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

The sections in this article are:

Thermodynamics

Maximal Work Is Attained on Reversible Paths

Changes in Free Energy Are Obtained from Chemical Potentials: Free Energy per Mole

Solutes in Phase Equilibrium

Osmotic Equilibrium

Chemiosmotic Coupling

Diffusion

Flux Is Proportional to the Concentration Gradient: Fick's First Law

Conservation of Matter: Fick's Second Law

Progress of Diffusion: Mean Square Displacement = 2 Dt

Randomly Diffusing Molecules Spread Out in a Normal Distribution

The Diffusion Front, Where Solute Depletion Ends and Accumulation Begins, Moves as

Diffusion Transients in Thin Membranes Are Very Rapid

Permeation through Membranes Takes Place in At Least Three Steps

The Exchange Time for Filling or Emptying of Cells Equals V/PA

In Simple Exponential Processes, the Time Constant = the Mean Residence Time

Cytoplasmic Diffusion Transients Are Rapid

Unstirred Layers Can Be a Significant Barrier

Membrane Diffusion Is Assumed to Be Rate‐Limiting for Plasma Membranes

Selective Permeability of Lipid Bilayers Is Determined Primarily by Solubility

Water Transport

Water Transport Can Be Driven by Three Different Gradients

In Lipid Membranes Water Transport Occurs by Solubility‐Diffusion Mechanism, with Pf= Posm= Pd

Osmotic Gradients Generate Hydraulic Pressure Gradients in Aqueous Channels

In Narrow Channels the RatioPosmotic/Pdiffusion= the Number of Water Molecules Contained within the Channel

Coupling of Solute and Solvent Transport Is Described by the Kedem‐Katchalsky Equations

Ionic Diffusion

Diffusion with Superimposed Drift Due to External Forces

Ions Transported by Simple Diffusion Follow the Ussing Flux Ratio Relation

Bulk Solutions Carry No Net Charge

The Constant Field Is a Convenient Idealization

Conductance Depends on Ionic Concentrations

Permeability Ratios Can Be Measured by Changes in Membrane Potential

An Electrogenic Pump Contributes to Ψm

Channels Can Be Incorporated into the Nernst‐Planck Formulation

Energy Barriers

The Born Energy Estimates the Work Required to Transfer an Ion from One Medium to Another

Born Energy, Image Forces, Dipole Potentials, and Hydrophobic Interactions Contribute to the Energy Barriers of Lipid Bilayers

Solvation Energies Are Important Determinants of Channel Accessibility

Surface Potentials Modify the Transmembrane Potential as Well as Local Ion Concentrations

Transport across Energy Barriers

Eyring Rate Theory: Rate Constants Depend on Ψ

Kinetic Approaches

Channels

Single‐Occupancy Channels with Binding Sites Show Saturation Kinetics

Competition, Unidirectional Flux, and the Ussing Flux Ratio

Single‐Occupancy Channels: Voltage Dependence in Symmetric Channels

Single‐Occupancy‐Channel Results Can Be Generalized to N Sites

Multiple Occupancy

Simple Carriers

Net Flux

Unidirectional Flux

Equilibrium at the Boundaries

Rate‐Limiting Steps at the Boundaries

Energy‐driven Simple Carrier Systems

Cotransport

Thermodynamics: Cotransport Can Move Solutes “Uphill”

Kinetic Description

General Net Flux

Unidirectional Fluxes

Kinetics of Simultaneous Binding of mA and nB Resembles Simple Carrier Kinetics When Carrier Concentrations Are Replaced with the Product AmBn

Relations between the Net Flux Equation Parameters and the Michaelis‐Menten—type Parameters for 1:1 Stoichiometry

Countertransport

Thermodynamics

Kinetic Models

Ping‐Pong Model

Sequential Model

Generalization to m‐n Stoichiometry for Simultaneous Binding

Fluctuating Barriers: Channels and Carriers

Channel Transport Properties Depend on the Rate of Transition between the Conformations

Channels with Fluctuating Barriers Do Not Show Michaelis‐Menten Kinetics

Channels with Fluctuating Barriers Can Show Carrier Kinetics