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Membrane Transport A Thermodynamic Perspective 4 ways to penetrate the Cell Membrane • Simple Diffusion • Passive transport (facilitated diffusion) • Active transport (energy-dependent) • Receptor-mediated endocytosis Simple Diffusion Initial Final High Low GA = the chemical potential of A (also called the partial molar free energy) GAo' = chemical potential of standard state GA – GAo = RTln[A] (free energy varies with conc. A) GA = GA(in) - GA(out) (final - initial) [A]in GA = GA(in) - GA(out) = RTln [A]out Thus: If [A]out is > [A] in, GA is negative exergonic If [A]out is < [A] in, GA is positive endergonic If [A]out = [A]in, GA is zero Thermodynamics (ENERGY) of Transport Diffusion Rule: A(out) A(in) Free energy is released when a solute moves from an area of high concentration to low concentration Spontaneous (out) High (in) GA(in) < GA(out) Low Final state – Initial state =G = negative (in) (out) Balanced Rule: Free energy change is zero when the concentration of A on both sides is the same GA(in) = GA(out) Final state – Initial state =G = 0 (out) (in) GA(in) > GA(out) Final state – Initial state =G= positive Rule: When chemical potential of A(in) is greater than A(out), energy must be provided to drive A across the membrane, i.e., make free energy change negative Energy = ATP or a proton gradient Rule: The movement of ions presents a separate challenge because not only must the mass difference (chemical potential) be taken into account, but also the charge differential (electrochemical potential) electrochemical potential refers to the state of (+) (-) charges on both sides of the membrane The electrochemical potential is referred to as the membrane potential when dealing with cells Chemical potential GA = GA(in) - GA(out) = RTln and [A]in [A]out Electrochemical potential GA = GA(in) - GA(out) = ZAF Membrane potential Total Energy GA = RTln [A]in [A]out + ZAF Text p398 Na Na + + + Na + _ _ _ + Na + + + Na + Na Na + + + + Na Na Na Na + Na Na + 15:1 +Na 150 mM GNa+ = RTln (- 60 mV) [A]in [A]out = 8.314 x (310 K) x ln = – 12.8 kJ/mole + 10 mM + ZAF [0.010] [0.150] + (1) 96,500 x -0.06 volts Na Na Na + + Out + Na + + + Na + Na Na + + Na Na + +Na Na In _ _ _ + + + 150 mM (+ 60 mV) GNa+ = RTln = 8.314 x (310 K) x ln = + 12.8 kJ/mole [A]out [A]in [0.150] [0.010] + + Na + Na + Na Na 10 mM + ZAF + (1) 96,500 x +0.06 volts – Out Cl – Cl – Cl – + Cl – Cl + – – Cl – + Cl Cl – Cl 150 mM In _ _ _ (+ 60 mV) GCl- = RTln [A]in [A]out Cl – Cl Cl – – 10 mM + ZAF [0.010] = 8.314 x (310 K) x ln + (1) 96,500 x +0.06 volts [0.150] = – 6.85 kJ/mole + 5.79 kJ/mol = – 1.06 kJ/mol – Out Cl – Cl – Cl – + Cl – Cl + – – Cl – + Cl Cl – Cl 150 mM In _ _ _ (– 60 mV) GCl- = RTln [A]out [A]in Cl – Cl Cl – – 10 mM + ZAF [0.150] = 8.314 x (310 K) x ln + (1) 96,500 x – 0.06 volts [0.010] = + 6.85 kJ/mole + (– 5.79 kJ/mol) = 1.19 kJ/mol Facilitated Diffusion (Mediated Transport) Modes of Transport ATP-Driven (Active) Transport [Ca2+-ATPase] Vesicle Trafficking • The secretory pathway • The trans-Golgi network • The signal hypothesis • Protein targeting Rule: Proteins destined for secretion from a cell or for relocation to a membrane or a specific organelle are synthesized on the rough endoplasmic reticulum (RER) Definition: The RER consists of ribosomes bound to membranes enclosing an internal hollow space or cisternae Selection: Proteins possess a signal sequence that is recognized by a receptor on the membrane Action: Proteins pass into the space and transit to the Golgi while entrapped in vesicles Protein inserted in plasma membrane Trans Secretory granule Golgi Cis Golgi RER Pre-lysosome Signal Hypothesis Proteins destined for secretion or transit to membranes and organelles, have a signal peptide that allows them to enter the RER cisternae The signal peptide is recognized by a receptor called the “signal recognition particle” (SRP) on the RER membrane Signal sequences on the N-terminal represent a string of leucine-rich hydrophobic amino acids that allow the peptide to dock with the receptor The signal peptide is removed after the peptide has penetrated the membrane Signal Hypothesis Docking SRP SRP receptor Signal Peptide cleaved Lumen of cisternae +NH 3 +NH Rough Endoplasmic Reticulum 3 Carbohydrate