CIE A Level Biology (9700) 2019-2021

Revision Notes

4.2.10 Osmosis in Plant Cells

Osmosis: Plant Cells

  • 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
  • If a plant cell is placed in pure water or a dilute solution, water will enter the plant cell through its partially permeable cell surface membrane by osmosis, as the pure water or dilute solution has a higher water potential than the plant cell
  • As water enters the vacuole of the plant cell, the volume of the plant cell increases
  • The expanding protoplast (living part of the cell inside the cell wall) pushes against the cell wall and pressure builds up inside the cell – the inelastic cell wall prevents the cell from bursting
  • The pressure created by the cell wall also stops too much water entering and this also helps to prevent the cell from bursting
  • When a plant cell is fully inflated with water and has become rigid and firm, it is described as fully turgid
  • This turgidity is important for plants as the effect of all the cells in a plant being firm is to provide support and strength for the plant – making the plant stand upright with its leaves held out to catch sunlight
  • If plants do not receive enough water the cells cannot remain rigid and firm (turgid) and the plant wilts

Osmosis of water into plant cell, downloadable AS & A Level Biology revision notes

Osmosis of water into a plant cell

  • If a plant cell is placed in a solution with a lower water potential than the plant cell (such as a concentrated sucrose solution), water will leave the plant cell through its partially permeable cell surface membrane by osmosis
  • As water leaves the vacuole of the plant cell, the volume of the plant cell decreases
  • The protoplast gradually shrinks and no longer exerts pressure on the cell wall
  • As the protoplast continues to shrink, it begins to pull away from the cell wall
  • This process is known as plasmolysis – the plant cell is plasmolysed

Plasmolysis of plant cell, downloadable AS & A Level Biology revision notes

Plasmolysis of a plant cell that has been placed in a solution with a lower water potential than the cell itself

Exam Tip

Remember – plant cell membranes are composed of a phospholipid bilayer and are partially permeable (only certain molecules can cross), whereas plant cell walls are made of cellulose and are freely permeable. Thus, in a plasmolysed cell, the external solution will be exerting pressure on the protoplast, that is, there is not an empty space between the cell wall and protoplast.

Pure water has a water potential of 0 kPa, so all other solutions will have negative water potentials.

Author:

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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