Movement of substances into & out of cells

Every cell is wrapped in a partially permeable membrane, and life depends on getting the right substances across it. In this lesson we meet the six ways things move in and out of cells: diffusion, facilitated diffusion, osmosis, active transport, endocytosis and exocytosis. For each one we ask three questions — which way along the gradient, is a transport protein needed, and is ATP used? — and pin them side by side in a comparison table. We then work out, with real numbers on the board, why big cells struggle: as a cube grows, its surface-area-to-volume ratio falls from 6:1 to 3:1 to 2:1. Next we explain water potential (psi) — pure water is zero, solutes make it negative, and water always moves from a higher, less negative potential to a lower, more negative one. Finally we see what this does to real cells: plant cells become turgid, flaccid or plasmolysed, while wall-less animal cells burst by haemolysis or shrink by crenation — and we use potato cylinders in sucrose to estimate a tissue's water potential.