CIE IGCSE Biology

Revision Notes

3.2 Osmosis

Osmosis Theory: Basics

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

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Osmosis Theory

  • 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
  • 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 notesHow osmosis works

Exam Tip

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 Experiments: Basics

  • The most common osmosis practical involves cutting cylinders of potato and placing them into distilled water and sucrose solutions of increasing concentration
  • The potato cylinders are weighed before placing into the solutions
  • They are left in the solutions for 20 – 30 minutes and then removed, dried to remove excess liquid and reweighed
  • The potato cylinder in the distilled water will have increased its mass the most
  • The potato cylinder in the strongest sucrose concentration will have decreased its mass the most
  • If there is a potato cylinder that has not increased or decreased in mass, it means there was no overall movement of water into or out of the potato cells
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Osmosis Experiments

  • The most common osmosis practical involves cutting cylinders of potato and placing them into distilled water and sucrose solutions of increasing concentration
  • The potato cylinders are weighed before placing into the solutions
  • They are left in the solutions for 20 – 30 minutes and then removed, dried to remove excess liquid and reweighed
  • The potato cylinder in the distilled water will have increased its mass the most as there is a greater concentration gradient in this tube between the distilled water (high water potential) and the potato cells (lower water potential)
  • This means more water molecules will move into the potato cells by osmosis, pushing the cell membrane against the cell wall and so increasing the turgor pressure in the cells which makes them turgid – the potato cylinders will feel hard

 

A turgid plant cell, IGCSE & GCSE Biology revision notesA turgid plant cell

 

  • The potato cylinder in the strongest sucrose concentration will have decreased its mass the most as there is a greater concentration gradient in this tube between the potato cells (higher water potential) and the sucrose solution (lower water potential)
  • This means more water molecules will move out of the potato cells by osmosis, making them flaccid and decreasing the mass of the cylinder – the potato cylinders will feel floppy
  • If looked at underneath the microscope, cells from this potato cylinder might be plasmolysed, meaning the cell membrane has pulled away from the cell wall

 

A plasmolysed plant cell, IGCSE & GCSE Biology revision notesA plasmolysed plant cell

 

  • If there is a potato cylinder that has not increased or decreased in mass, it means there was no overall net movement of water into or out of the potato cells
  • This is because the solution that cylinder was in was the same concentration as the solution found in the cytoplasm of the potato cells, so there was no concentration gradient

Exam Tip

Questions involving osmosis experiments are common and you should be able to use your knowledge of these processes to explain the results.

Don’t worry if it is an experiment you haven’t done – simply figure out where the higher concentration of water molecules is – this is the solution with the higher water potential – and explain which way the molecules move due to the differences in water potential.

Importance of Osmosis in Tissues

  • When water moves into a plant cell, the vacuole gets bigger, pushing the cell membrane against the cell wall
  • 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
  • The pressure created by the cell wall stops too much water entering and prevents the cell from bursting
  • If plants do not receive enough water the cells cannot remain rigid and firm (turgid) and the plant wilts
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Osmosis in Plant Tissues

  • Plant cells that are turgid are full of water and contain a high turgor pressure (the pressure of the cytoplasm pushing against the cell wall)
  • This pressure prevents any more water entering the cell by osmosis, even if it is in a solution that has a higher water potential than inside the cytoplasm of the cells
  • This prevents the plant cells from taking in too much water and bursting
  • Plant roots are surrounded by soil water and the cytoplasm of root cells has a lower water potential than the soil water
  • This means water will move across the cell membrane of root hair cells into the root by osmosis
  • The water moves across the root from cell to cell by osmosis until it reaches the xylem
    Once they enter the xylem they are transported away from the root by the transpiration stream, helping to maintain a concentration gradient between the root cells and the xylem vessels
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Osmosis in Animal Tissues

  • Animal cells also lose and gain water as a result of osmosis
  • As animal cells do not have a supporting cell wall, the results on the cell are more 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 and, as it has no cell wall to create turgor pressure, will continue to do so until the cell membrane is stretched too far and it bursts

 

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

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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