08_Membrane Transport

Membrane Transport Overview

  • Presented by Dr. Steven King

  • Affiliation: Integrative Biosciences, School of Dentistry, Oregon Health & Science University

Learning Objectives for Sessions 9 and 10

  • Define diffusion and the factors affecting net flux.

  • Compare diffusion through simple membranes against lipid bilayers and protein channels.

  • Understand ion movement regulation through channels, including factors affecting channel conformation and gating.

  • Define mediated-transport systems and transporters, including facilitated diffusion and active transport.

  • Describe characteristics of transport pathways across membranes.

Diffusion

Definition

  • Movement from one location to another due to random thermal motion.

  • Initial concentration in one region decreases as it moves to a region of lower concentration until uniformly distributed.

Magnitude and Direction

  • Flux: Amount of material crossing a surface over time.

  • Net Flux: Difference between two one-way fluxes; zero indicates diffusion equilibrium.

  • Flux always moves from higher to lower concentration.

Variables Affecting Net Flux

  1. Temperature: Higher temperatures increase movement speed.

  2. Molecular Mass: Greater mass decreases movement speed.

  3. Surface Area: Larger areas enhance diffusion.

  4. Medium: Faster movement in air than in water due to fewer collisions.

  • Diffusion time increases with the square of distance.

Diffusion Through Membranes

  • Net flux across a membrane stops when intracellular concentration equals extracellular concentration.

  • Equation: J = PiA(Co – Ci); J is net flux, Pi is permeability, A is surface area, Co and Ci are concentrations.

Lipid Bilayer Diffusion

  • Major limitation is the hydrophobic interior of the lipid bilayer; polar molecules diffuse slowly.

  • Nonpolar molecules dissolve easily in membrane's fatty acid chains, leading to higher permeability.

Ion Channel Diffusion

  • Ion channels made of polypeptide subunits; characteristics include ion selectivity based on channel diameter, surfaces, and hydration.

Electrical Forces on Ion Movement

Membrane Potential

  • Defined as the separation of electrical charges across plasma membranes.

  • Positive ions are attracted into the cell; negative ions are repelled.

Electrochemical Gradient

  • Ion fluxes depend on concentration and electrical differences across the membrane.

Ion Channel Regulation

  • Channels can be open or closed; rapid changes occur via channel gating.

Factors Affecting Channel Gating

  • Specific molecules can change channel shape (ligand-gated), membrane potential affects channel shape (voltage-gated), and physical deformation may alter conformation (mechanical-gated).

Mediated-Transport Systems

  • Integral membrane proteins called transporters mediate the transport of polar molecules and ions.

  • Substances bind to transporters causing conformational changes for transport across membranes.

Transporter Characteristics

  • Both transporters and ion channels involve membrane proteins and show specificity.

  • Ion channels facilitate faster ion movement than transporters.

  • Transporters require shape change for each ion/molecule, while ion channels allow continuous flow without shape change.

Factors Influencing Solute Flux

  • Flux depends on transporter saturation, number of transporters, and rates of conformational changes.

  • Diffusion flux increases with solute concentration and can reach a limit due to fixed number of channels.

Types of Mediated Transport

  1. Facilitated Diffusion: Moves solutes downhill concentration gradient without energy.

  2. Active Transport: Moves solutes uphill against gradient, using energy.

Facilitated Diffusion Example

  • Glucose is rapidly metabolized in cells, maintaining low intracellular levels for continuous uptake.

Active Transport Mechanisms

  • Primary Active Transport: Directly uses ATP; e.g., Na+/K+-ATPase pump.

  • Secondary Active Transport: Uses the electrochemical gradient created by primary transporters.

Primary Active Transport: Na+/K+-ATPase Model

Mechanism Steps

  1. Binding: Sodium binds, activating ATPase activity.

  2. Phosphorylation: Alters transporter conformation, releasing sodium to extracellular fluid.

  3. K+ Binding: K+ can bind due to the new conformation.

  4. Dephosphorylation: Returns transporter to original state, allowing K+ release.

Functionality of Na+/K+-ATPase

  • Establishes low intracellular sodium and high potassium; for each ATP hydrolyzed, moves 3 Na+ out and 2 K+ in, transferring positive charge outside.

Other Active Transporters

  • Ca2+-ATPase: Transports calcium from cytosol to extracellular fluid.

  • H+-ATPase: Moves protons out of cells maintaining pH.

  • H+/K+-ATPase: Pumps protons out, potassium in, particularly in stomach and kidney tissues.

Secondary Active Transport

  • Utilizes electrochemical gradients to transport additional solutes (e.g. glucose).

  • Sodium binding enhances solute binding and induces transporter conformational change, moving solute into the cell.

Cotransport and Countertransport

  • Cotransport: Solutes move in the same direction as sodium.

  • Countertransport: Solutes move in opposite directions.

Distribution of Substances Across Membranes

  • Unequal distribution driven by primary and secondary transporters, influencing ion gradients in living tissues.