Photosynthesis

Question 1

In chromatography, why should the origin be drawn in pencil rather than ink?

Answer 1

Ink is soluble in solvent so it would mix with the pigments and the line would move

Question 2

In chromatography, why should the point of origin should be above the level of the solvent?

Answer 2

Pigments are soluble in the solvent, so it would run off the paper / spots would dissolve into the 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 not the other

Question 4

What is an Rf value?

Answer 4

Distance moved by the spot / distance moved by the solvent front

Question 5

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

Answer 5

Once the solvent evaporates the solvent front isn’t visible

Question 6

Explain why the centre of each spot should be measured

Answer 6

Standardises the 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 solvents/papers/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/pigments move different distances and Rf values are constant so comparisons can be made

Question 9

Describe the role of the enzyme dehydrogenase in photosynthesis

Answer 9

Catalyses the reduction of NADP in the light dependent reaction

Question 10

Give examples of variables which could be controlled in required practical 8

Answer 10

Source of chloroplasts, volume of chloroplast suspension, volume / concentration of DCPIP

Question 11

Explain the purpose of control 1

Answer 11

Shows light is required for the reaction, and that chloroplasts can’t do it alone

Question 12

Explain why DCPIP in control 1 stays blue

Answer 12

There is no light available so photoionisation cannot take place, so no electrons are available to reduce DCPIP

Question 13

Explain the purpose of control 2

Answer 13

Shows chloroplasts are needed for DCPIP to decolourise and that light alone does not cause it

Question 14

Explain why DCPIP changed from blue to colourless

Answer 14

DCPIP is 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

Because the end point is subjective, you can use a colorimeter to measure light absorbance at set time intervals in comparison to the colour standard