AQA A Level Biology

Revision Notes

2.4.5 Osmosis

The Process of Osmosis

  • All cells are surrounded by a cell membrane which is partially permeable
  • Water can move in and out of cells by osmosis
  • Osmosis is the diffusion of water molecules from a dilute solution (high concentration of water) to a more concentrated solution (low concentration of water) across a partially permeable membrane
  • In doing this, water is moving down its concentration gradient
  • The cell membrane is partially permeable which means it allows small molecules (like water) through but not larger molecules (like solute molecules)

Osmosis & the partially permeable membrane, downloadable AS & A Level Biology revision notes

Osmosis and the partially permeable membrane

  • Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane
  • Water potential describes the tendency of water to move out of a solution. This term is used to avoid confusion between water concentration and concentration of a solution
    • A dilute solution has a high water potential (the right-hand side of the diagram below) and a concentrated solution has a low water potential (the left-hand side of the diagram below)

How-osmosis-works, IGCSE & GCSE Chemistry revision notes

How osmosis works

  • The water potential of pure water (without any solutes) at atmospheric pressure is 0kPa, therefore any solution that has solutes will have a water potential lower than 0kPa (it will be a negative value)

Exam Tip

Water can pass through the phospholipid bilayer because water molecules are small molecules that can pass between phospholipids in the cell membrane. Although water molecules are polar, they can still pass through the bilayer because of their small size.

When interpreting questions on water potential, remember – the more negative the water potential, the lower the water potential (the further it is away from pure water which has a water potential of 0 kPa).


Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.

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