Ion Transport Across Membranes (10.4) Transport of species across a membrane can be endergonic or exergonic – Passive transport (exergonic) occurs when a species diffuses across a membrane from high concentration to low concentration – Active transport (endergonic) occurs when a species diffuses across a membrane from low concentration to high concentration – Energetics are related to chemical potential on both sides of the membrane For charged species, the chemical potential is modified to an electrochemical potential – Electrochemical potential depends on charge of species and the electric potential Transport of charged species depends on concentration differences and on electric potential across the membrane – At equilibrium, the electric potential can be related to the concentration gradient

Donnan Potential (9.13) A potential can develop when a semi-permeable membrane is used to separate permeable species to impermeable speciessemi-permeable membrane – Small inorganic ions (e.g., Na + and Cl - ) may pass through the membrane, while large macromolecules (e.g., proteins) may not – If the macromolecule is charged and its counterion is found on both sides of the membrane, an equilibrium must be established based on chemical potential For two compartments separated by a semi-permeable membrane, transport of ions will occur under two conditions – Chemical potential of each permeable species tries to equilibrate (i.e., approach equal concentrations) – For every cation transported, an anion must accompany it (electrical neutrality) The impermeable species forces the concentrations of the permeable species to be unequal on both sides of membrane – Donnan potential is a measure of how far the ratio between permeable species on each side of the membrane deviates from one

Donnan Potential