CP 6 Flashcards
IV and dv
IV: Concentration of Sucrose Solution
DV: % Plasmolysed Cells
What is plasmolysis?
Where the protoplasm of the plant cell
begins to shrink away from the cell wall.
PROCEDURE
● Take five thin sections of the red part of the red onion. 1cm2 is a suitable size to use. (1 cell thick, so easy to view under microscope.
● Label five watch glasses and add 5 different concentrations of sucrose solution.
● Add a piece of tissue to each glass and leave for 20 mins
● Put a drop of each solution on the slide Remove each tissue piece using forceps and float each the matching slice.
● Cover each section with a coverslip and observe under the microscope.
● Count 25 cells and record how many of them show plasmolysis.
What is incipient plasmolysis?
Where the water potential of the cytoplasm is the
same as the solution.
The point at which plasmolysis just begins to
occur.
When 50% of cells in a tissue are plasmolysed.
Why are the plant cells left in the mineral salt
solution for 20 minutes before observation?
To allow time for osmosis to reach equilibrium
EVALUATION POINTS
● Difficult to obtain a single layer of red stained cells (too thick in places)
● Difficult to accurately measure just 25 cells by looking down the microscope.
● The pigment varies between different parts of the onion.
MATHS
Ψ = Π + P
P = 0 therefore Ψ = Π when 50% of the cells have been plasmolysed (incipient plasmolysis) so we can find the water potential from the sucrose concentration
BIOLOGICAL EXPLANATION
If a plant cell is placed in a solution with a lower water potential, it will lose water.
The living part of the cell or protoplast will shrink and pull away from the cell wall.
At this point, the pressure potential is zero and so the water potential of the cell is equal to its solute potential. This process is called plasmolysis and the cell is said to be plasmolysed.
The point at which the protoplast is just about to pull away from the cell wall is called incipient plasmolysis.
Why may the osmotic potential not be this calculated value?
no evidence of replicates, so data may not be valid
water potential = turgor pressure + osmotic potential and at 50% plasmolysis, there is no turgor pressure so
water potential = osmotic potential of cells
data for 1200 s suggest that osmotic potential of cell is equivalent to 0.3 M sucrose and more cells are plasmolysed at 1200 s
graphs are different suggesting that water is still moving in and out of the cells, data for 900 s suggests that the osmotic potential of cells is equivalent to less than 0.3 M
cannot be certain that osmotic potential is equivalent to 0.3 M sucrose unless cells are left in sucrose longer than 1200 s
the estimate is changing as time goes by so the cells would need to be left in solution until there is no further increase in the number of plasmolysed cells
