Inputs
Example Data Table
| Vm (mV) | Erev (mV) | I (pA) | Driving Force (mV) | Estimated g (pS) |
|---|---|---|---|---|
| -80 | -60 | -2 | -20 | 100.00 |
| -40 | -60 | 1 | 20 | 50.00 |
| 0 | -60 | 5.5 | 60 | 91.67 |
| 40 | -60 | 9 | 100 | 90.00 |
| 80 | -60 | 12 | 140 | 85.71 |
Formula Used
- G = V / R (conductance from voltage and resistance)
- g = I / (V − Erev) (conductance from current and driving force)
- I = g(V − Erev) (predict current at a given driving force)
- Gmacro = N × Po × g (optional macroscopic conductance)
How to Use This Calculator
- Select a calculation mode that matches your measurement.
- Enter values with units (mV/V, Ω, pA, etc.).
- Provide Erev when using driving-force formulas.
- Optionally enter N and Po to estimate macroscopic conductance.
- Press Calculate to see results, then export CSV or PDF.
Professional Notes on Ion Channel Conductance
1. Conductance as a Practical Measure
Conductance describes how easily ions cross a channel under an applied electrical driving force. In experiments, it is often more informative than current alone because it normalizes for voltage. The SI unit is Siemens (S), while single-channel values are commonly reported in picosiemens (pS) or nanosiemens (nS).
2. Linking Conductance to Ohm’s Law
For an ohmic pathway, current follows I = G·V. If you know voltage and resistance, the calculator uses G = V/R. This mode is helpful for equivalent-circuit checks, series resistance considerations, or quick sanity tests during setup.
3. Driving Force and Reversal Potential
In membrane electrophysiology, the relevant voltage is the driving force (V − Erev). Here, Erev is the potential where net ionic flux is zero. Using g = I/(V − Erev) lets you estimate conductance directly from patch-clamp data, including inward or outward currents.
4. Typical Ranges and Units
Many ligand-gated channels show single-channel conductances on the order of 10–100 pS, while some potassium channels can reach hundreds of pS. Macroscopic conductance can be much larger because it aggregates many channels. The calculator reports S, nS, and pS together to reduce unit-conversion mistakes.
5. Macroscopic Conductance from Channel Statistics
If you provide channel count (N) and open probability (Po), the tool estimates Gmacro = N×Po×g. This connects microscopic properties to whole-cell behavior. For example, doubling Po can double macroscopic conductance without changing the single-channel value.
6. Interpreting I–V Relationships
When conductance is constant across voltages, the I–V curve is linear and the slope equals conductance. Deviations suggest rectification, voltage-dependent gating, or changing ion selectivity. Use consistent sign conventions for current and voltage; the calculator will preserve signs in the computed conductance.
7. Measurement Quality and Common Pitfalls
Large errors often come from inaccurate Erev, uncompensated series resistance, or mixing units (mV vs V, pA vs nA). If the driving force is near zero, small noise can cause huge conductance estimates. The calculator warns when denominators are zero to prevent undefined results.
8. Where This Calculator Fits in Workflow
Use this tool for rapid validation during experiments, for report tables, and for teaching exercises on membrane biophysics. Exporting results to CSV supports lab notebooks and spreadsheets, while the PDF option supports quick sharing. For detailed modeling, combine these estimates with full I–V curve fitting and kinetic analysis.
FAQs
1) What is the difference between G and g?
g commonly refers to a single-channel conductance estimate. G is often used for macroscopic conductance of many channels or an equivalent circuit. The calculator reports conductance values consistently in S, nS, and pS.
2) Why do I need the reversal potential?
Erev sets the zero-current point for a given ion mixture. Using V−Erev gives the true driving force. Without Erev, conductance derived from current may be biased or even sign-flipped.
3) What should I enter if my current is inward?
Enter the measured current with its sign, using your lab’s convention. Inward currents are often negative in voltage-clamp. The computed conductance will reflect that sign relative to the driving force, helping you spot inconsistent inputs.
4) How do I estimate macroscopic conductance?
Provide N (number of functional channels) and Po (open probability). The calculator applies Gmacro = N×Po×g. This is a simplified estimate and assumes identical channels and independent gating.
5) Why does conductance blow up near V = Erev?
When V−Erev is close to zero, you are dividing by a tiny number. Noise in current or voltage produces unrealistically large conductance values. Measure farther from Erev or average multiple points for stability.
6) Which unit should I use for resistance?
Choose a unit that keeps the number readable: kΩ, MΩ, or GΩ are common in electrophysiology. The calculator converts everything internally to ohms and reports conductance in several convenient scales.
7) Is this suitable for non-ohmic channels?
It provides instantaneous or local estimates based on your inputs. For strongly rectifying or voltage-gated channels, conductance can vary with voltage. Use the calculator to compute point estimates, then analyze full I–V curves for complete characterization.