Biology Practicals

Dark purple pigment leaks from the cut ends of the beetroot for a while and then stops. Plant cells are surrounded by cellulose cell walls. When you cut through a piece of plant tissue, you cut through some of the cell walls and rupture the cell contents. The pigment then leaks out. The fact that it does not continue to leak suggests that further cells are not being damaged. If the beetroot tissue is treated with increasing temperatures, as the temperature rises, the phospholipid bilayer of the cell-surface membrane and the vacuole membrane will be disrupted. This means that the vacuole contents will more readily leak into the water in the test tube. The higher the temperature, the greater the disruption to the plasma membranes and the more pigment will leak out in 30 minutes.

The factors controlled in this test are the ones that are the same from one group to another – the size of the beetroot cores (their surface area and volume), the advance treatment of the beetroot cores, the volume of water in the test tubes, the pre-heating (or chilling) of the water in the test tubes using the water baths, the length of time the cores spend in the water baths, the treatment of the cores after heating. The experiment could be made more reliable by using more samples of beetroot, and by maintaining the temperature with thermostatically-controlled water baths.

The higher the temperature, the greater the amount of pigment released from the tissues.

Investigation

• a      Pipette 2 cm3 of the juice
• or vitamin C solution into a test tube.

• b     Using a graduated pipette or a
• burette, add 1% DCPIP drop by drop to the vitamin solution. Shake the tube
• gently after adding each drop. Add DCPIP until the blue colour just disappears.

• c      Record the exact amount of DCPIP
• solution that was added.

• d     Repeat the procedure and calculate an
• average result.

• e      Repeat with the juice to be tested. If
• the juice decolourises a large volume of DCPIP, dilute the fruit juice and
• repeat the test. If the juice has a strong colour that will interfere with
• determining the end point, dilute the juice before testing.

• f       Calculate the amount of vitamin C in
• the standard solution in mg cm-3. Calculate how much vitamin C there
• is in each of the fruit juices in mg cm-3.