Question

Let’s consider a spherical cell (it’s a simplified model) with conducting fluids inside and out

and an insulating membrane in between. The capacitance of the cell membrane is 90 pF; the resistance across the membrane is

30 MΩ. Under normal circumstances, the potential inside the cell is 70 mV than the potential outside. An action potential is triggered if the motion of charges across the

cell membrane changes this potential difference by 15 mV.

1) If the thickness of the cell membrane is doubled,

A. The resistance of the cell membrane increases, the capacitance increases.

B. The resistance of the cell membrane increases, the capacitance decreases.

C. The resistance of the cell membrane decreases, the capacitance increases.

D. The resistance of the cell membrane decreases, the capacitance decreases.

2) To one significant figure, how much charge must be transferred to trigger an action potential?

A. .1 pC

B. 1 pC

C. 10 pC

D. 100 pC

Answer 1

1) WHEN THICKNESS INCREASES,

Resistance increases and Capacitance decreases .

So B is the answer

Explanation(if required):-

Resistance = where l = length of resistance or distance between membranes , so if thickness is increased or doubled , l is doubled and hence Resistance is also doubled or increases.

Capacitance = where d = distance between, so if thickness increases , distance between the membranes also increases and hence Capacitance decreases as it is in inverse relation to distance between.

2)

We know,

Q [Charge] = C * V [ Capacitance * Voltage ]

Charge flowed = C * voltage difference

= 90 pF * 15 mV

= 1.35 pC

to one significant figure, it is 1 pF

Hence answer is B