AQA GCSE Biology

Revision Notes

1.3.4 Osmosis

Principles of Osmosis

Osmosis in Cells, downloadable IGCSE & GCSE Biology revision notes

Osmosis occurs when two solutions are separated by a partially permeable membrane

 

  • The movement of water into and out of cells occurs by osmosis
  • Osmosis is the diffusion of water molecules from a dilute solution to a concentrated solution through a partially permeable membrane
  • Osmosis is a just a special type of diffusion, so water molecules move about randomly just like any other particle that can diffuse
  • Water will always move from a more dilute solution to a more (solute) concentrated solution to even up the concentration of water molecules on each side of the membrane

Effect of solution concentration on diffusion table

Effect of solution concentration on diffusion table 1, downloadable IGCSE & GCSE Biology revision notes

 

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

Always identify which solution is more concentrated, and which is more dilute, to determine the movement of water by osmosis

Exam Tip

Osmosis refers only to the movement of water molecules, so if in an exam you are talking about the movement of water, make sure you mention osmosis as this will often earn you a mark.

The Effect of Osmosis on Plant Tissue

  • The movement of water by osmosis can easily be observed by placing two similar pieces of plant tissue into solutions with differing concentrations

 

Osmosis in Plant Tissue, IGCSE & GCSE Biology revision notes

Potatoes are usually used in osmosis experiments to show how the concentration of a solution affects the movement of water, but radishes can be used too

  • If the plant tissue gains mass:
    • Water must have moved into the plant tissue from the solution surrounding it by osmosis
    • The solution surrounding the tissue is more dilute than the plant tissue (which is more concentrated)
  • If plant tissue loses mass:
    • Water must have moved out of the plant tissue into the solution surrounding it by osmosis
    • The solution surrounding the tissue is more concentrated than the plant tissue (which is more dilute)
  • If there is no overall change in mass:
    • There has been no net movement of water as the concentration in both the plant tissue and the solution surrounding it must be equal
    • Remember that water will still be moving into and out of the plant tissue, but there wouldn’t be any net movement in this case

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
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