ions to move back into the cell along a voltage potential gradient. As it is more negative inside, potassium ions move back into the cell

Question 9

What happens to the overall net charge of potassium ions in a cell when the number of potassium ions leaving (Along a concentration gradient) is equal to those returning (along a voltage gradient)

Answer 9

There is no overall net charge and no net flow

Question 10

What type of leak do potassium ion channel have?

Answer 10

A slight sodium ion leak

Question 11

What is the Nernst potential?

Answer 11

This is when the amount of sodium in and out of the cell is equal in rate of ions moving in and out

There is no current and this makes a set membrane potential

Question 12

What is the nernst equation and what does everything mean?

Eion = RT/zF x Ln [ion in] / [ion out]

Note E ion means the nernst potential of a single ion.

Answer 12

R = gas constant this is usually always 61.5!
T = temp in K
Z = valence = the charge of the ion so if calcium it would be plus two
F = faradays constant
Ln = log base 10

Remember in all equations you need to be working in the SAME units and to WRITE the units down!

Question 13

Example: at body temp the IC of potassium was 145mm and EC was 6mm. What is the Nernst potential?

Eion = RT/zF x Ln [ion in] / [ion out]

Answer 13

-85mv

Zf = charge of ion = +1 for potassium

RT is always 61.5

Question 14

How to tell if there is more sodium or potasssium ion channels in the membrane of a cell using nernst potentials?

Answer 14

Look at membrane potential

Looks at nernst potentials of both ions

Look to see if the membrane potential is more similar to the nernst potential of sodium or potassium. Whichever the membrane potential is most similar to means there are more of those ions.

Nernst for sodium = 60mv

Nernst for potassium = 90mV

Question 15

Goldman equation. What does this tell us?
- learn this its quite easy to apply:

Vm = RT/Zf log PNa[Na out] + pk[k out] / pNA [NA in] + [k in]

Answer 15

This tells us the how the membrane potential is affected by multiple ions i.e. sodium and potassium. Not just how the potential changes for one ion.

Note in this equation pk means permability!

So you could have permeability of sodium of 2 and 150 mM of sodium moves out

This would be 2x[50] for the pNa [NA out] part.

You then log this equation.

Question 16

Action potentials. What happens with the sodium and potassium channels / pumps?

Answer 16

Remember the pump works for resting potential maintaining k+ and Na+ levels

At depolarisation = sodium channels open causing sodium to move into the cell until -70mV is reached for the OUTSIDE of the cell. Remember this isnt the intracellular potential.

These then shut - potassium channels open causing an efflux = potassium ions move out of the cell until hyperpolarisation when the channel shuts again

At hyperpolariastion = voltage gates potassium channels close

Sodium channels drive the membrane potential to being positive. Potassium ions channels bring the membrane potential back down

Repolarisation = membrane potential moves closer to the potassiums nernst.