Practicals

Question 1

Explain why the origin should be drawn in pencil rather than ink.

Answer 1

• Ink is soluble in solvent
• So ink would mix with pigments / line would move

Question 2

Explain why the point of origin should be
above the level of the solvent.

Answer 2

• Pigments are soluble in solvent
• So would run off paper / spots dissolve into solvent

Question 3

Explain why a pigment may not move up the chromatography paper in one solvent.

Answer 3

May be soluble in one solvent but insoluble in another

Question 4

Describe how pigments can be identified

Answer 4

• Rf value = distance moved by spot / distance moved by solvent front
• Compare Rf value to published value

Question 5

Explain why the solvent front should be marked
quickly once chromatography paper is removed.

Answer 5

• Once solvent evaporates, solvent front not visible

Question 6

Explain why the centre of each pigment spot
should be measured.

Answer 6

• standardises readings as pigment is spread out
• So allows comparisons to be made

Question 7

Explain why the obtained Rf values were similar,
but not identical, to the published values.

Answer 7

Different solvent / paper / running conditions may
affect Rf value

Question 8

Explain why Rf values are used and not the
distances moved by pigment spots.

Answer 8

• Solvent / pigment moves different distances
• Ri value is constant for same pigment / can be
compared

Question 9

Describe the role of the enzyme dehydrogenase in photosynthesis

Answer 9

• Catalyses the reduction of NADP in the light-dependent reaction
• NADP accepts (gains) electrons from photoionisation of chlorophyll / photolysis of water

Question 10

Give examples of variables that could be controlled.

Answer 10

• Source of chloroplasts
• Volume of chloroplast suspension
• Volume / concentration of DCPIP

Question 11

Explain the purpose of control 1
(tube A).

Answer 11

• Shows light is required for DCPIP to decolourise
• Shows that chloroplasts alone do not cause DCPIP to decolourise

Question 12

Explain why DCPIP in control 1 stays blue.

Answer 12

• No light so no photoionisation of chlorophyll
• So no electrons released to reduce DCPIP

Question 13

Explain the purpose of control 2
(tube B).

Answer 13

• Shows chloroplasts are required for DCPIP to decolourise
• Shows that light alone does not cause DCPIP to decolourise

Question 14

Explain why DCPIP changes from blue to colourless.

Answer 14

• DCPIP is a redox indicator / DCPIP gets reduced by electrons
• From photoionisation of chlorophyll

Question 15

Suggest a limitation with the method and how the experiment could be modified to overcome
this.

Answer 15

• End point (colour change) is subjective
• Use a colorimeter
• Measure light absorbance of sample at set time intervals
• Zero colorimeter using the colour standard