RP 4 - The effect of alcohol concentration on the diffusion of pigment from beetroot cells

Question 1

You were provided with beetroot discs that were washed thoroughly before the start of the investigation. Explain why it was important to wash the beetroot discs. (1)

Answer 1

• To wash off any pigment / betalins (on the discs);
• To show that any pigment released is from the effect of alcohol;

Question 2

In Part 2 of this investigation you used five concentrations of alcohol.

Describe a suitable control for your investigation. (1)

Explain why this control would be necessary. (1)

Answer 2

• (Distilled) water and (two) discs of beetroot;
• To show that the alcohol was causing the leakage of pigment / to compare it with the effects of the alcohol;

Question 3

3 (a) Explain why you were instructed to shake the test tubes every minute (step 11).

Answer 3

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

Question 4

Explain why you were instructed to pour the alcohol immediately from the experimental test tube into a clean test tube (step 13).

Answer 4

• 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;

Question 5

You were given discs taken from fresh beetroot in your investigation. Explain why your results would have been different if you had used cooked beetroot. (2)

Answer 5

• 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 6

You used a water bath in this investigation. Explain why a decrease in temperature of 5 °C would affect the results. (2)

Answer 6

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

Question 7

Two students carried out the same investigation as you did. They worked in the same practical session and followed the same procedure. They worked accurately but found their results were different. Suggest one reason for this.

Answer 7

• Variation in beetroot / variation in judging colour;

Question 8

The student suggested that point A (100% concentration) on the graph was an anomaly. What should he do to check this? (2)

After further investigation the student found the point was not an anomaly. Suggest why the curve levels out (1)

Answer 8

• Repeat experiment at 100 % alcohol concentration;
• Check to see if results are concordant / similar;
• Concentration of pigment inside and outside of the tissue reached equilibrium / an equal concentration of pigment inside and outside the beetroot / pigment diffusing in and out at the same rate;
• Maximum membrane damage;