MCAT Biological and Biochemical Foundations of Living Systems Flashcards: 2a Membrane Potential Electrochemical Gradients

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This deck focuses on 2a Membrane Potential Electrochemical Gradients, giving you a quick way to review the definitions, rules, and examples that matter most for MCAT Biological and Biochemical Foundations of Living Systems.

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Flashcard 1: What is the primary active transporter that maintains $Na^+$ and $K^+$ gradients in animals?

Answer: Na$^+$/K$^+$-ATPase. This pump actively maintains steep Na$^+$ and K$^+$ gradients essential for cellular function and excitability.

Flashcard 2: What is the Na$^+$/K$^+$-ATPase transport stoichiometry per ATP hydrolyzed?

Answer: $3\ Na^+$ out and $2\ K^+$ in. The unequal exchange creates concentration gradients crucial for resting potential and secondary transport.

Flashcard 3: What is the major extracellular anion in animal cells?

Answer: $Cl^-$. Chloride's abundance extracellularly balances cationic charges and contributes to Donnan equilibrium.

Flashcard 4: What is the major extracellular cation in animal cells?

Answer: $Na^+$. High extracellular Na$^+$ concentration is essential for osmotic balance and action potential generation.

Flashcard 5: What is the major intracellular cation in animal cells?

Answer: $K^+$. High intracellular K$^+$ concentration is maintained by active transport and supports resting membrane potential.

Flashcard 6: What does it mean for a membrane to be selectively permeable to $K^+$ at rest?

Answer: $P_{K}$ is high; $V_m$ is driven toward $E_{K}$. High potassium permeability allows K$^+$ fluxes to dominate, pulling the membrane potential toward its equilibrium.

Flashcard 7: Which direction does an anion move if $V_m - E_X$ is positive?

Answer: Into the cell (net inward anion flux). A positive driving force attracts negatively charged ions inward across the membrane.

Flashcard 8: Which direction does a cation move if $V_m - E_X$ is positive?

Answer: Out of the cell (net outward cation flux). A positive driving force repels positively charged ions outward across the membrane.

Flashcard 9: What is the electrochemical driving force expression for ion $X$?

Answer: $V_m - E_X$. This difference quantifies the net force driving ion movement away from its equilibrium potential.

Flashcard 10: What condition defines electrochemical equilibrium for ion $X$ across a membrane?

Answer: $V_m=E_X$ (no net driving force for $X$). At equilibrium, the membrane potential equals the ion's Nernst potential, resulting in zero net electrochemical force.

Flashcard 11: What is the definition of membrane potential ($V_m$) in a cell?

Answer: Voltage across the membrane: $V_{in} - V_{out}$. Convention defines membrane potential as the difference between intracellular and extracellular voltages.

Flashcard 12: What sign is the resting membrane potential of most neurons (inside relative to outside)?

Answer: Negative (inside is negative relative to outside). Ion gradients and selective permeability result in a negatively charged interior relative to the exterior at rest.

Flashcard 13: What is the typical resting membrane potential magnitude for many neurons?

Answer: Approximately $-70\ \text{mV}$. This value arises from the balance of ion concentrations and permeabilities, particularly high K$^+$ conductance.

Flashcard 14: What is the formula for the Nernst equilibrium potential for ion $X$?

Answer: $E_X=\frac{RT}{zF}\ln\left(\frac{[X]_{out}}{[X]_{in}}\right)$. The equation balances the chemical concentration gradient with the electrical potential difference at equilibrium.

Flashcard 15: At $37^\circ\text{C}$, what is the common base-$10$ Nernst form for a monovalent ion?

Answer: $E_X\approx\frac{61\ \text{mV}}{z}\log\left(\frac{[out]}{[in]}\right)$. At body temperature, the constant simplifies the natural log form to base-10 for easier physiological calculations.

Flashcard 16: If $V_m$ becomes more permeable to $Na^+$, which direction does $V_m$ shift?

Answer: Toward $E_{Na}$ (depolarizes; becomes more positive). Increased Na$^+$ permeability allows influx, shifting potential toward Na$^+$'s positive equilibrium value.

Flashcard 17: Which ion is closer to electrochemical equilibrium at rest if $V_m$ is near $-70\ \text{mV}$?

Answer: $K^+$ (since $V_m$ is near $E_K$). At resting potential, the driving force for K$^+$ is smaller than for other ions due to high K$^+$ permeability.

Flashcard 18: Identify the sign of $E_X$ for an anion when $[X]_{out} > [X]_{in}$.

Answer: Negative ($E_X<0$). Higher external concentration favors outward movement, but negative valence results in a negative equilibrium potential.

Flashcard 19: What does the valence term $z$ represent in the Nernst equation?

Answer: Ion charge (e.g., $+1$, $-1$, $+2$). Valence indicates the ion's charge magnitude and sign, influencing the electrical force in the equilibrium calculation.

Flashcard 20: Identify the sign of $E_X$ for a cation when $[X]_{out} > [X]_{in}$.

Answer: Positive ($E_X>0$). Higher external concentration creates an outward chemical gradient, yielding a positive equilibrium potential for cations.

Flashcard 21: What is the definition of an electrochemical gradient for an ion?

Answer: Combined chemical gradient and electrical gradient. It integrates concentration differences and electrical potential to determine the direction and magnitude of ion flux.

Flashcard 22: What change in $V_m$ is called hyperpolarization?

Answer: $V_m$ becomes more negative. Increased negativity results from net positive charge efflux or negative charge influx, inhibiting excitability.

Flashcard 23: What change in $V_m$ is called depolarization?

Answer: $V_m$ becomes less negative (moves toward $0$). Reduction in negativity occurs when net positive charge enters, as in Na$^+$ influx during excitation.

Flashcard 24: What is the net charge moved per cycle by the Na$^+$/K$^+$-ATPase?

Answer: Net $+1$ out (electrogenic pump). The unequal ion transport generates a net positive charge efflux, contributing to membrane hyperpolarization.