Ion Channels: Selectivity, Gating and the Evidence from Single Channels
A channel passes one ion species far better than another because of the chemistry lining its narrowest point, and it opens in response to voltage, a ligand or a mechanical pull. Recording one channel at a time shows it flicking between full open and shut rather than easing part way.
What a learner can do afterwards
- Explains selectivity from the pore lining rather than from the size of the hole alone.
- Reads a single-channel trace as open and closed states, and links their proportions to a whole-cell current.
- Names the three gating cues and gives a cell that uses each.
1 · Read
A channel passes one ion species far better than another because of the chemistry lining its narrowest point. Amino acids there space their charges so that matching ions shed their water and slip through, while others are held back. Selectivity is chemistry, not just the size of the hole.
These gates let you think, move, and feel touch, and they open to three cues. Voltage-gated channels, like the sodium channels of a neuron, open when the membrane passes threshold. Ligand-gated channels, like those on a muscle fiber at a synapse, open when a chemical docks. Mechanically gated channels, like those in a touch receptor cell, open when pulled.
Recording one channel at a time shows it flicking between full open and shut in square jumps, never easing part way. The whole-cell current is the number of channels times the single-channel current times the fraction of time each spends open, so more open time means more total current.
The action potential runs on the same all-or-nothing logic as one channel. Past threshold it fires its full spike on its own, like a light switch, and the rising phase is voltage-gated sodium channels opening so sodium rushes in. Later those sodium channels inactivate while potassium leaves, and the voltage falls back.
Pore chemistry picks the ion, three cues open the gate, and each channel is all or nothing.
2 · Watch
Take it off screen
Where it sits
Learn first
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.