Basic Principles of Transport
Authors: Macey Robert I. , Moura Teresa F. 
Published: November 2010

AbstractThe sections in this article are:ThermodynamicsMaximal Work Is Attained on Reversible PathsChanges in Free Energy Are Obtained from Chemical Potentials: Free Energy per MoleSolutes in Phase EquilibriumOsmotic EquilibriumChemiosmotic CouplingDiffusionFlux Is Proportional to the Concentration Gradient: Fick's First LawConservation of Matter: Fick's Second LawProgress of Diffusion: Mean Square Displacement = 2 DtRandomly Diffusing Molecules Spread Out in a Normal DistributionThe Diffusion Front, Where Solute Depletion Ends and Accumulation Begins, Moves asDiffusion Transients in Thin Membranes Are Very RapidPermeation through Membranes Takes Place in At Least Three StepsThe Exchange Time for Filling or Emptying of Cells Equals V/PAIn Simple Exponential Processes, the Time Constant = the Mean Residence TimeCytoplasmic Diffusion Transients Are RapidUnstirred Layers Can Be a Significant BarrierMembrane Diffusion Is Assumed to Be Rate‐Limiting for Plasma MembranesSelective Permeability of Lipid Bilayers Is Determined Primarily by SolubilityWater TransportWater Transport Can Be Driven by Three Different GradientsIn Lipid Membranes Water Transport Occurs by Solubility‐Diffusion Mechanism, with Pf= Posm= PdOsmotic Gradients Generate Hydraulic Pressure Gradients in Aqueous ChannelsIn Narrow Channels the RatioPosmotic/Pdiffusion= the Number of Water Molecules Contained within the ChannelCoupling of Solute and Solvent Transport Is Described by the Kedem‐Katchalsky EquationsIonic DiffusionDiffusion with Superimposed Drift Due to External ForcesIons Transported by Simple Diffusion Follow the Ussing Flux Ratio RelationBulk Solutions Carry No Net ChargeThe Constant Field Is a Convenient IdealizationConductance Depends on Ionic ConcentrationsPermeability Ratios Can Be Measured by Changes in Membrane PotentialAn Electrogenic Pump Contributes to ΨmChannels Can Be Incorporated into the Nernst‐Planck FormulationEnergy BarriersThe Born Energy Estimates the Work Required to Transfer an Ion from One Medium to AnotherBorn Energy, Image Forces, Dipole Potentials, and Hydrophobic Interactions Contribute to the Energy Barriers of Lipid BilayersSolvation Energies Are Important Determinants of Channel AccessibilitySurface Potentials Modify the Transmembrane Potential as Well as Local Ion ConcentrationsTransport across Energy BarriersEyring Rate Theory: Rate Constants Depend on ΨKinetic ApproachesChannelsSingle‐Occupancy Channels with Binding Sites Show Saturation KineticsCompetition, Unidirectional Flux, and the Ussing Flux RatioSingle‐Occupancy Channels: Voltage Dependence in Symmetric ChannelsSingle‐Occupancy‐Channel Results Can Be Generalized to N SitesMultiple OccupancySimple CarriersNet FluxUnidirectional FluxEquilibrium at the BoundariesRate‐Limiting Steps at the BoundariesEnergy‐driven Simple Carrier SystemsCotransportThermodynamics: Cotransport Can Move Solutes “Uphill”Kinetic DescriptionGeneral Net FluxUnidirectional FluxesKinetics of Simultaneous Binding of mA and nB Resembles Simple Carrier Kinetics When Carrier Concentrations Are Replaced with the Product AmBnRelations between the Net Flux Equation Parameters and the Michaelis‐Menten—type Parameters for 1:1 StoichiometryCountertransportThermodynamicsKinetic ModelsPing‐Pong ModelSequential ModelGeneralization to m‐n Stoichiometry for Simultaneous BindingFluctuating Barriers: Channels and CarriersChannel Transport Properties Depend on the Rate of Transition between the ConformationsChannels with Fluctuating Barriers Do Not Show Michaelis‐Menten KineticsChannels with Fluctuating Barriers Can Show Carrier Kinetics