International A Level Biology / The nervous system and neurones
The resting potential
Edexcel International A Level Biology (YBI11), chapter 23: The nervous system and neurones, specification point 8.4. Free lesson.
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The resting potential is the potential difference across a neurone membrane when it is not conducting an impulse. It is about −70 mV, with the inside negative relative to the outside, and keeps the neurone ready to respond.
What the resting potential means
A potential difference is a difference in electrical charge between two places. Across a neurone membrane at rest, it is about −70 mV. The inside is negative relative to the outside, so the membrane is polarised. Give the value, unit and side: about −70 mV, inside relative to outside. Saying only that the neurone is negative is not precise enough.
How the membrane controls ion movement
Ions are charged, so they cannot pass through the lipid part of the phospholipid bilayer. They cross the membrane through proteins. At rest, many potassium ion channels are open, but nearly all sodium ion channels are closed. The membrane is therefore much more permeable to potassium ions than to sodium ions. The sodium-potassium pump is a carrier protein that uses energy from ATP hydrolysis. ATP is broken down to ADP and phosphate. For each ATP used, the pump moves 3 sodium ions out and 2 potassium ions in. This is active transport.
How the inside becomes negative
The pump first sets up concentration gradients: sodium ions are more concentrated outside, and potassium ions are more concentrated inside. Because potassium channels are open while most sodium channels are closed, potassium ions can move out more easily. Their movement leaves the inside more negative. Potassium ions diffuse out through open potassium ion channels, down their concentration gradient. This is facilitated diffusion. Each potassium ion carries a positive charge out, so the outside becomes more positive and the inside is left more negative. The pump sets up the gradients, but…
Which way the ions move
Potassium ions do not diffuse into the axon at rest. They are pumped in, which keeps their concentration higher inside, and then diffuse out through open channels. Keeping the direction and the word ions clear is important for accurate answers. Sodium ions are pumped out. Potassium ions are pumped in, then diffuse out through potassium ion channels.
What happens when ATP is unavailable
Cyanide stops respiration producing ATP. Without ATP, the sodium-potassium pump stops. Ions continue to leak across the membrane, so the concentration gradients run down and fewer potassium ions diffuse out. The resting potential becomes less negative, moving closer to 0 mV, and the membrane becomes less polarised.