Biology › Section C · Cell structure and membranes › Topic C.7
Passive transport
Membranes are picky, but the cell does not pay for everything that crosses them. Passive transport is the free ride: molecules drift down their gradients — from crowded to uncrowded — and the membrane either lets them through or provides a protein to help, spending no ATP either way.
In this topic
- The free ride downhill
- Diffusion: the default method
- Worked example: tracing oxygen's downhill run
- Facilitated transport: proteins as doorkeepers
- Osmosis: water runs its own gradient
- Tonicity: what the solution does to the cell
- Worked example: predicting four cells in four solutions
- Tonicity in living systems
- Common mistakes to avoid
- Key takeaways
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