Transport Across Membranes: Overcoming the Permeability Barrier Chapter 8: Transport Across Membranes: Overcoming the Permeability Barrier

Movement Across a Membrane With concentration gradient Simple Diffusion Facilitated Diffusion – transport proteins required Against concentration gradient Active transport – transport proteins required

Simple Diffusion

Osmosis

Limits of Simple Diffusion Size Polarity Charge Rate is affected by concentration gradient

Rate of Diffusion

Facilitated Diffusion Large molecules need help crossing the membrane Channel proteins Carrier proteins

Channel Proteins Form hydrophilic channels through the membrane No major conformational change Examples: Ion Channels and Aquaporins (AQPs)

Ion Channels Very selective Mechanism not well understood Most are gated Voltage Ligand Mechanosensitive

Aquaporins Help water rapidly cross some membranes Just large enough for water molecules to pass through one at a time

Facilitated Diffusion (cont.) Large molecules need help crossing the membrane Channel proteins Carrier proteins

Carrier Proteins Bind solutes on one side of membrane Undergo conformational change Deposit solutes on other side of membrane Alternating Conformation Model Analogous to enzymes (but no catalytic function!) Can carry one or two types of solutes Examples: The Glucose Transporter and The Erythrocyte Anion Exchange Protein

One vs. Two Types of Solutes

The Glucose Transporter

The Erythrocyte Anion Exchange Protein

Active Transport Sometimes molecules need to move up their concentration gradient Requires energy Has directionality Three major functions Cellular uptake of nutrients Allow products to exit cell Maintain non-equilibrium concentrations of ions Direct vs. Indirect

Direct Active Transport AKA primary active transport Transport is tied directly to an exergonic reaction Usually involves hydrolysis of ATP (ATPases)

Indirect Active Transport AKA secondary active transport Simultaneous transport of two solutes Energy to move molecule “A” up its concentration gradient tied to moving molecule “B” down its concentration gradient

Direct vs. Indirect Active Transport

Direct Active Transport (cont) 4 types of ATPases 1. P type = phosphorylation (e.g. Na/K pump) 2. V-type = vacuole 3. F-type = factor (AKA ATP synthases) 4. ABC-type = ATP-binding cassette

Na+/K+ Pump Structure

Indirect Active Transport (cont) Simultaneous transport of two solutes Energy to move molecule “A” up its concentration gradient tied to moving molecule “B” down its concentration gradient Usually coupled to an ion Example: Na/Glucose symporter