BRAINCAKEOsmosis
Osmosis is the net movement of water molecules across a partially permeable membrane, from a region of higher water potential to a region of lower water potential.
It is a special case of diffusion, involving only water. It is passive: the energy comes from the kinetic energy of the water molecules, and no ATP is used.
Why does water move? A partially permeable membrane lets water molecules through but not larger solute molecules such as sucrose. On the side with more solute, many water molecules are held around the solute particles, so fewer are free to move. More free water molecules hit the membrane from the dilute side, so there is a net movement of water towards the more concentrated side.
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Key terms in this lesson
- water potential
- A measure of the tendency of water molecules to move out of a solution; water moves from a higher to a lower (more negative) water potential.
More in Cell Transport Mechanisms
- Passive and Active Transport
- Diffusion
- Facilitated Diffusion
- Water Potential ψ
- Modelling Osmosis
- Osmotic Concentrations
- Osmosis in Animal Cells
- Water Potential in Plant Cells
All 18 lessons in Cell Transport Mechanisms · All Edexcel AS-level Biology topics