Edexcel IGCSE Biology

Revision Notes

2.4.3 Osmosis

Osmosis Theory

  • 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 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
  • 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 and the partially permeable membrane, IGCSE & GCSE Biology revision notes

Osmosis and the partially permeable membrane

  • It can get a little confusing to talk about the ‘concentration of water’ when we also talk about solutions being ‘concentrated’ (having a lot of solute in them) so instead, we can say that 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

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 best explanations to do with osmosis will refer to water potential, so if you are aiming for a 7, 8 or 9 you will need to understand the concept and use it in your explanations.

Osmosis in Animal Cells

  • Animal cells lose and gain water as a result of osmosis
  • As animal cells do not have a supporting cell wall, the results of osmosis can be severe
  • If an animal cell is placed into a strong sugar solution (with a lower water potential than the cell), it will lose water by osmosis and become crenated (shrivelled up)
  • If an animal cell is placed into distilled water (with a higher water potential than the cell), it will gain water by osmosis as it has no cell wall to create turgor pressure
  • It will continue to gain water until the cell membrane is stretched too far and it bursts

Effect of osmosis on animal cells, IGCSE & GCSE Biology revision notes

Effect of osmosis on animal cells

Osmosis and the human body

  • It is important that osmosis is carefully controlled in organisms to avoid damage to cells through lysis
  • The human body is adapted to maintain the optimum osmotic balance using processes such as sweating or increasing and decreasing urine concentration
    • This is all part of osmoregulation

Osmosis in Plant Cells

  • Plant cells lose or gain water as a result of osmosis
  • As plant cells have a supporting cell wall, they are protected from cell lysis
  • If a plant cell is placed into a strong sugar solution (with a lower water potential than the cell), it will lose water by osmosis
    • The vacuole gets smaller and the cell membrane shrivels away from the cell wall
    • It becomes flaccid or plasmolysed (shrivelled up)
  • If a plant cell is placed into distilled water (with a higher water potential than the cell), it will gain water by osmosis
    • The vacuole gets bigger, pushing the cell membrane against the cell wall
    • The plant cell is described as being turgid or as containing a high turgor pressure (the pressure of the cytoplasm pushing against the cell wall)

Osmosis in plant cells, downloadable AS & A Level Biology revision notes
The effect of osmosis on plant cells

Osmosis and plant cells

  • Water entering the cell by osmosis makes the cell rigid and firm
  • This 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

Author: Ruth

Ruth graduated from Sheffield University with a degree in Biology and went on to teach Science in London whilst also completing an MA in innovation in Education. With 10 years of teaching experience across the 3 key science disciplines, Ruth decided to set up a tutoring business to support students in her local area. Ruth has worked with several exam boards and loves to use her experience to produce educational materials which make the mark schemes accessible to all students.
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