History of Fluid Mosaic Model For more on this history, see: 2/membrane.htm

Fluid Mosaic Model

Movement Across Membranes Passive Transport Simple diffusion (passive diffusion) Facilitated diffusion Transporters Uniporters Cotransporters Channel proteins Active Transport Pumps Group Translocation Electrochemical Gradients

Movement Across Membranes Passive Transport Simple diffusion (passive diffusion) Facilitated diffusion Transporters Uniporters Cotransporters Channel proteins Active Transport Pumps Group Translocation Electrochemical Gradients

Primer on Diffusion Diffusion is movement from area of high concentration to low Diffusion is Passive Transport: no energy is required

Simple Diffusion Across Bilayers Passive diffusion - unaided by transport proteins - no metabolic energy expended - movement is down chemical concentration gradient - diffusion rate is proportional to concentration gradient and hydrophobicity - rate limiting step is movement across hydrophobic portion of membrane - the greater the hydrophobicity of a water-soluble molecule, the faster it diffuses across phospholipid bilayer.

Diffusion Across Membranes

Osmosis: H 2 0 Down Conc. Gradient

Equilibrium

Tonicity: H 2 0 Down Conc. Gradient

Movement Across Membranes Passive Transport Simple diffusion (passive diffusion) Facilitated diffusion Transporters Uniporters Cotransporters Channel proteins Active Transport Pumps Group Translocation Electrochemical Gradients

Protein-Mediated Diffusion

Proteins Involved in Membrane Transport Transport of molecules and ions across the cell membrane is mediated by transport proteins - integral (intrinsic) membrane proteins (transmembrane; contain multiple transmembrane  helices). - Peripheral (extrinsic) membrane proteins (examples: spectrin and actin in RBC; protein Kinase C) It is Integral Membrane Proteins that are involved in Membrane Transport

Membrane Transport Proteins All are integral membrane proteins All exhibit high degree of specificity for substance transported Three major classes: 1. ATP powered pumps- ATPases that use energy of ATP hydrolysis to move ions or small molecules across membrane against a chemical concentration gradient or electrical potential – active transport 2. Channel proteins – form water filled pores across the bilayer through which ions move down their concentration or electrical potential gradients at rapid rates 3. Transporters – bind only specific # substrate molecule at a time (binding of substrate induces conformational change) -if transported molecule crosses membrane passively then “downhill” process called passive transport or facilitated diffusion

Movement Across Membranes Passive Transport Simple diffusion (passive diffusion) Facilitated diffusion Transporters Uniporters Cotransporters Channel proteins Active Transport Pumps Group Translocation Electrochemical Gradients

Active Transport Active transport requires energy to move substances from areas of lower concentration to areas of higher concentration

Sodium-Potassium Pump

Movement Across Membranes Endocytosis Phagocytosis Pinocytosis Receptor mediated Exocytosis These are mechanisms that involve movement into and out of the lumen of the endomembrane system Not movement directly across membranes Movement Across Membranes

Endocytosis Note that the substance enters the Endomembrane System but not the Cytoplasm

Exocytosis

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