Topic 2 Practicals

Question 1

Describe method for the practical investigating membrane permeability

Answer 1

1. Cut beetroot into 6-10 identical cubes using a scalpel.
2. Wipe/rinse to clean off any pigment released as a result.
3. Create a dilution series of ethanol using distilled water. Ethanol concentrations should range from 0-100% ethanol.
4. Leave the samples for 20 minutes - the pigment will leak out of the beetroot.
5. Set the colorimeter to a blue filter and zero using a cuvette with distilled water.
6. 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.

Question 2

How does measuring absorbance indicate membrane permeability?

Answer 2

Beetroots contain a pigment called betalain, which is only able to diffuse out of the cell membrane when it becomes more permeable

Question 3

What effect does ethanol have on the cell membrane?

Answer 3

Ruptures the cell membrane by creating larger gaps, so more ethanol = more permeable cell membrane

Question 4

Describe method into the practical investigating enzyme and substrate concentrations

Answer 4

1)Set up two water baths, one with the reaction tube containing reaction mixture, connected by delivery tube to syringe barrel collecting tube
2) Cut 10 discs of potato, each 0.2 mm thick. Place these in the boiling tube with 5cm^3of buffer solution.
3) Add 5 cm^3 of hydrogen peroxide solution to the potato discs. Immediately place the bung and delivery tube firmly into the boiling tube. Place the other end of the delivery tube under the collecting tube.
4) Start a stopclock as soon as the first bubble of oxygen enters the collecting tube from the delivery tube. Collect any gas produced at suitable time intervals for 5 minutes.
Shake the boiling tube gently throughout the reaction period to keep the contents well mixed. Measure the volume of oxygen produced by raising the collecting tube so that the water level in the tube is level with the surrounding water level in the beaker. Wash out the boiling tube thoroughly.
5) Plot a graph of volume of gas produced against time. Use the graph to determine the initial rate of reaction
6) Repeat steps 1 to 5 of the experiment using a range of numbers of potato discs, ensuring that other conditions are unchanged.

Question 5

Why is hydrogen peroxide used?

Answer 5

Because the enzyme catalase in potatoes breaks down hydrogen peroxide into water and oxygen