Transport in and out of cells
Biology · GCSE · The School
Three ways in, three signatures
DIFFUSION: any particle, down its gradient, no energy. OSMOSIS: water only, through a partially permeable membrane, no energy. ACTIVE TRANSPORT: any particle, AGAINST its gradient, and it costs energy from respiration — which is why cells doing a lot of it, like root hair cells, are packed with mitochondria. Identify which is happening by asking two questions: what is moving, and which way relative to the gradient?
What happens to a cell in water
An ANIMAL cell in pure water takes water in and can burst, because nothing holds it. A PLANT cell in the same water also takes water in, but the cellulose wall resists and the cell becomes turgid — firm, which is how a plant stands up without a skeleton. In concentrated solution both lose water: the animal cell shrivels, and the plant cell plasmolyses as its membrane pulls away from the wall.
Surface area to volume ratio
A small organism can rely on diffusion across its surface. As something gets bigger, volume grows faster than surface area, so the surface can no longer supply the inside — which is why large organisms need lungs, gills and a circulation. Exchange surfaces answer the same problem every time: large area, thin, good blood supply, and a way to keep the gradient steep.
Identical potato cylinders go into sugar solutions of different concentration for an hour, and each is weighed before and after. In WEAK solution the cylinder GAINS mass: water moved in by osmosis, because the potato cells were more concentrated than the solution. In STRONG solution it LOSES mass: water moved out. Somewhere between, one cylinder does not change at all — and that point matters, because at zero change the solution has the same concentration as the inside of the potato cells. That is how the experiment measures something you cannot see directly. Note also why PERCENTAGE change is plotted rather than grams: the cylinders were not all exactly the same starting mass, and percentage removes that difference.