Required practical 4 - investigating permeability of cell membranes

Question 1

What is the independent variable?

Answer 1

Concentration of alcohol - would be increasing

Question 2

Why is beetroot usually used in the practical?

Answer 2

• first of all, it is a plant cell so it has cell membranes
• secondly, it contains purple pigment, you can observe the leaching of this pigment outside the cells

Question 3

Why are color standards used in the first stage of the practical

Answer 3

to act as a comparison to the test tubes with the beetroot discs.

Question 4

Why should the beetroot extract be mixed?

Answer 4

• to make sure beetroot extract is evenly suspended and hasn’t sunk to the bottom of conical flask

Question 5

How should you set up the color standards ?

Answer 5

• should be first labeled with a number ( 0,2,4,6,8)
  0: 0% = 5ml water
  2: 20% = 1 ml beetroot + 4ml of water
  4: 40% = 2ml beetroot + 3ml of water
  6: 60% = 3ml beetroot + 2ml of water
  8: 80% = 4ml beetroot + 1ml of water
  10: 100% = 5ml beetroot

Question 6

What is the second stage of the practical

Answer 6

• putting evenly cut beetroot discs into different concentrations of ethanol

Question 7

With the second set of test tubes why do they all have to be at 30 degrees Celsius? ( the ethanol)

Answer 7

because diffusion of the beetroot pigment is affected by temperature. The higher the temperature the faster the rate of diffusion, so the ethanol must be at the same temperature for all the test tubes before placing beetroot discs. This can be checked using a thermometer.

Question 8

Why should you blot the beetroot discs?

Answer 8

• cell walls and cell membranes of the beetroot discs have been damaged by the cutting, so some of the pigment will have already leaked out ( this is not affected by the alcohol concentration)

Question 9

When you put the test tubes containing the beetroot disc in a water bath why do you need to give them a shake every one minute?

Answer 9

• the beetroot disc in the ethanol might stick together and that reduces the rate of diffusion of pigment.
• pigment will build up around the discs reducing the concentration gradient between the beetroot disc and the pigment outside. A lower conc gradient will also slow down the rate of diffusion.

Question 10

As well as the 5 test tubes containing ethanol, why should you have one control tube of purely water with the two beetroot discs ?

Answer 10

You can compare the control tube to the tubes containing ethanol to see if the pigment leakage is due to the effect of the ethanol or another factor.

Question 11

Why do you have to be very quick at putting the test tubes in and removing them ?

Answer 11

To make sure the beetroot discs are in contact with ethanol for the same amount of time

Question 12

Analysing the results what can you say about increasing the conc of alcohol on cell membranes ?

Answer 12

• As concentration of ethanol increases permeability of membrane increases because theanol disrupts the cell membrane and so more pigment leaks out of the beetroot, and escapes cell into the ethanol.

Question 13

How could you improve the experiment with a colorimeter?

Answer 13

• the experiment is semi-quantitative because it compares two numbered color standards.
• colorimeter can be used to directly measure the absorbance of light of each of the tubes, this would give you a quantitative measure of color intensity.