Cell Potassium Concentration and Anions

Potassium Ions and Concentration Gradients

• Inside a cell, there is a higher concentration of potassium (around 150 millimoles per liter) compared to outside the cell (around five millimoles per liter).
• Due to the concentration gradient, potassium ions leave the cell, leaving behind anions that create a negative charge.
• The negative charge attracts potassium ions back into the cell.

Membrane Potential

• The negative charge created by the anions is around -92 millivolts, which offsets the concentration gradient.
• If a positive charge is introduced into the cell, it reduces the negative charge, making the membrane potential less negative.
• As potassium ions continue to leave the cell, the anions left behind contribute to the negative charge, causing the membrane potential to return to -92 millivolts.

Thought Experiment

• If a cell is only permeable to potassium ions, adding a positive charge or changing the concentration gradient does not affect the membrane potential, which will always return to -92 millivolts.
• Two important conditions are required for this: a concentration gradient and permeability to only potassium ions.

Diagram Explanation

• Four scenarios are possible: no concentration gradient and no permeability, concentration gradient but no permeability, permeability but no concentration gradient, and both concentration gradient and permeability.
• A membrane potential is only possible when there is both a concentration gradient and permeability to potassium ions.

Formula for Membrane Potential

• The formula for membrane potential (Vm) is Vm = 61.5 x log([K_out] / [K_in]), where [K_out] and [K_in] are the concentrations of potassium outside and inside the cell, respectively.
• The formula can be used to calculate the membrane potential for other ions, such as sodium, chloride, and calcium, by plugging in their respective concentrations.

Other Ions

• Sodium has a membrane potential of +67 millivolts, chloride has a membrane potential of -86 millivolts, and calcium has a membrane potential of +123 millivolts.
• Calcium has a two-plus charge, so the formula is adjusted to 30.75 instead of 61.5.

Recap of potassium concentration inside and outside cells, and its relation to anions. Understand the concept of concentration gradient and its effects.