Generating and Maintaining Ion Gradients and Membrane Potential

Flashcards covering the generation of ion gradients, membrane potential, the Nernst equation, electrochemical driving force, ionic currents, and the phases of an action potential, based on lecture notes.

How are ion gradients generated across a plasma membrane?

Ion gradients are generated by the action of active transporters and the impermeability of the plasma membrane to ions.

What is membrane potential?

Membrane potential is a difference in electrostatic potential between the two sides of a cell membrane, arising from charged ion gradients, acting like a battery.

Define 'driving force' in the context of molecular movement.

A driving force is a net force felt by an individual molecule, related to its distance from system equilibrium and the tendency of a system to move towards increased entropy.

What is the role of the Na+/K+ ATPase in establishing ion gradients?

The Na+/K+ ATPase uses energy from ATP hydrolysis to pump Na+ out of the cell and K+ into the cell, creating Na+ and K+ gradients.

What is the equilibrium potential for a given ion?

The equilibrium potential is the membrane potential at which there is no net electrochemical gradient for a given ion; forces drawing the ion out are balanced by forces drawing it in.

What is the Nernst equation used for?

The Nernst equation is used for calculating the equilibrium potential of a given ion.

What key factors determine an ion's equilibrium potential according to the Nernst equation?

The equilibrium potential depends on the presence, size, and direction of the ion gradient, as well as the charge (valence) of the ion.

What is the approximate equilibrium potential for Na+ at 37°C with typical intracellular and extracellular concentrations?

The approximate equilibrium potential for Na+ is +66.5 mV.

How is the electrochemical driving force for an ion calculated?

The electrochemical driving force (ΔV) is the difference between the cell's current membrane potential (Vm) and the ion's equilibrium potential (Veq), i.e., ΔV = Vm - Veq.

According to Ohm's Law, what two main factors determine the magnitude of an ionic current across a membrane?

Ionic current depends on the magnitude of the electrochemical driving force (Vm-Veq) and the magnitude of ion conductance (membrane permeability to that ion).

If all Na+ channels are closed, what is the Na+ current across the membrane?

If all Na+ channels are closed, the Na+ conductance is zero, therefore the Na+ current is 0.

What are some intracellular components that contribute to the resting negative charge inside a cell?

Negatively-charged DNA and anionic proteins inside the cell contribute to the resting negative charge.

What forces act on K+ ions that draw them both into and out of the cell?

K+ is drawn into the cell by negatively-charged proteins and pumped in by Na+/K+ ATPase, while it is drawn out by its concentration gradient.

Why is the resting membrane potential negative and close to the equilibrium potential for K+?

The resting membrane potential is negative and close to K+ Veq because of the persistent K+ leak channels that are always open at rest, allowing a K+ current.

What equation is used to calculate the cell membrane potential by integrating the contributions of all available ionic currents?

The Goldman-Hodgkin-Katz (GHK) equation is used for this purpose.

What is the primary driver of the membrane potential at rest?

At rest, the membrane potential is primarily driven by K+ leak current.

Why is the cell's resting membrane potential close to, but not exactly at, K+ equilibrium potential?

The Na+/K+ ATPase continuously pumps K+ into the cell, opposing the outward K+ leak, preventing K+ from fully equilibrating.

What happens to the Na+ and K+ gradients, osmotic balance, and resting membrane potential when Na+/K+ ATPase is inhibited?

Inhibition of Na+/K+ ATPase leads to loss of the Na+ gradient (Na+ accumulates), loss of the K+ gradient (K+ leaks out), loss of osmotic balance, and the resting membrane potential trending to zero.

What is the function of Inward Rectifying K+ (KIR) channels?

KIR channels open at very negative membrane potentials to support an inward K+ current, which counteracts outward K+ leak current and stabilizes the membrane potential.

What are the two main roles of membrane potential in cells?

Membrane potential stores energy harnessed by secondary active transporters in all cells, and in excitable cells, stereotyped changes communicate information via action potentials.

What event triggers the rapid depolarization phase of an action potential?

Depolarization of the cell beyond a threshold potential triggers the opening of voltage-gated Na+ channels, causing rapid depolarization.

What causes the repolarization phase during an action potential?

Repolarization is caused by the slower opening of voltage-gated K+ channels, leading to an outward K+ current that drags the cell towards the K+ equilibrium potential.

What happens during the absolute refractory period of an action potential?

During the absolute refractory period, voltage-gated Na+ channels are inactivated and cannot reopen, preventing another action potential from being triggered immediately.

How can a change in a cell's resting membrane potential alter its excitability?

Changing the resting membrane potential can make a cell more or less excitable by bringing the membrane potential closer to (or farther from) the threshold potential for activating an action potential.