Very simply Action potential past paper question Watch

Leah.J
Badges: 13
Rep:
?
#1
Report Thread starter 4 months ago
#1
Are the first 2 mName:  Screenshot (39).png
Views: 44
Size:  38.1 KBarking points describing what's in the textbook picture ?Name:  Screenshot (38).png
Views: 54
Size:  36.4 KB
Name:  Screenshot (40).png
Views: 42
Size:  64.5 KB
Last edited by Leah.J; 4 months ago
0
reply
Jpw1097
Badges: 15
Rep:
?
#2
Report 4 months ago
#2
(Original post by Leah.J)
Are the first 2 mName:  Screenshot (39).png
Views: 44
Size:  38.1 KBarking points describing what's in the textbook picture ?Name:  Screenshot (38).png
Views: 54
Size:  36.4 KB
Name:  Screenshot (40).png
Views: 42
Size:  64.5 KB
The textbook image shows all of the points set out in the mark scheme. In an action potential, sodium ions move into the axon, depolarising the adjacent membrane which causes voltage-gated sodium channels to open and sodium moves into the axon, and then this depolarises the adjacent membrane and so on. This cycle repeats, allowing the action potential to propagate along the entire axon. It looks as though this is for an unmyelinated axon, in a myelinated axon the process is essentially the same however there are much larger gaps between voltage-gated sodium channels (at the nodes of Ranvier, at gaps between the myelin sheath) - this is known as saltatory conduction and explains why action potentials are transmitted much more quickly in a myelinated axon.
0
reply
Leah.J
Badges: 13
Rep:
?
#3
Report Thread starter 4 months ago
#3
(Original post by Jpw1097)
The textbook image shows all of the points set out in the mark scheme. In an action potential, sodium ions move into the axon, depolarising the adjacent membrane which causes voltage-gated sodium channels to open and sodium moves into the axon, and then this depolarises the adjacent membrane and so on. This cycle repeats, allowing the action potential to propagate along the entire axon. It looks as though this is for an unmyelinated axon, in a myelinated axon the process is essentially the same however there are much larger gaps between voltage-gated sodium channels (at the nodes of Ranvier, at gaps between the myelin sheath) - this is known as saltatory conduction and explains why action potentials are transmitted much more quickly in a myelinated axon.
so the answer is for an unmyelinated nerve fibre ? I'm finding it a little difficult to understand what you mean by adjacent membrane. To me it looks like 1 membrane only.
Also
Can you help me in part d ? Name:  Screenshot (41).png
Views: 55
Size:  31.1 KB Name:  Screenshot (42).png
Views: 41
Size:  37.1 KB
I could almost swear my book and my teacher said during that time the K ion channels close and the pump acts to restore the resting potential
0
reply
Jpw1097
Badges: 15
Rep:
?
#4
Report 4 months ago
#4
(Original post by Leah.J)
so the answer is for an unmyelinated nerve fibre ? I'm finding it a little difficult to understand what you mean by adjacent membrane. To me it looks like 1 membrane only.
Also
Can you help me in part d ? Name:  Screenshot (41).png
Views: 55
Size:  31.1 KB Name:  Screenshot (42).png
Views: 41
Size:  37.1 KB
I could almost swear my book and my teacher said during that time the K ion channels close and the pump acts to restore the resting potential
Yes, it's the same membrane, but the adjacent part of the membrane. No the principles are the same for both myelinated and unmyelinated nerve fibres - in a myelinated nerve fibre, the sodium channels are located at the nodes of Ranvier.

Yes that's right for part (d), voltage-gated potassium channels close and the sodium-potassium pump restores the resting membrane potential.
0
reply
Leah.J
Badges: 13
Rep:
?
#5
Report Thread starter 4 months ago
#5
(Original post by Jpw1097)
Yes, it's the same membrane, but the adjacent part of the membrane. No the principles are the same for both myelinated and unmyelinated nerve fibres - in a myelinated nerve fibre, the sodium channels are located at the nodes of Ranvier.

Yes that's right for part (d), voltage-gated potassium channels close and the sodium-potassium pump restores the resting membrane potential.
But it says that the membrane remains permeable in the mark scheme, and that the potassium ions move into the axon because of the charge difference, so I'm guessing they mean by diffusion
0
reply
Jpw1097
Badges: 15
Rep:
?
#6
Report 4 months ago
#6
(Original post by Leah.J)
But it says that the membrane remains permeable in the mark scheme, and that the potassium ions move into the axon because of the charge difference, so I'm guessing they mean by diffusion
Do you know the sequence of events that happen during an action potential? If not, I would suggest having a look.
0
reply
Leah.J
Badges: 13
Rep:
?
#7
Report Thread starter 3 months ago
#7
(Original post by Jpw1097)
Do you know the sequence of events that happen during an action potential? If not, I would suggest having a look.
I swear I do. Or at least I think I do ?
An electrical impulse reaches the neurone and causes Na ion channels to open resulting in an influx of Na ions which depolarise the neurone. If this depolarisation is above a threshold value, voltage gated Na ion channels will open and sodium ions will diffuse into the axon depolarising the adjacent part of the membrane then that causes Na ion channels in that part to open and so on and so on.
Then the neurone or axon needs to get repolarised so the K ion channels open and the Na ion channels close, K ions diffuse outside the axon repolarising the neurone. When the resting potential is reached, K ion channels close but take time to do so resulting in hyperpolarisation, to restore the resting potential, the K+ channel proteins close and the Na K pump K+ ions inside.


That's all I know on action potential
Is there anything missing or incorrect ?
This is from the examiner reportsName:  Screenshot (62).png
Views: 41
Size:  68.4 KB
and I'm ... I don't know I'm done with life and with edexcel and I'm just confused
1
reply
JacobBob
Badges: 10
Rep:
?
#8
Report 3 months ago
#8
Leah.J
Your questions are great. Jpw1097 your answers are greater
(Original post by Leah.J)
I swear I do. Or at least I think I do ?
An electrical impulse reaches the neurone and causes Na ion channels to open resulting in an influx of Na ions which depolarise the neurone. If this depolarisation is above a threshold value, voltage gated Na ion channels will open and sodium ions will diffuse into the axon depolarising the adjacent part of the membrane then that causes Na ion channels in that part to open and so on and so on.
Then the neurone or axon needs to get repolarised so the K ion channels open and the Na ion channels close, K ions diffuse outside the axon repolarising the neurone. When the resting potential is reached, K ion channels close but take time to do so resulting in hyperpolarisation, to restore the resting potential, the K+ channel proteins close and the Na K pump K+ ions inside.


That's all I know on action potential
Is there anything missing or incorrect ?
This is from the examiner reportsName:  Screenshot (62).png
Views: 41
Size:  68.4 KB
and I'm ... I don't know I'm done with life and with edexcel and I'm just confused
1
reply
Jpw1097
Badges: 15
Rep:
?
#9
Report 3 months ago
#9
(Original post by Leah.J)
I swear I do. Or at least I think I do ?
An electrical impulse reaches the neurone and causes Na ion channels to open resulting in an influx of Na ions which depolarise the neurone. If this depolarisation is above a threshold value, voltage gated Na ion channels will open and sodium ions will diffuse into the axon depolarising the adjacent part of the membrane then that causes Na ion channels in that part to open and so on and so on.
Then the neurone or axon needs to get repolarised so the K ion channels open and the Na ion channels close, K ions diffuse outside the axon repolarising the neurone. When the resting potential is reached, K ion channels close but take time to do so resulting in hyperpolarisation, to restore the resting potential, the K+ channel proteins close and the Na K pump K+ ions inside.


That's all I know on action potential
Is there anything missing or incorrect ?
This is from the examiner reportsName:  Screenshot (62).png
Views: 41
Size:  68.4 KB
and I'm ... I don't know I'm done with life and with edexcel and I'm just confused
That’s fine, I think you have a very good idea about action potentials.
0
reply
Leah.J
Badges: 13
Rep:
?
#10
Report Thread starter 3 months ago
#10
(Original post by Jpw1097)
That’s fine, I think you have a very good idea about action potentials.
I swear, last thing, just answer part d please.
Did you read the examiner reports ? It says that K Na pumps do not restore the resting potential and that that's a misconception which "obviously" doesn't get credit ... everywhere else on earth, Na K pumps restore resting potential, did I misunderstand part d ? Or do I not know sth about restoring the resting potential ?
0
reply
Reality Check
Badges: 22
Rep:
?
#11
Report 3 months ago
#11
(Original post by Leah.J)
I swear, last thing, just answer part d please.
Did you read the examiner reports ? It says that K Na pumps do not restore the resting potential and that that's a misconception which "obviously" doesn't get credit ... everywhere else on earth, Na K pumps restore resting potential, did I misunderstand part d ? Or do I not know sth about restoring the resting potential ?
You need to look a the role of leak channels here to fully understand why the Na-K-ATPase doesn't do exactly what you think it does here.
0
reply
Jpw1097
Badges: 15
Rep:
?
#12
Report 3 months ago
#12
(Original post by Leah.J)
I swear, last thing, just answer part d please.
Did you read the examiner reports ? It says that K Na pumps do not restore the resting potential and that that's a misconception which "obviously" doesn't get credit ... everywhere else on earth, Na K pumps restore resting potential, did I misunderstand part d ? Or do I not know sth about restoring the resting potential ?
Yes, you’re right, I tried to simplify it for your level but since you’re asking I will go into it. The sodium-potassium pump is important in MAINTAINING the resting membrane potential. Yes, the misconception is that the resting membrane potential (usually around -70mV) is due to the fact that the sodium-potassium pump pumps 3 Na+ ions out of the cell and 2 K+ ions into the cell, thereby making the inside of the cell more negative relative to the outside - this is FALSE. This mechanism only accounts for a few mV. The main reason that the resting membrane potential is around -70mV is because the membrane, at rest, is far more permeable (around 40 times more) to potassium ions than sodium ions. At rest, voltage-gated ion channels are closed however, so-called ‘leak channels’ are open, and allow sodium and potassium ions to move across the membrane. However, there are far more leak potassium channels than leak sodium channels, and this is why the membrane is more permeable to potassium ions.

Now consider the factors that determine the rate at which ions will move across the membrane. One of these factors is the electrochemical gradient of a particular ion (the difference in charge and concentration across the membrane) and the other is the permeability of the membrane to the ion.

Imagine a scenario where the membrane is permeable ONLY to potassium ions. Potassium ions would move from inside the cell to the outside via these leak channels via their concentration gradient. As a result, the outside of the cell would become more positive and the inside of the cell would become more negative (as positive ions are leaving the cell) - making the resting membrane potential more negative. Now, the build up of positive charge on the outside of the cell would begin to reduce the rate at which potassium ions are leaving the cell across the membrane as the positive charge outside the cell would repel the positively charged potassium ions. The membrane potential at which there would be no net flow of potassium ions (the equilibrium potential) would be around -84mV.

Now imagine if the membrane were only permeable to sodium ions. Sodium ions would move from the outside to the inside of the cell (down its concentration gradient). This would make the inside of the cell more positive relative to the outside, however, the build up of sodium ions inside the cell would make the membrane potential positive. This would then begin to slow down the movement of sodium ions from the outside to the inside of the cell. The membrane potential at which the chemical and electrical forces (due to the concentration and electrical gradient) would be balanced, and therefore there would be no net flow of sodium ions, would be around +62mV.

However, since the membrane is more permeable to potassium, the resting membrane potential is much closer to the equilibrium potential of potassium (-80mV) rather than sodium (+62mV). It’s quite complicated and I’m not sure if I have explained it very well, but hopefully that gives you an idea of the reason why the resting membrane potential is the value that it is.
0
reply
X

Quick Reply

Attached files
Write a reply...
Reply
new posts

All the exam results help you need

1,488

people online now

225,530

students helped last year

University open days

  • University of Aberdeen
    General Open Day Undergraduate
    Tue, 27 Aug '19
  • Norwich University of the Arts
    Postgraduate (MA) Open Day Postgraduate
    Sat, 31 Aug '19
  • University of Lincoln
    Guardian Offices, Kings Cross, London Postgraduate
    Mon, 2 Sep '19

How are you feeling about GCSE Results Day?

Hopeful (220)
12.45%
Excited (166)
9.39%
Worried (308)
17.43%
Terrified (389)
22.01%
Meh (184)
10.41%
Confused (39)
2.21%
Putting on a brave face (244)
13.81%
Impatient (217)
12.28%

Watched Threads

View All
Latest
My Feed