Total body water ~(60% of body mass): Intracellular fluid ~2/3 or ~65% Extracellular fluid ~1/3 or ~35% fluid. Interstitial.

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1 Physiology of the Cell Membrane Membrane proteins and their roles (channels, transporters, receptors and structural proteins). Why molecules move across membranes? Basic laws of membrane permeability. Trans-membrane transport in the cell. Dr. Sergey Kasparov room E9 The main text for this lecture is: Germann and Stanfield. Some additions from Vander & Luciano and Cooper s textbooks If this was a cell membrane, which part of the Winnie The Pooh could be expected to be the most hydrophobic? Total body water ~(60% of body mass): Intracellular fluid ~2/3 or ~65% Extracellular fluid ~1/3 or ~35% Interstitial Plasma of fluid the blood ~28% of TBW ~7% of TBW (~80% of ECF) (~20% of ECF) 1

2 Some Functions of Some Integral Membrane proteins (from the Chemistry of the cell ) Anchoring/connecting Receptor detects chemical signals A channel (leak) effect Chemical reactions Carriers A gated channel Concentration of selected solutes in intracellular fluid and extracellular fluid in millimols mm Inside the cell In extracellular fluid 20 0 If the membrane was freely permeable to these molecules The first message: the cell membrane is NOT freely permeable to polarised/ionised/hydrophilic molecules 2

3 Polar molecules and ions cannot freely pass through the membrane Glucose Amino Acids H Na Cl - Ca 2 Small, uncharged molecules pass through the membrane O 2 CO 2 N 2 Ethanol Glycerol Steroids Uncharged, non-polar molecules can pass through the membranes because they can temporarily dissolve in the lipid bi-layer IN OUT K Na Mg Ca Cl HCO Inorganic P Amino acids /- 8 2 Glucose / ATP /- 4 0 Protein / The second message: the cell is able to actively move certain molecules across its membrane. Proteins and ATP are synthesised inside the cell, therefore are highly concentrated there. Forces which determine the direction of transport across the membrane Passive vs active transport Passive Active - along the concentration - against the concentration gradient and using the energy gradient of this gradient - uses energy supplied by the cell to special proteins called PUMPS CO 2 These guys are called pumps! 3

4 4 Chemical vs Electrical Driving Force Chemical vs Electrical Driving Force Chemical vs Electrical Driving Force Chemical vs Electrical Driving Force

5 Chemical and Electrical Driving Forces may combine to create the Electrochemical Driving Force 140 mm -70 mv 4mM K What will happen if the potential of this membrane decreases to -10 mv? Conclusion: movement of charged particles such as ions, across the membrane depends on electro-chemical driving force (the sum of the force generated by chemical gradient and the force generated by electric field). Net flux V 5

6 Net flux V Net flux V Equilibrim potential (or reversal potential) The Reversal Potential (same as equilibrium potential): Potential of the membrane at which the electrical driving force is exactly equal the chemical driving force and therefore THE NET FLUX of charged particles (of one particular kind) is NIL. 6

7 The reversal (equilibrium) potential can be calculated using Nernst equation: ion = 61.5mV Z Eion Log 10 (C out /C in ) IMPORTANT: 61.5 is a calculated constant derived from universal gas constant, the temperature (37 o C) and Faraday electrical constant. For 20oC it is For 25oC it is 60. This is why different textbooks sometimes give you different values (In Vander s physiology it is 60!). Z is a valence of an ion. Remember, for a negative ion it will be negative. You MUST look through the relevant chapters in the textbooks: E.g.: - Chapter 6 in Vander s Human Physiology (10 th ed) or -Chapter 6 in Boron Boulpaep Textbook Be prepared to calculate the reversal potentials of the ions. Also read about Goldman Equation which describes membrane potential when more than one ion is involved: P Vm = 61.5 * Log K [K o ] P Na [Na o ] P Cl [Cl i ] 10 P K [K i ] P Na [Na i ] P Cl [Cl o ] Mechanisms of Passive Transport Transport Passive Active 1. Simple Diffusion 2. Facilitated Diffusion 3. Diffusion through Ion Channels HANDOUTS: PAGE Transport 7

8 Factors affecting the speed of simple diffusion: 1. The magnitude of the driving force 2. Surface area of the membrane 3. Permeability of the membrane Fick s law: Net flux = P(permeability) A(area) (ΔC) Net Flux Concentration Rest O 2 CO 2 Exercise O 2 Net Flux CO 2 Concentration But for glucose Net Flux Concentration Facilitated diffusion 8

9 For glucose the maximum rate can be ~ molecules per second Factors affecting the speed of facilitated diffusion: 1. The magnitude of the driving force 2. Transport rate of the individual carriers 3. The number of available carriers 3. Diffusion through Ion Channels A leak channel A gated channel Both leak and gated channels allow movement of molecules (mainly inorganic ions) down the electrochemical gradient. So, if the gradient reverses, the ions will flow in the opposite direction. 1. The channels are aqueous pores through the membrane. 2. The channels are usually quite selective, for example some only pass Na, others K, still others Cl - 3. Gated channels may be opened or closed by various factors (for example electrical potential of the membrane). Factors affecting the speed of diffusion through ion channels: 1. The magnitude of the electro-chemical driving force 2. The transport rate of the individual channels (conductance) 3. The number of available channels 4. State of channels (open/close) in case they are gated (ADD TO YOUR HANDOUTS!!!) Passive transp.flv 9

10 Active transport IN OUT Key features: K Occurs irrespective of the chemical Na and/or electrical gradient 2. Requires an extra chemical source of energy, supplied by the cell. Directly or indirectly this is ATP Active transp.flv These guys are called pumps! The Key Points: 1. Polar & charged molecules cannot freely diffuse through the cell membrane but use special molecular pathways. Na ions are concentrated in the extracellular space, K inside the cells. 2. Small lipid-soluble molecules can enter the cell because they can pass through the lipid bi-layer. 3. Molecules move down their concentration gradient. Ions in addition move according to the electrical field. Therefore for ions we need to consider the electro-chemical gradients. 4. There are passive and active means of molecular transport across the membrane. Passive transport uses the energy of chemical (or electrochemical) gradient. Active transport uses the energy supplied by the cell and requires specialised proteins called pumps. 5. Simple passive diffusion occurs in non-saturating manner because it does not involve any special membrane proteins. All other ways of transport saturate, because the available number of carrier molecules is always finite. 6. Ion channels may exist in either open or closed conformation, this can be regulated by various factors, for example electrical potential of the membrane. THERE IS A WEB-BASED TUTORIAL FOR YOU ON THIS TOPIC! OSMOSIS! 10

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