Practical 6 - Determination of water potential by measuring changes in mass or length Flashcards
What are the different ‘solutions’ that have different water potentials in this experiment?
The cytoplasm and the cell sap of a cell
What separates cytoplasm and cell sap within in a cell?
A selectively permeable membrane
Whats the selectively permeable membrane between cytoplasm and cell sap?
The cell membrane
What is the cell membrane to the cytoplasm and the cell sap within a cell?
The selectively permeable membrane
Within a cell, where will water move through a selectively permeable membrane (between what?)?
Into the solution with the lowest water potential
(Cell sap or cytoplasm)
Which substance do we use which consists of different concentration levels?
Sodium chlorie
What do we use as our tissue sample?
A vegetable - eg potato
Why do we use potato in this experiment?
Contains millions of cells which would have an increase in mass if it gained water by osmosis
+Cells will stretch a small amount = increased length
How many cylinders of tissue do we chop in this experiment + describe these?
15, of equal length
What do we use to remove any periderm from the potato?
A scalpel
Periderm
Skin
Word for the skin on a potato
Periderm
Why do we have to remove the periderm (skin) off of the potato?
Its Suberin makes it waterproof and would prevent osmosis
What may prevent osmosis in the potato?
The periderm (skin), as its Suberin makes it waterproof
How much of each solution do we place in a test tube?
30cm^3
What do we need to do to the potato samples before placing them within the test tubes?
Either…
Measure the length to the nearest mm
Measure the mass to the nearest 0.01g
What do we measure the mass to the nearest?
0.01g
What do we measure the length to the nearest?
mm
How long do we leave the potato samples in the solutions?
At least 45 minutes or overnight at 4 degrees Celsius
At what temperature would we leave the potato samples in their solutions if let overnight?
4 degrees Celsius
What do we do to the potato samples after removing them from their solutions?
Gently blot them and re-measure/re-weigh and record in a table
What do we plot on the graph?
Mean percentage change against concentration of the solution
How do we figure out the solute potential of the tissue?
Using a standard table
Convert from where the concentration of NaCl solution causes no change in mass (hits the x-axis)
Which point on the graph shows where the concentration of NaCl solution causes no change in mass?
Where the line hits the x-axis
What would be the results of this experiment and why?
Potato in lower concentrations of salt:
The water potential of the outside of the potato is higher than the inside of the cell tissue, therefore water moves into the cell via osmosis, causing it to increase in mass/size
What is the change in mass calculated as?
A % change
Why is the change in mass calculated as a percentage change?
As the potato pieces are at different lengths and masses to start - we need to be able to make a comparison
How can we ensure that the we can make comparisons in the changes in mass of the potato pieces?
Calculate it as a % change
% change of mass formula
Change in mass ( end-start )
—————————————
Start mass or length
What do we need to remember to write with the % change in mass or length if it’s LOST mass or length?
Negative value
When would the mass or length of the potato sample decrease?
In high concentrations of NaCl, as the water potential of the outside of the cell is lower, so water diffuses out of the cell via osmosis
Where does the line cross the x-axis on the graph?
Where there’s no change in mass or length
What goes along the x and y axis on the graph?
x - concentration of bathing solution
y - % change
What’s actually occurring when there’s no change in mass or length?
There’s no net movement of water in or out of the potato tissue, as the water potentials of the tissue + bathing solution are equal
What are equal when there’s no net movement of water in or out of the potato tissue?
The water potentials of the tissue and the bathing solution
What else is equal when the water potentials of the tissue and bathing solution are equal?
The solute and water potentials
When are the solute and water potentials equal?
When the water potentials of the tissue and bathing solution are equal
What are the cells in when the water potentials of the solute and water are equal?
Incipient plasmolysis
When are cells in incipient plasmolysis?
When the solute and water potentials are equal
Whats the pressure potential when the cell is in incipient plasmolysis?
0kPa
When is the cells pressure potential 0kPa?
When cells are in incipient plasmolysis
How would this experiment be different using a sweet potato and why?
No decrease in mass
(Very high sucrose concentration - very low water potential)
How would we alter the experiment for a sweet potato?
Increase the sucrose concentration of the external medium until a point is reached where water will flow out of the sweet potato
