Required Practical 4

Question 1

State 2 factors the affect the permeability of cell membranes

Answer 1

• Temperature
• Concentration of solvents (ethanol)

Question 2

How is beetroot used to measure the permeability of cell membranes?

Answer 2

The higher the permeability, the more red pigment that leaks out into the surrounding solution within a given time. A colorimeter can be used to determine the absorbance, hence concentration of pigment

Question 3

Outline the procedure to investigate the effect of temperature on permeability of cell membrane

Answer 3

1. Cut beetroot into 6 identical cubes with a scalpel
2. Place each cube in a different test tube with equal volumes of distilled water
3. Place each test tube into water baths ranging from 30-80c. Leave for 20 minutes
4. Filter each solution out into a cuvette and measure the absorbance using a colorimeter

Question 4

What are the safety hazards involved in testing the effect of ethanol concentration on membrane permeability?

Answer 4

• Ethanol is irritant and flammable, keep away from naked flames, wear eye protection
• Keep sharp scalpel away from fingers
• Handle hot liquid with care

Question 5

What is the effect of temperature on membrane permeability?

Answer 5

Increasing temperature results in increase membrane permeability

Question 6

What is the effect of ethanol concentration on membrane permeability?

Answer 6

Increasing ethanol concentration leads to increased permeability

Question 7

Explain why its important to wash the beetroot discs

Answer 7

To wash off any pigment. To show that any pigment released is from ethanol

Question 8

Suggest a suitable control variable

Answer 8

• Distilled water and two discs of beetroot
• To show that any pigment released was from the alcohol/ to compare it with the effects of alcohol

Question 9

Explain why your results would have been different if you had used cooked beetroot.

Answer 9

• Heating/cooking would damage the (plasma) membranes;
• Valid explanation of damage e.g. disrupts phospholipid bilayer / denatures proteins in cell membrane;
• A lot of pigment released during cooking / less pigment present (in tissue);

Question 10

Explain why a decrease in temperature of 5 °C would affect the results.

Answer 10

Less kinetic energy;
Molecules move slower;
Lower rate of diffusion;
Membranes less fluid;

Question 11

Explain why you were instructed to shake the test tubes every minute

Answer 11

Increase the contact of all surfaces with the alcohol / prevent discs sticking together / maintain a diffusion gradient for the pigment

Question 12

Explain why you were instructed to pour the alcohol immediately from the experimental test tube into a clean test tube

Answer 12

To limit the variation in time discs are in alcohol /make sure all the discs were in contact with alcohol for the same length of time / so no more pigment can diffuse out of the discs