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Ion concentration gradient

Resting Membrane Potential Simulation

– Section 2 –

Main Affecting Factors

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Introduction to the Affecting Factors

Two factors primarily determine the resting membrane potential of excitable cells:

  • the concentration gradients of potassium (K+), sodium (Na+), and chloride (Cl) ions,
  • and the permeability of the cell membrane to these ions.

Concentration Gradients

What are Concentration Gradients?

A concentration gradient for an ion occurs when the number of that particular ion differs on either side of a cell membrane.

It is also commonly referred to as a chemical gradient because it concerns the quantity of particles, irrespective of their charge.

In excitable cells, sodium and chloride ions are more concentrated outside the cell membrane.

In contrast, potassium ions and large anions, such as phosphates and proteins, are more concentrated inside the cell membrane.

The differences in concentration cause the ions to move through the cell membrane along their respective gradients from areas of higher concentration to regions of lower concentration.

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How Do Concentration Gradients Form?

The concentration gradients for Na+, K+, and Cl ions result from different physiological processes.

1. The Na+/K+ pump (ATPase) causes Na+ ions to accumulate outside the membrane and K+ ions inside the membrane.

2. Cl ions passively accumulate outside the cell because they are drawn to the positive charge outside the membrane when an excitable cell is at rest.

3. Large intracellular anions (phosphates and proteins) are produced inside the cell and accumulate there due to their size, which prevents them from passing through the cell membrane.

 Analysis Questions

1. What is an ion concentration gradient?

2. What is another name for ion concentration gradients, and why is this name applied?

3. Which ions have a higher concentration outside the cell membrane?

4. Which ions have a higher concentration inside the cell membrane?

5. How do the ion concentration gradients affect the resting membrane potential?

6. What maintains the gradients for Na+ and K+?

7. How is the gradient for Cl ions established?

8. Why do large anions like phosphates and proteins accumulate inside the membrane?

Membrane Permeability

What Determines Cell Membrane Permeability?

The permeability of a membrane is determined by the number of open ion channels.

The permeability of a cell membrane to ions is comparable to how floodgates control water flow through a dam. The number of open floodgates determines the amount of water that can flow through the dam, just as the number of open ion channels influences the rate of ion movement across the cell membrane.

Water flows through the floodgates due to the force of gravity that acts on the water behind the dam. It flows from a higher level on one side of the dam to a lower level on the other.

Water flowing through open dam floodgates from an higher level area to a lower level area
Ions flowing through open channels in a cell membrane from an area of higher concentration to an area of lower concentration

Similarly, ions flow through open channels in the cell membrane in response to their concentration gradient. Ions move from areas of higher concentration to areas of lower concentration.

Cell Membrane Permeability Values

The associated slider activity displays a table of K+, Cl, and Na+ ions and their corresponding membrane permeability values.

The cell membrane of excitable cells is most permeable to K+, which is why its permeability value is the highest of the three ions.

  • PK = 1.

This high permeability value indicates that the cell membrane has many open (leak) channels through which K+ can pass.

The Na+ and Cl membrane permeability values are relatively lower than K+, meaning there are fewer open channels for these ions.

  • PCl = 0.3 – 0.5.
  • PNa = 0.01 – 0.05.
Slide
Potassium channels embedded in cell membrane
Potassium ion (K+)membrane  permeability
Most permeable ion, value = 1

Permeability Value
1

Potassium (K+)

Sodium channels embedded in cell membrane
Lowest permeability ion, value = 0.01 to 0.05
Sodium ion (Na+) membrane permeability

Sodium (Na+)

Permeability Value
0.01 – 0.05

Chloride channels embedded in cell membrane
Low permeability ion, value = 0.3 to 0.5
Chloride ion (Cl-) membrane permeability

Permeability Value
0.3 – 0.5

Chloride (Cl)

Ion Membrane Permeability
Slide
Potassium channels embedded in cell membrane

Most Permeable

Most Permeable

Potassium ion, relative permeability =
Sodium channels embedded in cell membrane

Sodium (Na+)

Permeability Value
0.01 – 0.05

Chloride channels embedded in cell membrane

Chloride (Cl)

Permeability Value
0.3 – 0.5

Ion Membrane Permeability

Slide
Potassium channels embedded in cell membrane

Most Permeable

Most Permeable

Potassium ion, permeability = 1

Permeability Value
0.01 – 0.05

Sodium (Na+)

Chloride channels embedded in cell membrane

Less Permeable

Less Permeable

Potassium ion, membrane permeability = 1, most permeable ion
Ion Membrane Permeability

Ion Membrane Permeability
Slide
Potassium channels embedded in cell membrane
Sodium channels embedded in cell membrane
Chloride channels embedded in cell membrane
Ion Membrane Permeability

Most Permeable

Most Permeable

Less Permeable

Less Permeable

Least Permeable

Least Permeable

Ion Membrane Permeability
Potassium ion
Potassium ion, membrane permeability = 1, most permeable ion
Sodium ion (Na+), membrane permeability = 0.01 to 0.05, lowest membrane permeability

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 Analysis Questions

1. How much less permeable is the membrane to Na+ ions than to K+ ions? Determine your answer using PK / PNa.

2. How much less permeable is the membrane to Cl ions than to K+ ions? Determine your answer using PK / PCl.

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References

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