CORE PRACTICAL 3: INVESTIGATE MEMBRANE STRUCTURE, INCLUDING THE EFFECT OF ALCOHOL CONCENTRATION OR TEMPERATURE ON MEMBRANE PERMEABILITY

Question 1

Aim:

Answer 1

To investigate membrane structure, including the effect of alcohol concentration or temperature on membrane permeability.

Question 2

Independent Variable:

Answer 2

The temperature of the water in the water bath (Degrees Celcius)

Question 3

Dependent Variable:

Answer 3

The percentage transmission of light through the resulting solution

Question 4

Control Variables:

Answer 4

1. Volume of distilled water – 10cm³ of distilled water should be used to fill the boiling tubes each time
2. Time left in water – leave each boiling tube containing beetroot for 30 minutes
3. Size of beetroot piece – use a ruler and knife to cut cylindrical beetroot pieces of 1cm in length
4. Colorimeter used – same colorimeter should be used on the same blue/green setting each time, measuring percentage absorbance. Calibration with distilled water should be carried out each time
5. Volume of beetroot solution – 2cm³ of beetroot solution should be added to a cuvette each time

Question 5

Method:

Answer 5

1. Use a cork borer and knife to cut cylinders of beetroot in equal sizes over a white tile.
2. Place all the cut pieces in a beaker of distilled water and leave overnight to remove any dye (betalains) released when the beetroot was cut.
3. Wash and blot dry (with filter paper or a tissue) the 8 pieces of beetroot.
4. Fill 8 boiling tubes each with 10cm³ of distilled water with different temperatures or different concentrations of alcohol and place them into 8 separate water baths of different temperatures (e.g. 0°C, 10°C, 20°C, 30°C, 40°C, 50°C, 60°C, 70°C).
5. Once at the desired temperature, add a piece of beetroot to each boiling tube and leave for 30 minutes.
6. Remove the beetroot pieces gently with a pair of forceps and then shake the tubes to disperse the dye.
7. Set a colorimeter to percentage absorbance on the blue/green filter. Calibrate by filling a cuvette with distilled water first then add 2cm³ of beetroot solution from the first temperature to a new cuvette.
8. Place this cuvette into the colorimeter to read the percentage absorbance. Repeat this for all other pieces.

Question 6

Results & Calculations:

Answer 6

In order to obtain the percentage transmission for each beetroot solution in the colorimeter, we can use the following equation:

Percentage transmission = 100 – percentage absorbance

Recordings can be noted down in an appropriate table as well as a graph.

Question 7

Conclusion:

Answer 7

An increase in temperature caused further destruction of the cell membrane, which allowed more pigment to leak out via diffusion.

Question 8

What are some RANDOM ERRORS that can occur?

Answer 8

1. Some beetroot may have skin on affecting surface area – use a bigger beetroot and use cork borer to obtain pieces free from skin
2. Difficulty in maintaining temperature – set water baths for higher temperatures and set refrigerators for lower temperatures
3. Accurate size of beetroot – cut many different pieces from different beetroot and use the most similar sized pieces for the experiment
4. From the different parts of the root – use a large beetroot and take all samples from the round part of the root
5. Ensuring same amount of time at the different temperatures – have 7 other helpers to make sure all 8 boiling tubes are extracted at the same time after 30 minutes

Question 9

What are some SYSTEMATIC ERRORS that can occur?

Answer 9

1. Accurate reading of the colorimeter – use more precise colorimeter and close cap to ensure outside light does not interfere with reading. Make sure that distilled water is used for calibration