concentration/charge gradients

Membrane Potential

• Membranes are selective about which ions they allow to pass through.
• The difference in charge between the inside and outside of a cell is called the membrane potential.
• Most cells have a negative membrane potential due to the presence of macromolecules inside the cell.

Ion Concentrations

• Potassium (K+) concentration:

  • Inside the cell: $150$ mmol
  • Outside the cell: $4$ mmol
  • This creates a concentration gradient for K+ to flow out of the cell.

• Calcium (Ca2+) concentration:

  • Outside the cell: $2.5$ millimolar
  • Inside the cell: very low
  • This creates a concentration gradient for Ca2+ to flow into the cell.

• Sodium (Na+) concentration:

  • Outside the cell: $145$ millimolar
  • Inside the cell: $20$ millimolar
  • This creates a concentration gradient for Na+ to flow into the cell.

• These concentration gradients are established due to the membrane's regulation of ion flow.

  • Allow for rapid ion flow due to the steepness of the gradients.

Cardiomyocyte Example: Resting Membrane Potential

• A cardiomyocyte (heart muscle cell) has a resting membrane potential of about $-96$ mV.
• To induce muscle contraction, the cell needs to become more positive.
• Sodium channels are opened, allowing Na+ to flow into the cell due to the high concentration gradient.
• The membrane potential can shoot up to about $+30$ mV, signaling contraction.
• To return to resting potential, potassium (K+) is pumped out of the cell, removing positive charge and making the cell more negative.
• Calcium is also involved, but its role is not detailed here.

Ion Regulation

• Charged ions are highly regulated by the membrane.
• The combined effect of concentration and charge gradient across the membrane dictate the Electrical potential and ion flow for charged ions.
• Neutral ions are only dictated by concentration gradients

Concentration and Charge Gradients

• Concentration gradient: The difference in ion concentration across the membrane (e.g., $145$ to $20$ mmol for Na+).
• Charge gradient: The electrical attraction or repulsion due to ion charges, for instance, negatively charged cell attracting positive ions.
• Example: Sodium ions are both attracted by concentration gradient and the negative charge inside the cell.
• Chloride ions are higher concentration outside the cell but will not flow into the cell as fast as sodium ion due to the repelling negative charge.