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Download MEMBRANE PERMEABILITY ! membranes are highly impermeable
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MEMBRANE PERMEABILITY ! membranes are highly impermeable to ions and polar molecules ! bilayer core is hydrophobic, so these species must shed their water of hydration to get across ! dehydration is energetically very unfavourable (like a high activation energy barrier for a chemical reaction) ! species such as Na+, Cl!, sugars, amino acids, cannot cross membranes at a significant rate ! small uncharged molecules, such as glycerol and ethanol, cross slowly ! hydrophobic molecules, and gases such as O2, CO2 , N2 , cross rapidly ! water crosses membranes the fastest of all (not really understood why) ION GRADIENTS ACROSS MEMBRANES ! the ionic composition of the cytosol is very different from that of extracellular fluids ! large ion concentration gradients exist across plasma membrane ! gradients also exist across organelle membranes e.g. endoplasmic reticulum lumen has high Ca2+ (1-5 mM) ! generated by membrane transporters TYPES OF MEMBRANE TRANSPORT 1. Simple diffusion ! compounds move from high c to low c free energy of solution of concentration c G = G0" + RT ln c ! free energy change takes place when moving a molecule from c1 (high) to c2 (low) c1 c2 ΔG = RT ln c2 - RT ln c1 = RT ln (c2 /c1 ) if c1 > c2, ln (c2/c1) is !ve, so ΔG is !ve ! diffusion occurs spontaneously from high to low concentration (down gradient), no energy input required ! equilibrium is reached when c1 = c2 2. Membrane channels ! when open, forms doughnut-like pore through which solutes flow rapidly by diffusion ! always move from high c to low c (down gradient) ! transport rate # substrate concentration, not saturable ! ΔG !ve, spontaneous, no energy required ! animal cells have many ion channels; highly selective, only let specific ions pass through ! channels open and close in response to specific stimuli 3. Membrane transporters ! no continuous pore through membrane ! like enzymes; they transport their substrates one molecule at a time ! rate of transport is saturable at high substrate concentrations (like enzyme catalysis) ! specific binding site(s) for substrate(s) exists ! substrate binds to site on one side of membrane, conformational change takes place, and site now opens to other side of membrane, releasing substrate Passive transporters ! transport their substrates DOWN a concentration gradient; ΔG !ve, spontaneous ! e.g. passive glucose transporter, found in all cells, moves glucose from blood into cytosol - GLUT1 protein Active transporters ! transport their substrates UP a concentration gradient; ΔG +ve, energy input required ! often called "pumps" (pump substrate uphill) ! many active transporters of ions are powered by ATP hydrolysis; large family of ion pumping ATPases ! generate ion gradients across membranes MEMBRANE POTENTIAL ! when transporter moves a charged species across the membrane, get a charge imbalance across the membrane; membrane potential Δψ (in Volts) ! typical cell, Δψ = !60 mV, negative inside ! when Δψ exists, free energy of charged species is different on each side of membrane ΔG = zF Δψ F = Faraday constant (96.48 J V-1mol-1) z = unit charge on solute for transport of a charged species: ΔG = RT ln (c2/c1) + zF Δψ + + - - + + - - + + - - + - + + - - + + - - + + - + favourable free energy released + + unfavourable extra free energy required