Biology Practical Exam Notes - Salt Concentration Effect on Onion Cells
Part A: Effect of Salt Solutions on Onion Cells
Background: In the Negev, brackish water exists underground, less salty than seawater but saltier than drinking water.
Experiments explore using brackish water for irrigation, showing some crops (tomatoes, melons) can grow in it.
Researchers are studying the effects of brackish water irrigation on onion crops.
Experiment Setup
Prepare
two onion leaf strips.
Place one strip in a dish with water and another in a dish with a 2% salt solution.
Ensure the outer side of the leaf faces up, and the inner side is submerged.
After 5 minutes, remove the strips and place them on a labeled paper towel.
Observation (Based on video up to 1:50)
Observe the difference in shape between the leaf strip soaked in water and the one in salt solution.
Leaf Structure
The outer epidermis has tightly packed cells with rigid walls. Therefore, changes in the external solution minimally affect its volume.
The inner layer has more flexible cell walls, making its volume susceptible to changes in the concentration of the external solution.
Questions and Answers
Observation: Note the difference in shape between the onion strip in water vs. the onion strip in the salt solution.
Based on information:
(a) What is the effect of soaking the inner layer cells in pure water?
(b) What is the effect of soaking the epidermis cells in pure water?
Explanation: Explain the experimental results, referencing the difference observed in question 1.
Relationship: Describe how the properties of the epidermis relate to its function.
Part B: Testing the effect of salt solutions at varying concentrations on onion leaf strips
Preparing Salt Solutions
Add distilled water and 2% salt solution to cups A and B as per Table 1.
Gently stir.
Table 1: Preparing different salt concentrations
Cup | Distilled Water (mL) | 2% Salt Solution (mL) | Total Volume (mL) | Final Salt Concentration (%) |
|---|---|---|---|---|
A | 95 | 5 | 100 | |
B | 50 | 50 | 100 |
Calculate the final salt concentration for each solution and fill in Table 1.
Note: The initial salt solution concentration is 2%.
Copy the calculated results to Table 2 (in question 5).
Transferring Solutions
Transfer 30 mL of solution from cup A into each of the dishes 1 and 2.
Transfer 30 mL of solution from cup B into dish 3.
Preparing Onion Strips
Prepare 8 onion leaf strips (as in Part A).
Experiment Stages
Stage 1: Soaking in identical salt concentration for 8 minutes
Transfer 4 onion leaf strips to each of the dishes 1 and 2 (outer side facing up).
Leave them in the solution for 8 minutes.
Answer question 5.
Question 5
(a) In column B of Table 2, write the salt concentrations in dishes 1 and 2 (from cup A).
(b) In column F of Table 2, write the salt concentrations in dish 3 (from cup B).
Table 2: Change in distance between the ends of onion leaf strips after soaking in different salt concentrations
Stage I - First 8 minutes | Stage II - Second 8 minutes | ||||
|---|---|---|---|---|---|
Dish | Salt Concentration (%) | Distance between ends of onion leaf strip (cm) | Average | Salt Concentration (%) | Distance between ends of onion leaf strip (cm) |
Strip 1 | Strip 2 | Strip 3 | Strip 4 | ||
1 | |||||
2 |
Measuring Strip Lengths
After 8 minutes, remove the 4 strips from dish 1 and place them on the designated area of the paper towel.
Note that some strips may fold (Figure 2b), causing the ends to get closer. Some may even curl into a circle (Figure 2c).
Possible Strip Configurations (Figure 2)
(a) Unfolded
(b) Folded
(c) Curled into a circle
Repeat the process with strips from dish 2, placing them on the paper towel.
Assessing Stage 1 Results
Watch the video, pause at 2:46, and measure the distance between the ends of each of the 4 strips from dish 1:
If unfolded (Figure 2a), record "5".
If folded (Figure 2b), measure the distance.
If curled (Figure 2c), record "0".
Record these measurements in column C of Table 2.
Watch the video, pause at 3:08, and repeat the measuring process for the strips from dish 2. Record in column C of Table 2.
Stage 2: Soaking the Strips in Different Concentration Salt Solutions for 8 More Minutes
Return the strips from dish 1 back to dish 1.
Transfer the strips from dish 2 to dish 3.
Watch the video, pause at 3:42. Measure the distance between the ends of the strips in dish 1 and record in column G of Table 2.
Watch the video, pause at 3:45. Measure the distance between the ends of the strips in dish 3 and record the measurements in column G of Table 2.
Answer questions 6–9.
Questions 6-9
(a) Calculate the average distance between the ends of the onion leaf strips in dishes 1 and 2 (after the first soaking stage). Write the results in column D of Table 2.
(b) Calculate the average distance between the ends of the onion leaf strips in dishes 1 and 3 (after the second soaking stage). Write the results in column H of Table 2.
(a) Describe the results in dish 1 at the end of Stage II, compared to the results in the same
dish at the end of Stage I.
(b) Describe the results in dish 3 at the end of Stage II, compared to the results in dish 2 at the end of Stage I.
Explain the changes observed in the strips transferred to dish 3. Base your explanation on the results obtained in dish 1 at the end of Stage II.
The onion leaves used in this experiment were taken from onion plants of identical genetic origin. Explain why maintaining genetic uniformity of the plants is important in this experiment.