Donnan Effect & The Sodium/Potassium ATPase Pump (E3)

Question 1

What is the Donnan effect?

Answer 1

The consequence(s) of having multiply charged macromolecules trapped inside a compartment enclosed by a semipermeable membrane; it is effectively an osmotic threat to living cells

Question 2

Which types of ions are permeable across the PM? Which ones are impermeable?

Answer 2

Permeable - small, monoatomic ions (e.g., M⁺ & A^-)

Impermeable - large polyanions (e.g., P^n-)

Question 3

How do cells maintain electroneutrality when large polyanions are trapped inside?

Answer 3

Each P^n- is balanced with a number “n” of M⁺

Question 4

How does the impermeable solute within the cell, while the cell itself is bathed in a permeable solution of ions, affect the state of the cell?

Answer 4

The extracellular solution becomes hypotonic, forcing water into the cell, and causing uncontrolled swelling and rupture

Question 5

How does the Na⁺/K⁺ ATPase pump resolve the osmotic cell rupture issue caused by the Donnan effect?

Answer 5

The pump “transforms” a permeable solute (e.g., Na⁺) into an impermeable one. Na⁺ is actively pumped out as quickly as it is pumped in. This pump utilizes 1/3 of the cell’s ATP – 2/3 in a neuron – to maintain osmotic balance by constantly removing the small cationic solutes that would otherwise be attracted to the impermeable polyanion. A stable cell volume is ensured