PG1005 Lecture 7 Membrane Poten4al Dr. Neil Docherty My Teaching Objec4ves •  To define membrane poten6al •  To describe how charge separa6on across the membrane arise and is maintained •  To highlight the relevance of the membrane poten6al in biology, using the example of how it is harnessed to control insulin ac6vity. Diffusion Across Lipid Membrane Barriers; Important Considera4ons •  Chemical gradient •  Size of molecule •  Hydrophobic/hydrophilic compa6bility (Par66on co-­‐efficient) •  Thickness of membrane (5nm) •  Existence of pores/transporters for charged par6cles •  Channel open probability/ transporter satura6on •  Electrochemical gradients •  Osmo6c movement of water The Electrochemical Gradient •  Chemical Gradient -­‐Energy in the differen6al solute concentra6on on either side of the membrane •  Electrical Gradient -­‐Energy associated with movement of charged par6cles across the membrane when a voltage difference exists (membrane poten6al) Electrochemical Poten6al The Membrane Poten4al •  The plasma membrane of all cells possesses a membrane poten6al (mV) •  Meaning there is a rela6ve charge separa6on on either side of the membrane •  There is a rela6ve nega6ve charge along the inside (cytosolic side) of the membrane. WHY DOES THIS ARISE? HOW IS IT MAINTAINED? WHY IS IT IMPORTANT? Why Does This Arise? Differences in Conductance 3 key proper6es of membrane: 1)  Low Na+conductance 2)  High K+ leak 3)  Na+/K+ ATPase ac6vity 4)  (2 posi6ve charges in for 3 out) How is It Maintained-­‐? Na+/K+ ATPase ac6vity (Primary Ac6ve Transport) Why is It Important? -­‐The energy in poten6al differences can be used to regulate/
drive cellular processes e.g. 1.  Ac6on Poten6als 2.  Secre4on ( note example of insulin on the next slide) 3.  Contrac6lity Example -­‐Insulin Release • Glucose uptake increases ATP in pancrea6c beta cells • ATP closes a K+ leak channel • This depolarising force ac6vates a calcium channel (voltage sensi4ve) • Calcium promotes fusion of insulin containing granules with membrane (regulated exocytosis) • Insulin free to travel to target sites through the blood (endocrine ac4on) N.B. Develop a familiarity with the terms in bold and an ability to explain their meaning Your Learning From Today Should focus on being able to; •  Define what is meant by membrane poten6al •  Describe how differences in membrane conductance and lead to charge separa6on and how enzyme driven, primary ac6ve transport sustains these differences •  Illustrate how insulin release from the endocrine pancreas is dependent on the phenomenon of the membrane poten6al