Three out, two in
"Three out, two in" is a free, interactive Class 11 Biology simulation for NEET, built on the lesson "Resting potential" in Neural Control and Coordination. Each cycle of the sodium-potassium pump carries some Na⁺ out of the axon and some K⁺ in. Run a cycle and count what crossed each way. The proteins inside carry negative charge and cannot leave. The pump moves 3 Na⁺ out for 2 K⁺ in. The inside keeps high K⁺ and negative proteins with low Na⁺; the outside has high Na⁺ and low K⁺. The potential difference across this resting membrane is the resting potential.
From the lesson Resting potential in Neural Control and Coordination. Change the values and watch what happens.
The idea behind it
NCERT §18.3.1
- Neurons are excitable because their membranes are polarised. The neural membrane carries different ion channels, each selectively permeable to particular ions.
- At rest, the axonal membrane is fairly permeable to K⁺, nearly impermeable to Na⁺, and impermeable to the negatively charged proteins of the axoplasm.
- So the axoplasm has a high concentration of K⁺ and negative proteins and a low concentration of Na⁺, while the fluid outside has low K⁺ and high Na⁺: a concentration gradient.
- The sodium-potassium pump maintains these gradients by active transport, moving 3 Na⁺ out of the cell for every 2 K⁺ it brings in.
- The result is a positive charge on the outside of the axonal membrane and a negative charge on its inner face: the membrane is polarised.
- This potential difference across the membrane of a resting neuron is called the resting potential.
Questions
2 answers
- What does the "Three out, two in" simulation show?
- Each cycle of the sodium-potassium pump carries some Na⁺ out of the axon and some K⁺ in. Run a cycle and count what crossed each way. The proteins inside carry negative charge and cannot leave.
- Is this simulation free?
- Yes. It is free and runs in the browser on a phone or a laptop. It belongs to the lesson "Resting potential" in Neural Control and Coordination (Class 11 Biology).
Embed this simulation
Free, with a link back
Teaching this idea? Put the simulation on your own page or LMS with this code.