Passive transport: diffusion and osmosis
No ATP required.
Simple diffusion. Net movement of particles from a region of higher concentration to lower concentration, down their concentration gradient, until equilibrium. Driven by random kinetic motion.
Crosses the bilayer directly: small non-polar molecules — O₂, CO₂, urea, lipid-soluble vitamins.
Facilitated diffusion. Passive (no ATP), but mediated by membrane proteins:
- Channel proteins form a hydrophilic pore (e.g. aquaporins for water; ion channels).
- Carrier proteins bind the molecule and change shape (e.g. GLUT transporters for glucose).
Used for polar molecules and ions too large or charged to cross the bilayer.
Osmosis. Net movement of water across a partially-permeable membrane from a region of higher water potential () to lower water potential.
- Hypotonic solution: lower solute, higher water potential than cell → water moves IN.
- Isotonic solution: equal water potential → no NET movement.
- Hypertonic solution: higher solute, lower water potential than cell → water moves OUT.
In plant cells with a wall: turgid (full), flaccid (limp), plasmolysed (membrane pulls from wall in hypertonic). Animal cells lyse in hypotonic (no wall to resist).
- Diffusion: passive, down gradient.
- Facilitated: through proteins; still passive.
- Osmosis: water moves from high ψ to low ψ.
- Tonicity decides cell fate: lysis vs crenation vs normal.