Required Practical 03

Question 1

RP3 water potential of plant tissue method:

Answer 1

1. make a series of dilutions of 20 cm cubed of 1.0 mol dm^-3 of sucrose solution ( control variable). The concentration of these should be 0.0, 0.2, 0.4, 0.6, 0.8, 1.0 mol dm^-3. Dilute these using distilled water.
2. Place 6 test tubes in a boiling tube rack, label each test tube with a concentration go sucrose solution that will be added to it using a marker pen, mark these tubes form left to right with increasing concentration values.
3. For each concentration use a new, clean, dry 10 cm^-3 graduated pipette add 5cm ^-3 of each dilution into a separate boiling tubes ( make sure you add the correct conc to the correct labelled boiling tubes)
   - Use a cork borer to cut 6 cylinders of potato tissue with the same diameter .
   - Use a ruler and a scalpel to cut all the cylinders to the same length. This ensures all potato cylinders are the same size and have the same surface area. Remove any skin
   - Measure and record the initial mass of each potato cylinder
   - Place each potato cylinder into different boiling tubes containing sucros solutions on known concentrations.
   - Leave the cylinders in the solution for a set time (e.g. 20 minutes)
   - Remove, blot dry and remeasure the mass
   - calculate the percentage change in mass, using thr equation
   - plot results on a graph with concentration of sucrose on the x axis and percentage change in mass on the y axis

Question 2

Percentage change in mass equation :

Answer 2

(Change in mass / initial mass ) x100

Question 3

For RP3 water potential fo plant tissue:
Explain why a percentage change in mass is calculated rather than an absolute change in mass

Answer 3

• Initial masses of potatoes are never the same
• as it is impossible to cure every potato to the exact same weight as the amount of water inside of them are different.
• By calculating percentage changed in mass you cancel out any original mass difference
• so a 1g increase in a lighter potato is not treated the same as a 1g increase in a heavier potato
• Thsi allows the comparing of different concentration to be equal

Question 4

RP3 water potential fo plant tissue :
Results and interpretations

Answer 4

increase in mass : the solution has a higher water potential than the potato (hypotonic), so water moves in by osmosis
No change in mass : the solution is isotonic to the potato, meaning the water potential is equal inside and outside of the cells, so there is not net movement of water (this part is importnst water does move in and out but there is no NET movement)
Decrease in mass : The solution has a lower water potential than the potato (hypertonic) so water moves out by osmosis

Question 5

How to use graph to find the water potential of the potato tissue?

Answer 5

• Find the point where the percentage change in mass is 0
• This is where the line of best fit crosses the x axis
• At this concentration, the solution is isotonic to the potato tissue , therefore there was no net gain or loss of water by osmosis and no cnge in mass
  -Teh water potential of that equates to the concentration of sucrose can then be looked up in a table.
• This will be the water potential of the potato tissue

Question 6

In RO 3 water potential of pant tissue :
Why are the potato cylinders blotted dry after removing them from the sucrose solution?

Answer 6

• To remove excess sucrose solution fo the surface of the potato cylinders that could artificially increase mass measurements
• Ensures tat only Teh water absorbed by osmosis is measured

Question 7

For RP 3 water potential of plant tissue :
Controlled variables

Answer 7

• Same volume of sucrose solution for each potato cylinder
• Same temperature ( osmosis affected by temperature)
• Same duration for all potato cylinders in the solution
• Use of the same type / age of potato ( as different potatoes may have different initial water potential)
• Same surface area of potato cylinders ( cut to equal sizes using a cork borer)
• All potato clingers have all skin removed,

Question 8

Processed data =

Answer 8

Using raw data that has even recorded (while doing the experiment ) to complete calculations. The results from the calculations can be analysed or plotted on graphs to create conclusions

Question 9

RP3 water potential of plant tissues :
In this investigation you were given potato cylinders take from potatoes of the same species, why was this important?

Answer 9

As different species will have different initial sucrose concentrations so therefore different water potential so this would not be a fair test

Question 10

RP3 potato skin was removed, why could have the potato skin affected osmosis

Answer 10

It could have reduced osmosis as the skin stops the movement of water as it is impermeable, so the rate of the movement of water particle decreases down a water potential gradient across a partially permeable membrane

Question 11

RP 3 why was potato cylinders cut to the same length used ?

Answer 11

If they are the same length then they would have had the same surface area to volume ratio this is important this means that thinvestugatiin is fair.