RP 7 - Use of chromatography to investigate the pigments isolated from leaves of different plants e.g., leaves from shade-tolerant and shade intolerant plants or leaves of different colours

Question 1

You were told to use a pencil to mark the origin. It was important not to use a pen to mark this line. Suggest why

Answer 1

Ink would dissolve in solvent / ink would be confused with substances on chromatogram / ink will run; Accept converse statements for pencil.

Question 2

You were told to draw a line to show the solvent front before the filter paper dried.
Explain why it was necessary to draw the line before the filter paper dried

Answer 2

Cannot see (solvent);

Question 3

You could use Rf values to find out if a pigment on your chromatogram was the same as a pigment on another student’s chromatogram. Explain why it would be better to use the Rf value to do this and not the distances moved by the pigment spots (2)

Answer 3

1 The solvent / pigment moves different distances;
2 Rf value is distance moved (by spot) divided by distance moved by solvent front / Rf value is a ratio of distance moved (by spot) to distance moved by solvent front/Rf is a proportion;
3 Rf value constant (for the same pigment) / Rf values can be compared;

Question 4

You were told not to move the bottle once you had put the TLC paper in it.
Suggest why this is important

Answer 4

Pigments / solvent would not run straight / would run off side of paper / pigments could wash off paper;

Question 5

You were told to find the distance moved by the pigment by measuring to the centre of the pigment spot. Explain why you should measure to the centre of the pigment spot. (2)

Answer 5

1 Would standardise readings;
2 Allow comparisons to be made;
3 Pigment is spread out / moves different distances;
4 It is the mean / average distance moved by the pigment;

Question 6

The technique you used to extract pigments from the leaves may not produce good results if it is used with the leaves of xerophytic plants. Use your knowledge of the leaves of xerophytic plants to suggest why. (2)

Answer 6

1 Thick cuticle;
2 Prevents pigments penetrating / prevent pigment being transferred to paper;
OR
1 Thicker / stronger leaves;
2 Lower concentration of pigment / lower amount of pigment / more cells without pigment / more tissue without pigment / harder to crush;
OR
1 Leaves are spines;
2 Have little / no pigment;

Question 7

Leaves change colour in the Autumn. This is partly due to loss of chlorophyll. Describe how you could use chromatography to show that chlorophyll is lost. (2)

Answer 7

1 Carry out chromatography at different times;
2 Find Rf for chlorophyll;
3 Look for substance at predicted position / look for green substance;

Question 8

Holly usually has dark green leaves. Some cultivated varieties of holly have yellow leaves. Explain why these cultivated varieties grow more slowly than holly with dark green leaves. (3)

Answer 8

1 Yellow leaves have less / no chlorophyll;
2 Less / no photosynthesis;
3 Products of / named product of photosynthesis needed for growth / synthesis;