Key facts & rules

• Diffusion: net movement of particles from HIGH to LOW concentration — no energy needed (passive).
• Osmosis: net movement of WATER molecules from high water potential (dilute solution) to low water potential (concentrated solution) through a selectively permeable membrane — passive.
• Active transport: movement from LOW to HIGH concentration — requires ENERGY (ATP) and carrier proteins.
• Factors affecting diffusion rate: concentration gradient, surface area, temperature, distance, molecule size.
• Turgid cell: full of water, firm (plant cells in dilute solution).
• Plasmolysed cell: loses water, membrane pulls away from cell wall (plant cells in concentrated solution).
• Crenated/shrunken: animal cells in concentrated solution (they have no cell wall).