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Unformatted text preview: Membrane Permeability: Movement of Small Molecules Across Lipid Bilayers Membrane is a Partially Permeable Barrier Permeability Properties of Lipid Bilayer (Pure Lipid with No Permeability Protein) Protein) Permeability Properties of Membranes Mechanisms of Transport and Structures of Molecules Mechanisms Involved Types of Molecules Types Gas, amino acid, nucleotide, sugar, ion Types of Membranes Plasma Membrane Mitochondrial Membrane Lysosome membrane Endoplasmic Reticulum Endoplasmic
Diffusion Diffusion Diffusion Facilitated Diffusion Fick’s Law of Diffusion Diffusion is Diffusion the random continuous movement of molecules driven by kinetic energy Fick’s Law Fick’s Js = - DA dC/ dX Js = flux of
substrate s substrate C= concentration concentration D = diffusion diffusion coefficient coefficient Lipid Bilayer Different Solutes Exhibit Different Rates of Diffusion Across a Lipid Bilayer Fast: Small NonPolar Moderate: Small Polar Slow:Medium Polar (<400) No Transport: Charged Ions Cell Membranes Movement of Molecules into Cells Molecules like glucose and ions that cannot Molecules pass across a lipid bilayer by diffusion can cross cell membranes. Must have special structures/mechanisms structures/mechanisms Diffusion Coefficient Not Always Applicable Distance X across membrane not known. Distance Define Permeability Coefficient, P. Js = PA C (units of P are distance/time) (units Transport Experiment
Measure Permeability of Different Molecules Measure Vary Concentration Outside Measure accumulation inside cell (flux) Calculate P Transport Data Transport Experiment on Cell Membranes Js = Slope = flux = Moles S/sec Diffusion: straight line through origin Diffusion: Carrier Mediated Transport: Approach to a Carrier maximum value (saturation) value
Is it possible to have a positive flux when the Is 3 types of transport types concentration gradient is negative? concentration Three Types of Transport Three Table Characteristics of Transport Solute Transported Active Transport Active
Non-Polar Solute yes Polar MW > 400 yes Solute Specificity Solute Thermodynamics down gradient down no effect on entropy effect decrease decrease requires energy intrinsic direction Diffusion Diffusion
yes Facilitated Diffusion
yes no yes yes yes yes increase no no yes yes yes yes no no yes increase no no Mechanism of Carrier Mediated Transport Mechanism
Glucose transport in red blood cell Amino acid transport in bacteria Structure conserved: 12 transmembrane domains Active Transport Active Transport: Movement Of Molecules Against a Concentration Gradient Gradient
Sodium Potassium ATPase Maintains high K+ inside Maintains low Na+ inside Transports Na+ and K+ against gradient against Electrogenic Requires ATP Strict ordering of steps Similar pumps for H+ and Ca++ No ATP hydrolysis without transport Transport Cycle Mechanism of P Type Active Transport Pumps
P type pump is phosphorylated during transport Binding of nucleotide to protein induces a conformational change Nucleotide state of enzyme controls conformation and affinity for ion (high or low) Domains of Pump Calcium Active Transport Pump Transmembrane Transmembrane Region Region Nucleotide Domain Phosphorylation Phosphorylation Domain Domain Activator Domain Changes in nucleotide ligand induce conformational change from E1 to E2 States E1: inward facing, and high affinity for calcium; E2: outward facing and low affinity for calcium 2 structures Conformational Changes in Calcium Pump Coupled Transport Similar Structures of Pumps for Ca+ + , Na+/K+, and H+.
See Nature 450, 597-599. (December 2007). Coupled Transport Coupled Electrochemical gradient drives transport. Symport: Na+/glucose Antiport: Na+/H+ Sodium gradient drives transport of other solutes. Energy supplied indirectly by Na+/K+ ATPase Lac permease Mechanism of Coupled Transport Mechanism Lac Permease: Structure and Function
gradient drives lactose uptake. tside Facing: and lactose bind side Facing: and lactose released.
ansmembrane helices t and shift. Lac Y structure Science 301, 603-604; Science Science 301 610-615 (2003) (2003) Epithelial Cell A Single Cell has Multiple Pumps and Transporters Working Together Rates Rate of Transporters , Pumps, and Channels
Pumps: slow Transporters: moderate Channels: fast ...
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- Spring '08