Gibbs–Donnan effect

Gibbs–Donnan Effect

Diagram illustrating the Gibbs–Donnan effect

The Gibbs–Donnan effect, also known as the Donnan effect, describes the behavior of charged particles near a semi-permeable membrane that does not allow certain ions to pass through. This phenomenon is named after the American physicist Josiah Willard Gibbs and the British chemist Frederick George Donnan.

Principles

The Gibbs–Donnan effect occurs when a semi-permeable membrane separates two solutions, one of which contains a non-diffusible ion. This situation leads to an unequal distribution of diffusible ions across the membrane, resulting in an osmotic pressure difference and an electric potential difference between the two sides.

Ion Distribution

In a typical scenario, consider a membrane that separates two compartments. One compartment contains a solution with a non-diffusible anion, such as a large protein, while the other compartment contains a simple salt solution. The presence of the non-diffusible anion causes an imbalance in the distribution of diffusible ions, such as sodium (Na_) and chloride (Cl_) ions, across the membrane.

Osmotic Pressure

The unequal distribution of ions results in an osmotic pressure difference. Water tends to move from the side with lower osmotic pressure to the side with higher osmotic pressure, which can lead to swelling or shrinkage of cells or compartments.

Electric Potential

The Gibbs–Donnan effect also creates an electric potential difference across the membrane. This potential difference is due to the unequal distribution of charged ions, which can affect the movement of other ions and molecules across the membrane.

Biological Significance

The Gibbs–Donnan effect is important in various biological systems. It plays a crucial role in maintaining the cell membrane potential and the distribution of ions across cell membranes. In particular, it affects the behavior of red blood cells, neurons, and other cells that rely on ion gradients for their function.

Red Blood Cells

In red blood cells, the Gibbs–Donnan effect contributes to the maintenance of cell volume and the balance of ions such as potassium (K_) and sodium (Na_) across the cell membrane.

Neurons

In neurons, the effect is part of the mechanism that establishes the resting membrane potential, which is essential for the transmission of nerve impulses.

Related Pages

• Osmosis
• Membrane potential
• Electrochemical gradient
• Ion channel