Movement of Substances — Study Notes

Drop a teabag into a mug of hot water and don't stir. Within a minute the colour has spread through the whole mug on its own. Nobody pushed it there — and that same "spreading without being pushed" trick is exactly how oxygen gets into your blood, how minerals get into a plant root, and how food particles cross your gut wall.

Diffusion: Movement That Costs No Energy

Particles — whether in a gas or dissolved in liquid — are never sitting still; they are constantly jostling about in random directions. Diffusion is what happens when far more of those particles happen to start out packed tightly in one place: purely by the numbers, more of them wander away from the crowd than wander back into it, so overall there is a steady drift away from the packed area until particles are equally spread everywhere. That drift keeps going for as long as a concentration gradient — a difference in how crowded particles are between two places — still exists. Once the spread is even, random jostling continues, but the outward and inward drifts now balance, leaving nothing left to shift.

Nothing forces this along — the particles were already moving before diffusion began — so it costs an organism zero energy. That single fact separates diffusion from the pumped transport method covered later in this topic, which does cost the cell energy.

Worked Example — Why an Amoeba Needs No Lungs

1. An amoeba is a single tiny cell living in pond water, with no lungs, gills or blood vessels of any kind.
2. Respiration keeps using up oxygen inside the cell, so its internal level stays lower than in the pond water — that gap is the gradient driving oxygen inward across its outer membrane.
3. Respiration also keeps producing carbon dioxide, raising the level inside above the level outside, so carbon dioxide diffuses out along its own, opposite gradient.
4. Being so small, the amoeba's outer membrane sits close to every part of it — no point is far from that surface, so both gases arrive fast enough for respiration, with no pump needed.

Scale the amoeba up to a much bigger animal and the trick fails — the journey from surface to interior grows too long for diffusion alone, which is exactly why bigger organisms evolved lungs, gills and blood to finish the trip.

Diffusion is only one of three delivery methods cells rely on — osmosis (its water-only cousin) and active transport (the energy-costing one) work alongside it. The full lesson below walks through both, with an audio walkthrough, further worked examples and a practice worksheet.