A neuron has 150 mmol/L of potassium ions inside and 5 mmol/L outside…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A neuron has mmol/L of potassium ions inside and mmol/L outside. Use and the Nernst equation.
- What is the potassium potential?
- A person has too much potassium in the blood, and the outside concentration rises to mmol/L. What is the potassium potential now?
- By how many millivolts has it moved towards zero?
Need a reminder? The key ideaShow
At rest the membrane is more permeable to K+ than to Na+, and the pump moves out and in. The outside is positive and the inside is negative. A stimulus makes the membrane freely permeable to Na+, the polarity reverses, and this is the action potential. K+ then leaves and the resting potential returns. For one ion, , and at 310 K.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Given. mmol/L. mmol/L, then mmol/L. mV.
Plan. Use the Nernst equation twice, then subtract.
Step 1. First potential. .
Step 2. Second potential. .
Step 3. The shift. towards zero.
Answer: ; ; .
Check. More potassium outside leaves less of a gradient, so the potential is smaller. A cell with a less negative resting potential fires more easily.
Watch out. Doubling the ratio does not double the potential, because the potential follows the logarithm.
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