Concept preview

What you will be able to do

1. 01Describe the fluid mosaic model of the cell membrane.
2. 02Distinguish passive transport (diffusion, osmosis, facilitated diffusion) from active transport.
3. 03Predict the direction of water movement and the fate of a cell in hypertonic, isotonic and hypotonic solutions.
4. 04Explain how and why cells use energy (ATP) for active transport.

Every cell is wrapped in a cell membrane — a boundary only a few nanometres thick, yet it decides the fate of everything that tries to enter or leave. It is selectively permeable: it lets some substances through freely, controls others tightly, and blocks the rest. This control is what lets a cell keep its inside different from its surroundings — concentrating the molecules it needs and expelling wastes — which is the physical basis of homeostasis at the cellular level. Understanding how things cross this membrane explains everything from how your nerves fire to why a wilted plant revives in water.