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How does a surrounding sucrose solution affect the shape of onion cells?
Equipment
1. 2 microscope slides
2. 2 cover slips
3. Red onion
4. Microscope
5. 2 pipettes
6. Tweezers
7. White tile
8. Distilled water
9. Sucrose solution
Method
1. Firstly, using the tweezers, peel a small layer, one cell thick, from the
onionskin and place onto the microscope slide. This step should be
repeated twice. This procedure should be done on a white tile.
2. Using a pipette add 3 drops of distilled water to one of the slides.
3. Again using a pipette add 3 drops of sucrose solution to the other slide.
4. The cover slip may need to be briefly left to dry in order to avoid water/
sucrose solution bubbles, which may obstruct the view of cells under the
microscope.
5. Add a cover slip to each of the microscope slides and observe the onion
cells under the microscope
Results
In Distilled water
In sucrose solution
Protoplasm pulls away from cell
wall
Conclusion
The results of the experiment show that the onion cells become plamolysed
when in sucrose solution however remain turgid when in distilled water. This
because when in distilled water the water potential outside the cell is equal to or
higher than the water potential inside the cell, therefore no water moves out of
the cell. Moreover because the plant is already turgid, the potential pressure of
the cell wall against the protoplasm means that no more water can move into the
cell via osmosis.
However when in sucrose solution the water potential is higher inside than
outside of the cell, therefore due to osmosis water moves down the potential
gradient (from high to low) and out of the cell causing a loss of turgidity, through
the gated protein channels . As a result of this the protoplasm shrinks causing it
to pull away from the rigid cell wall, which doesn’t change shape, as seen in the
pictures.