What is Osmosis?
The movement of water molecules through a selectively permeable membrane from a region of higher water concentration (lower solute concentration) to a region of lower water concentration (higher solute concentration).
Osmosis is a special type of diffusion — but it only involves water molecules. It is a passive process, meaning it requires no energy from the cell.
The key ideas to remember:
- Only water molecules move during osmosis
- Movement happens across a selectively permeable membrane
- Water moves from dilute → concentrated solution (in terms of solutes)
- The process continues until equilibrium is reached (equal concentrations on both sides)
Think of osmosis like a crowd at a concert. If one side of a barrier has lots of people (solute) and the other side is empty, water molecules are like tiny helpers trying to "even things out" — they squeeze through the gaps in the barrier to the crowded side to balance everything up.
Selectively Permeable Membranes
A membrane that allows some molecules to pass through but not others. It allows small water molecules to pass freely, but blocks larger solute molecules such as glucose or proteins.
Cell membranes are selectively permeable. This property is essential for controlling what enters and exits the cell.
Why can water pass through but not solutes?
- Water molecules () are very small and can fit through tiny pores in the membrane
- Solute molecules (e.g. glucose, salt ions, proteins) are either too large or interact with the membrane in ways that prevent free passage
You may also see the terms semi-permeable membrane or partially permeable membrane — these all mean the same thing in the context of osmosis.
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