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

Sodium-Potassium Pump (10.4) Sodium and potassium ions are actively passed through cell membranes – Sodium has a higher concentration outside the cell, potassium has a higher concentration inside the cell – Pumping sodium out of a cell and pumping potassium ions into the cell requires energy (endergonic ATP hydrolysis is necessary for the pump to work since the process is endergonic – An imbedded protein serves to transport the ions across the membraneimbedded protein – Protein is phosphorylated during ATP hydrolysis, changing the structure of the protein, allowing ions to pass through the protein Binding of the ions to the protein depends on the nature of the protein Binding – When sodium binds to the protein, it can be phosphorylated – Phosphorylated protein binds preferentially to potassium, and hydrolyzes to change the structure of the protein, allowing potassium to pass through the protein

Donnan Potential

Cellular Membrane

Sodium-Potassium Pump Cycle