Diffusion

Understand the idea

Diffusion is the movement of molecules from higher concentration to lower concentration until the mix is even. It happens in solids, liquids, and gases. Unlike osmosis, it does not require a membrane and both solute and solvent can move, in all directions. You cannot stop ordinary diffusion by squeezing one side.

Simple diffusion crosses a membrane or space without a helper protein, as when oxygen enters cytoplasm. Facilitated diffusion is still passive, but a carrier protein helps a substance cross from high to low concentration. Temperature, gradient steepness, distance, particle size, and membrane type change the rate.

Diffusion lets carbon dioxide leave cells, moves ions, brings soil water and minerals toward root hairs, and helps nerve signals. In plants it also moves gases through stomata and spreads flower scent. Tea in hot water, perfume in air, and incense smoke are everyday pictures of the same rule.

What to remember

• Diffusion: high concentration → low, any state of matter.
• Simple = no protein helper; facilitated = carrier, still passive.
• Osmosis needs a membrane and concerns water; diffusion is broader.

Recap

Diffusion equalises concentration. Add a membrane and water only, and you have osmosis. Add a carrier protein but no energy cost, and you have facilitated diffusion.