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Electrochemical gradient

Resting Membrane Potential Simulation

– Section 3 –

Equilibrium Potentials

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What is an Equilibrium Potential?

An equilibrium potential is a state in which the concentration gradient of an ion is balanced by an opposing electrical gradient.

It refers to the voltage across the cell membrane that exactly counters the force driving the diffusion of a specific ion across the membrane.

At this state, ions continue to move across the cell membrane in both directions, but there is no net gain in the number of ions on either side of the membrane.

This is because the opposing forces driving the ion movement are equal.

Diagram of an equilibrium potential

Electrical Gradients

What is an Electrical Gradient?

An electrical gradient develops when the electrical charge between the interior and exterior of a cell membrane differs.

How do Electrical Gradients Form?

This imbalance occurs when ions move across the cell membrane along their concentration gradient, causing particles with the same charge to accumulate on the opposite side of the membrane. The increase in charge on the opposite side of the membrane repels similarly charged ions from entering.

Concentration and Electrical Gradient Interactions

Let us investigate the interaction between gradients and equilibrium potentials using three activities. Each will focus on K+ ions because the cell membranes of excitable cells are most permeable to this ion, which significantly affects the resting membrane potential.

Activity 1: Concentration Gradient and No Opposing Electrical Gradient

The associated activity shows a cell membrane consisting of a lipid bilayer without channels or other embedded proteins. 

A higher concentration of K+ ions exits inside the membrane than outside. In an actual cell, this would be due to the action of the Na+/K+ pump (ATPase), although no Na+ ions are shown here. 

As a result, the unequal distribution of K+ ions produces an outward concentration gradient.

Adding negatively charged particles (anions) eliminates the charge on either side of the membrane. 

Each side of the membrane is neutral because the number of anions equals the number of cations. 

Therefore, there is no electrical gradient, and the membrane potential is 0mV.

Use this activity to create content.

 Analysis Questions

1. What conditions make both sides of the membrane neutral?

2. Why is there no electrical gradient?

3. Why is the membrane equilibrium potential 0 mV?

Activity 2: Concentration Gradient and a Partial Opposing Electrical Gradient

Adding potassium leak channels to the membrane provides a pathway for K+ ions to exit the cell along their concentration gradient while the anions remain in place. 

The outward flow of K+ ions makes the cell’s external environment positively charged and the internal environment negatively charged, creating an inward electrical gradient.

As K+ ions continue their outward flow, the charges on either side of the membrane increase.

As a result, the growing inward electrical gradient creates an opposing force that makes it increasingly difficult for K+ ions to move across the cell membrane.

The outer positive charge increasingly repels the K+ ions, while the inner negative charge increasingly attracts them.

Use this activity to create content.

 Analysis Questions

1. What pathway allows the K+ ions to cross the membrane?

2. What causes the K+ ions to move outside the cell membrane?

3. Does a concentration gradient exist for the anions? If so, what direction is the gradient?

4. What prevents the anions from crossing the membrane?

5. What causes an electrical gradient to develop for the K+ ions?

6. In what direction is the electrical gradient for K+?

7. What causes the magnitude of the electrical potential to grow?

Activity 3: Concentration Gradient and a Fully Developed Opposing Electrical Gradient

An ion, like K+, attains an equilibrium state when the force of its electrical gradient equals the force of its opposing concentration gradient.

The electrical potential at this stage is the ion’s equilibrium potential, and its magnitude is directly proportional to the initial ion concentration. 

The larger the initial concentration gradient, the greater the equilibrium potential to offset the flow of ions through the membrane.

Although ions continue to move across the membrane at equilibrium, the exchange remains balanced, with no net gain on either side.

Use this activity to create content.

 Analysis Questions

1. At what point is an equilibrium achieved between an ion’s concentration and electrical gradients?

2. Do ions stop moving across the membrane once equilibrium is attained?

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