The Capacity to Compare Capacitors and Cell Membranes
Objectives: Compare a cell membrane of a neuron to an RC circuit.
Introduction:
A capacitor can not produce new electrons – it only stores them. The movement of these stored
electrons is important in providing short bursts of energy. In biology, the movement of charges is
important in nerve impulses.
Test you knowledge with a Capacitor Quiz: http://electronics.howstuffworks.com/capacitorquiz.htm
If you need a review about active transport in the cell membrane, watch the following video and
answer the questions?
http://www.youtube.com/watch?v=fxOIcWwdl-o
1) How many sodium ions are removed from the cytoplasm by one protein molecule?
2) Does the movement of sodium ions require energy?
3) How many potassium ions enter the cell by one protein molecule?
Use the following graphics to answer the question below:
How does a cell membrane compare to a capacitor? (Include the structure acting as the dielectric).
An RC (resistor, capacitor) equivalent circuit model is a good model for a patch of passive neural
membrane. The capacitor represents the fact that cellular membranes are good electrical
insulators. The battery represents the sodium-potassium pump that acts to hold the electrical
potential of the inside of the cell below that of the outside. This voltage difference is called the
resting potential of the neuron. The resistor represents the leakage of current through the
membrane
1.5V
1.5V
Capacitor
Lamp
Activity:
Materials: Two 1.5 V batteries in series, 1 F capacitor, single 2.5 V light bulb, wire leads,
digital volt meter, switch
Procedure: Charging Capacitor
1) Hook batteries, light bulb, and capacitor in series. See diagram. LEAVE one
lead disconnected at a battery terminal (switch). Make sure the capacitor’s
polarity is inserted correctly (negative toward negative part of battery).
2) Using leads connect the DVM (digital volt meter) across the capacitor. Set
DVM to 20 V DC max scale.
3) CLOSE the circuit by attaching the lead to the battery or closing the switch.
4) Observe the brightness of bulb and describe:______________________
5) Observe the voltage on DVM when it stops: _______________
Capacitor is now fully charged
6) Disconnect both leads from battery.
7) Momentarily (for 1 second) connect the leads then disconnect them and
observe light bulb and DVM (See diagram below). Disconnect, and reconnect
3-4 times, letting the bulb stay on for 1 second each time.
Light bulb:_________________________________________
DVM:_____________________________________________
Capacitor
Lamp
Questions:
1. How does connecting the leads together resemble a sodium ion channel in a cell
membrane?
2. How does disconnecting the leads resemble a sodium ion channel? Can you
relate this to action potential?
3. What biological process does the charging of the capacitor represent?
4. What does the battery represent in the neuron?
Extension:
Using a battery, switch, capacitor, and potentiometer, can you show how the action
potential increases during depolarization and then decreases during repolarization?