Summary OCR A/AS-Level Biology 2.5.2-3 Diffusion Across Membranes

Diffusion Across
Membranes
Simple Diffusion
 Follows concentration gradient
 High to low concentration
 Small, lipid-soluble molecules
 Integral (not metabolic) energy
 Through phospholipid bilayer

Facilitated Diffusion
 Follows concentration gradient
 High to low concentration
 Small, water-soluble molecules
 Integral (not metabolic) energy
 Through channel proteins

Active Transport
 Against concentration gradient
 Low to high concentration
 Insoluble molecules
 Metabolic (not integral) energy
 Through carrier proteins

Osmosis
 Follows water-potential gradient
 High to low water-potential (high to low water concentration)
 Low to high solute concentration
 Water molecules only
 Integral (not metabolic) energy