3 Leaky Capacitor The membrane of a living cell is an insulator that separates two conducting fluids. Thus, it functions as a capacitor. The membrane is not a perfect insulator, however. It has a small conductance, making it a leaky capacitor. In this problem, you will estimate the RC time constant of the cell membrane. (a) A cell membrane typically has a capacitance per unit area on the order of 1 μF/cm²- i.e., 1 cm² of the membrane material would have a capacitance of 1 µF. It is believed that the membrane material is a dielectric with x ≈ 3. What thickness does this imply for the cell membrane? (b) Electric measurements indicate that the resistance of 1 cm² of cell membrane is R 1000. What is the resistivity p of the membrane material? (c) Find an expression for the time constant 7 = RC of the membrane in terms of p, x, and €0. Show that it is inde- pendent of the area of the membrane. This should be a symbolic result, not a numerical value.

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3 Leaky Capacitor
The membrane of a living cell is an insulator that separates two conducting fluids. Thus, it functions as a capacitor. The
membrane is not a perfect insulator, however. It has a small conductance, making it a leaky capacitor. In this problem,
you will estimate the RC time constant of the cell membrane.
(a) A cell membrane typically has a capacitance per unit area on the order of 1 µF/cm2 – i.e., 1 cm? of the membrane
material would have a capacitance of 1 µF. It is believed that the membrane material is a dielectric with x 3.
What thickness does this imply for the cell membrane?
(b) Electric measurements indicate that the resistance of 1 cm? of cell membrane is Rz 1000 n. What is the resistivity
p of the membrane material?
(c) Find an expression for the time constant 7 = RC of the membrane in terms of p, x, and €o. Show that it is inde-
pendent of the area of the membrane. This should be a symbolic result, not a numerical value.
(d) What is the numerical value of the time constant of the cell membrane? (If there were no active charge transport,
this is the time it would take the potential across a cell membrane to decay to about 37 percent of its initial value.)
Transcribed Image Text:3 Leaky Capacitor The membrane of a living cell is an insulator that separates two conducting fluids. Thus, it functions as a capacitor. The membrane is not a perfect insulator, however. It has a small conductance, making it a leaky capacitor. In this problem, you will estimate the RC time constant of the cell membrane. (a) A cell membrane typically has a capacitance per unit area on the order of 1 µF/cm2 – i.e., 1 cm? of the membrane material would have a capacitance of 1 µF. It is believed that the membrane material is a dielectric with x 3. What thickness does this imply for the cell membrane? (b) Electric measurements indicate that the resistance of 1 cm? of cell membrane is Rz 1000 n. What is the resistivity p of the membrane material? (c) Find an expression for the time constant 7 = RC of the membrane in terms of p, x, and €o. Show that it is inde- pendent of the area of the membrane. This should be a symbolic result, not a numerical value. (d) What is the numerical value of the time constant of the cell membrane? (If there were no active charge transport, this is the time it would take the potential across a cell membrane to decay to about 37 percent of its initial value.)
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