Membrane Properties, Channels and Transporters W2D1H3 W2D1H4 MCBD

Overview Intro: Why this topic? Learning Objectives ( 5 min) IRAT review (5-10 min) Review “lecture” slides (30 min) Case #1: peptic ulcer (15 min) Break (10 min) Case #2: nephropathy (15 min) GRAT (10 min) Optional: Case #3: digoxin Nernst practice problems MCBD

Why This Topic? All cells are enclosed within a membrane that separates the inside of the cell from the outside. Understanding the components and roles of this barrier will allow you to understand how a cell’s internal environment is optimized to support the biochemical reactions required for normal function. Once a basic grasp of the channels and transporters has been met, you can begin to appreciate that all cells selectively modify the ionic composition of their internal environment to create a membrane potential. This process creates an electrochemical driving force for diffusion that can be used to move charged solutes across the membrane. Later we will learn how the ions can be modified transiently in excitable cells to create an electrical signal called the action potential. MCBD

Learning Outcomes LO1 Describe structure and topology of cellular membranes. LO2 Contrast ion channels and ion transporters and the forces that drive them. LO3 Know the normal balance of Na+, K+, Cl- and Ca2+ with respect to the plasma membrane. LO4 Describe elements of ionic electricity: ions, charge, potential gradients, forces, current, membrane capacitance. LO5 Describe elements of ionic electricity: ions, charge, potential gradients, forces, current, membrane capacitance. LO6 Recognize that fluxes needed to make membrane potential are extremely small. LO7 Explain with Na+, K+, Cl- and Ca2+ what contribution each could make individually to potential changes in plasma membrane of excitable cells. LO8 Describe how pumps without channels make a membrane potential in mitochondria MCBD

Rhodopsin Structure, Function, and Topography Paul A. Hargrave Investigative Ophthalmology & Visual Science January 2001, Vol.42, 3-9. doi: Rhodopsin embedded in bovine photoreceptors MCBD

http://thebrain. mcgill. ca/flash/i/i_06/i_06_m/i_06_m_mou/i_06_m_mou http://thebrain.mcgill.ca/flash/i/i_06/i_06_m/i_06_m_mou/i_06_m_mou.html MCBD

LO1 Describe structure and topology of cellular membranes. Briefly discuss the structure of the plasma membrane. It might be good to compare/contrast with other organelles a bit and remind them that the nuclear membrane is a double bilayer, and phospholipid content varies in different organelles (and make sure they know what organelles are membrane bound!). MCBD

LO1 Describe structure and topology of cellular membranes. Briefly discuss membrane proteins MCBD

LO2 Contrast ion channels and ion transporters and the forces that drive them. Review different types of transport MCBD

Types of Transport Proteins MCBD Hille 2016 Mechanisms of transport across cell membranes Downhill flow Cotransporter--secondary active transport Active transport ATPase Pump Ion channel often gated Coupled carrier

Ion-selective channels are pores KcsA 2x Hille MCBD 2016 Enhancement slide Ion-selective channels are pores OUT Cell mem- brane IN N K+ ions in pore C This bacterial K+ channel protein is a tetramer of 4 subunits. One subunit has been removed so we can look into the pore in the middle. (Doyle, ..., MacKinnon, 1998)

LO2 Contrast ion channels and ion transporters and the forces that drive them. Question: Which best describes the Na+/K+ ATPase pump? 1 An exchanger 2 Primary active transport 3 A cotransporter 4 Diffusion 5 Facilitated Diffusion 3 easy questions to make sure everyone is awake! Answer: 2 MCBD

LO2 Contrast ion channels and ion transporters and the forces that drive them. Question: Which best describes movement of glucose across a membrane in a thermodynamically favorable direction? 1 An exchanger 2 Primary active transport 3 A cotransporter 4 Diffusion 5 Facilitated Diffusion 3 easy questions to make sure everyone is awake! Answer:5 This is correct, but the story for glucose is a bit more complex….. MCBD

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LO2 Contrast ion channels and ion transporters and the forces that drive them. Question: Which best describes the Na+/Ca2+ exchanger that removes cytosolic Ca2+? 1 An exchanger 2 Primary active transport 3 A cotransporter 4 Diffusion 5 Facilitated Diffusion 3 easy questions to make sure everyone is awake! Answer: 3 MCBD

LO3 Know the normal balance of Na+, K+, Cl- and Ca2+ with respect to the plasma membrane. Review briefly MCBD

LO3 Know the normal balance of Na+, K+, Cl- and Ca2+ with respect to the plasma membrane. This is how balances are established. Note: Ca2+ pumped out (partly by Na+ gradient) and chloride redistributes (partly in response to negative charge inside cell- many impermeant anions in the cell like amino and nucleic acids cannot move but Cl can). MCBD

LO3 Know the normal balance of Na+, K+, Cl- and Ca2+ with respect to the plasma membrane. Na+ IN K+ OUT Tease them with the action potential MCBD

LO4 Describe elements of ionic electricity: ions, charge, potential gradients, forces, current, membrane capacitance. 20 Jan 1775 – 10 June 1836 18 Feb 1745 – 5 March 1827 Charge: Ions have a charge given in multiples of one elementary electron charge. Cations (Na+, K+, Ca2+) are positive and anions (Cl-) are negative. Two forces: Charges are moved by the force of electric fields: cations towards a negative pole and anions towards a positive pole. Opposites attract. Ions are also moved by diffusion down their concentration gradients. Current: A net movement of charge (flow of ions) is an electric current (ion current) measured in amperes (A) and symbolized by I. The direction of current is defined as the direction that positive charges are moving. Voltage: If cations are removed from a compartment (the cell), the compartment becomes more negative; a negative electrical potential (or negative voltage) will be set up there. In electro­physiology, the words voltage and electrical potential will mean the same thing. A voltage is measured in units of volts (V) and symbolized by V. It is defined as the amount of electric work it would take to remove one more cation. Review this briefly MCBD

voltage (volt) = current (amp) x resistance (ohm) LO4 Describe elements of ionic electricity: ions, charge, potential gradients, forces, current, membrane capacitance. voltage (volt) = current (amp) x resistance (ohm) 20 Jan 1775 – 10 June 1836 18 Feb 1745 – 5 March 1827 16 Marcg 1789 – 6 Juky 1854 Review this briefly voltage (V) = current (A) x resistance (Ω) MCBD

LO4 Describe elements of ionic electricity: ions, charge, potential gradients, forces, current, membrane capacitance. Review this briefly If we can separate a charge Q by applying an electrical potential V across the membrane, the membrane has by definition a capacitance C=Q/V . In fact, because the membrane is so thin (only two molecules thick, with a total thickness of about 6×10−9m), we don't need much voltage to separate the charges and therefore the membrane capacitance is quite high; per unit area, it is C = C/S ≈ 10−2 Fm−2 MCBD

LO5 Describe elements of ionic electricity: ions, charge, potential gradients, forces, current, membrane capacitance. Based on a classic experiment by Nernst showing ion movement/gradients could be responsible for charge across a membrane MCBD

LO6 Recognize that fluxes needed to make membrane potential are extremely small. In this experiment the concentration of ions do not change significantly and they do not during an action potential. MCBD

LO7 Explain with Na+, K+, Cl- and Ca2+ what contribution each could make individually to potential changes in plasma membrane of excitable cells. The gas constant (R, joules/K) is a proportionality constant that relates the energy scale to the temperature scale. Temperature (T, K) is body temperature, which is 310 K. The Faraday constant (F, C/mol) represents the amount of electric charge carried by one mole of electrons. Formula for the Nernst equation. Students will use this in cases to consider several pathologies. Remind students that there is a practice document and encourage them to use this if the material is unfamiliar. MCBD

Enhancement: How did Nernst derive his equation in 1988? Hille MCBD 2016 Enhancement: How did Nernst derive his equation in 1988? At equilibrium the electrical work for moving an ion across the membrane electrical field (E) in one direction would balance the chemical work resulting from the movement of ions in the other direction caused by the difference in concentration. Electrical work per mole = zionFΔE Chemical work per mole = RT ln [Kout]/[Kin] Electrical Work = Chemical Work zionFΔE = RT ln [Kout]/[Kin] Solve for ΔE ΔE = RT ln [Kout]/[Kin]

LO7 Explain with Na+, K+, Cl- and Ca2+ what contribution each could make individually to potential changes in plasma membrane of excitable cells. Nernst values for several ions. They will come back to this in cases and in the AP material. MCBD

Vm (mV) = 61 log PK[K+]out PNa[Na+]out PCl[Cl-]in + PK[K+]in LO7 Explain with Na+, K+, Cl- and Ca2+ what contribution each could make individually to potential changes in plasma membrane of excitable cells. The Goldman-Hodgkin-Katz (GHK) equation is used to calculate the resting membrane potential Vm (mV) = 61 log PK[K+]out PNa[Na+]out PCl[Cl-]in + PK[K+]in PNa[Na+]in PCl[Cl-]out Therefore, the resting membrane potential is determined by the combined contributions of the (concentration gradient) X (membrane permeability) for each ion. If the membrane permeability to an ion is zero, it has no effect on the resting membrane potential – e.g., Ca2+ MCBD

LO8 Describe how pumps without channels make a membrane potential in mitochondria. Ask the students where [H+] are higher/lower. They should be able to answer based on production of ATP. MCBD

From: Molecular Biology of the Cell. 4th edition MCBD