Core practicals

Question 1

Vitamin C

Answer 1

1. Transfer 1cm3 of DCPIP solution into a test tube with a pipette.
2. Add Vitamin C solution drop wise to the DCPIP solution. Shake after each drop.
3. Record the volume of Vitamin C that is required to change the colour of the DCPIP. BLUE –> COLOURLESS
4. Repeat the experiment 5x and replace the Vitamin C solution with other fruit juices.
5. Use a calibration curve to get accurate results.

Question 2

Observing mitosis

Answer 2

1. Cut a 5mm sample of the root tip using a scalpel.
2. Transfer root tip to sample tubes containing HCl and leave for 5 minutes.
3. Dry root tips on filter paper.
4. Place tip on a microscope slide. Macerate with a needle to spread the cells out to 1 cell thick.
5. Add a drop of toluidine blue to the slide and leave to stain for 2 minutes.
6. Lower the cover slip down carefully onto the slide. Make sure there are no air bubbles in the slide which may distort the image.
7. Adjust microscope using fine focus knob.
8. To calculate mitotic index = no. of cells undergoing mitosis / total no. of cells.

Question 3

Hill reaction + photosynthesis

Answer 3

1. Collect leaf samples. Cut into small sections. Grind sample using a pestle and mortar and place into a chilled isolation solution.
2. Place several layers of muslin cloth into funnel and wet with isolation medium to filter sample into a beaker.
3. Suspend the beaker in an ice water bath to keep sample chilled.
4. Transfer to centrifuge tubes and centrifuge at high speed for 10 minutes. This will separate chloroplasts into the pellet.
5. Remove supernatant and add pellet to fresh isolation medium.
6. Store isolation solution on ice.
7. Set the colorimeter to the red filter. Zero it using a cuvette containing chloroplast extract and distilled water.
8. Place test tube in rack 30cm from light source and add DCPIP. Immediately take a sample and add to cuvette.
9. Measure the absorbance of the sample using the colorimeter
10. Take a sample and measure its absorbance every 2 minutes for 10 minutes.
11. Plot results on a graph.
12. Repeat for different distances from lamp up to 100 cm. This will vary the light intensity.
13. Do a control experiment in a dark room.

Question 4

Conclusion for Hill reaction

Answer 4

1. As light intensity decreases the rate of photosynthesis decreases as well.
2. Meaning at further distances from light it will take longer for DCPIP to change from blue to colourless.
3. So absorbance rate decreases slower with decreasing light intensity. (When the DCPIP is blue, the absorbance is higher).

Question 5

Membrane permeability of beetroot

Answer 5

1. Cut beetroot into 8 identical cylinders using a cork borer and wipe/rinse to clean off any pigment released as a result.
2. Place each of the cylinders of beetroot in 10 ml of distilled water. Place each test tube in a water bath at a range of temperatures between 0 and 70°C.
3. Leave the samples for 15 minutes- pigment will leak out of the beetroot.
4. Record the exact temperature of the water bath using the thermometer.
5. Remove the test tubes from the water baths and remove the cylinders of beetroot from them. Decant the liquid into clean test tubes.
6. Set the colorimeter to a blue filter and zero using a cuvette with distilled water. Filter each sample into a cuvette using filter paper.
7. Measure the absorbance for each solution. A higher absorbance indicates higher pigment concentration, and hence a more permeable membrane.