Question 1

Which of the following best describes the two main components that contribute to the Gibbs free energy change ($\Delta G_{transport}$) for the movement of a charged ion across a membrane?

Accepted Answer

The chemical gradient and the electrical gradient

Question 2

Given the equation for the Gibbs free energy change for ion transport: $\Delta G_{transport} = RT \ln \left( \frac{[C]_{out}}{[C]_{in}} \right) + zF\Delta \Psi$, what does the term $zF\Delta \Psi$ represent?

Accepted Answer

The energy contribution from the electrical gradient

Question 3

A cell has a membrane potential ($\Delta \Psi$) of $-60$ mV (inside negative) and a temperature of $37^\circ$C. If $1$ mole of $Na^+$ is moved from outside (150 mM) to inside (10 mM), what is the sign of $\Delta G_{transport}$?

Accepted Answer

Negative, because both the chemical and electrical gradients favor influx

Question 4

Which of the following is the correct formula for calculating the energy required to move a charged ion across a membrane?

Accepted Answer

$\Delta G_{transport} = RT \ln \left( \frac{[C]_{out}}{[C]_{in}} \right) + zF\Delta \Psi$

Question 5

If the membrane potential ($\Delta \Psi$) is $-60$ mV (inside negative), what is the effect on the movement of $Ca^{2+}$ ions into the cell?

Accepted Answer

It favors the influx of $Ca^{2+}$ due to the negative interior

Question 6

A scientist wants to calculate the energy required to move $1$ mole of $K^+$ from inside a cell ($[K^+]_{in} = 140$ mM) to outside ($[K^+]_{out} = 4$ mM) at $37^\circ$C with $\Delta \Psi = -70$ mV (inside negative). Which step should be performed first?

Accepted Answer

Determine the charge of the ion ($z$)

Question 7

Which of the following best explains why the movement of $Na^+$ into a neuron is energetically favorable under typical physiological conditions?

Accepted Answer

Both the chemical and electrical gradients drive $Na^+$ into the cell

Question 8

If $RT = 2.48$ kJ/mol at $25^\circ$C, $F = 96,485$ C/mol, and $\Delta \Psi = -0.06$ V, what is the value of $zF\Delta \Psi$ for $Ca^{2+}$?

Accepted Answer

$-11.6$ kJ/mol

Question 9

A mutation causes the membrane potential of a cell to become more positive inside. How would this affect the $\Delta G_{transport}$ for $K^+$ influx?

Accepted Answer

$\Delta G_{transport}$ would become less negative, making influx less favorable

Question 10

Which of the following scenarios would result in the largest positive $\Delta G_{transport}$ for $Na^+$ movement?

Accepted Answer

$[Na^+]_{in} \gg [Na^+]_{out}$ and $\Delta \Psi$ is positive inside

Question 11

A patient is given a drug that doubles the cytosolic concentration of $Ca^{2+}$ without changing the extracellular concentration or membrane potential. What is the effect on $\Delta G_{transport}$ for $Ca^{2+}$ influx?

Accepted Answer

$\Delta G_{transport}$ becomes less negative, making influx less favorable

Question 12

Which of the following best describes why $\Delta G_{transport}$ for $Cl^-$ is typically positive when moving into a cell with a negative membrane potential?

Accepted Answer

The electrical gradient opposes $Cl^-$ influx

Question 13

Calculate the $\Delta G_{transport}$ for moving $1$ mole of $Na^+$ from inside the cell ($5$ mM) to outside ($150$ mM) at $37^\circ$C and $\Delta \Psi = -60$ mV (inside negative). Use $R = 8.314$ J/(mol·K), $T = 310$ K, $F = 96,485$ C/mol.

Accepted Answer

$+11.6$ kJ/mol

Question 14

Which of the following changes would make $\Delta G_{transport}$ for $K^+$ efflux more negative?

Accepted Answer

Increasing $[K^+]_{in}$ relative to $[K^+]_{out}$

Question 15

A researcher observes that $\Delta G_{transport}$ for $Na^+$ influx is zero. What does this indicate about the system?

Accepted Answer

The chemical and electrical gradients are exactly balanced

Thermodynamics of Membrane Diffusion: Charged Ions

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