simple diffusion
/ SIM-pul dih-FYOO-zhun /
Drop a teaspoon of ink into a glass of still water and walk away. With no stirring at all, the ink slowly spreads until the whole glass is faintly colored. That natural spreading-out from where there is a lot to where there is little is diffusion, and it happens simply because molecules are always jiggling and bumping randomly. There is no pump and no plan — it is just the unavoidable result of countless random nudges.
Simple diffusion is when a substance crosses a membrane this same way: straight through the fatty bilayer, with no protein helper, driven only by the difference in concentration between the two sides. Molecules wander randomly, but because there are more of them on the crowded side, more happen to wander toward the empty side than the other way, so the net flow is always downhill, from high concentration to low. This continues until both sides are equal, a balance called equilibrium.
Only certain molecules can do this, because they have to dissolve into and squeeze through the oily core. Small, uncharged, fat-friendly molecules — oxygen, carbon dioxide, and small fatty substances — pass freely. This is how the oxygen in your lungs reaches your blood and how carbon dioxide leaves it: no energy spent, no protein needed, just the quiet downhill drift of diffusion working in your body every second.
In your lungs, the air sacs are crowded with oxygen and your blood arriving there is nearly empty of it, so oxygen diffuses straight across the thin membranes into the blood — no energy required.
Oxygen flows downhill, from crowded air into empty blood, all by itself.
Diffusion never stops once balanced; molecules keep crossing both ways, but in equal numbers, so there is no further net change — equilibrium means even, not frozen.