Osmosis in Animal & Plant Cells (OCR A Level Biology): Revision Note
Osmosis in Animal & Plant Cells
Osmosis in animal cells
Animal cells can lose and gain water as a result of osmosis
As animal cells do not have a supporting cell wall (unlike plant cells), the results of this loss or gain of water on the cell are severe
Animal cells losing water
If an animal cell is placed in a solution with a lower water potential than the cell (such as a concentrated sucrose solution)
Water will leave the cell through its partially permeable cell surface membrane by osmosis and the cell will shrink and shrivel up
This is crenation (the cell has become crenated), which is usually fatal for the cell
Crenation occurs when the cell is in a hypertonic environment (the solution outside of the cell has a higher solute concentration than the inside of the cell)
Animal cells gaining water
If an animal cell is placed in pure water or a dilute solution, water will enter the cell through its partially permeable cell surface membrane by osmosis, as the pure water or dilute solution has a higher water potential
The cell will continue to gain water by osmosis until the cell membrane is stretched too far and the cell bursts (cytolysis), as it has no cell wall to withstand the increased pressure created
This is fatal for the cell
Lysis occurs when the cell is in a hypotonic environment (the solution outside of the cell has a lower solute concentration than the inside of the cell)
This is why a constant water potential must be maintained inside the bodies of animals
Animal cells in isotonic environments
If an animal cell is in an isotonic environment (the solution outside of the cell has the same solute concentration as the inside of the cell)
The movement of water molecules into and out of the cell occurs at the same rate (no net movement of water) and there is no change to the cells
Effect of osmosis on animal cells
Osmosis in plant cells
Like animal cells, plants cells can also lose and gain water as a result of osmosis
As plant cells have a supporting cell wall, the results of this loss or gain of water on the cell are less severe than in animal cells
Plant cells losing water
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 a plant cell that has been placed in a solution with a lower water potential than the cell itself
Plant cells gaining water
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 from 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 a plant cell
Comparing & Contrasting the Movement of Water by Osmosis in Plant & Animal Cells Table
Examiner Tips and Tricks
Be careful with your scientific terminology – animal cells do not plasmolyse because they do not have a cell wall. In a solution with a lower water potential than the cell itself, animal cells will shrink. Plasmolysis only occurs in plant cells.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 (fully) permeable. Thus, in a plasmolysed cell, the external solution will be exerting pressure on the protoplast (there is not an empty space between the cell wall and protoplast).
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