Photosynthetic pigmentsEdexcel A-Level Biology B: Subtopic test

1. 1
   A student investigated the effect of wavelength on the rate of photosynthesis. A piece of pondweed in water containing dissolved sodium hydrogencarbonate was placed 15 cm from a lamp. Coloured filters were placed between the lamp and the pondweed in turn, and the number of oxygen bubbles released in five minutes was counted.

   (a)

   Which pair of wavelength regions is absorbed most strongly by chlorophyll a?

   [1 mark]

   • ARed and blue-violet
   • BGreen and yellow
   • CGreen and red
   • DYellow and orange

   (b)

   Why was sodium hydrogencarbonate added to the water?

   [1 mark]

   • ATo absorb heat from the lamp
   • BTo provide extra oxygen for respiration
   • CTo increase the pH of the water
   • DTo supply carbon dioxide so that it does not limit the rate

   (c)

   The student found that very few bubbles were released when a green filter was used. Explain this result.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A researcher measured how strongly a purified extract of chlorophyll a absorbed light of different wavelengths, and the rate of photosynthesis of whole leaves at the same wavelengths. The extract absorbed most strongly at about 430 nm and 660 nm. The leaves also photosynthesised at a substantial rate at 480 nm, where chlorophyll a absorbs very little.

   (a)

   What does an action spectrum show?

   [1 mark]

   • AThe wavelengths of light reflected by a leaf
   • BThe proportion of light absorbed by a single pigment at each wavelength
   • CThe rate of photosynthesis at different wavelengths of light
   • DThe light intensity at different depths of water

   (b)

   Which of these best explains the photosynthesis at 480 nm?

   [1 mark]

   • AAccessory pigments such as carotenoids absorb light at this wavelength
   • BThe enzyme RUBISCO absorbs the light
   • CChlorophyll a absorbs strongly in the stroma
   • DThe NADP absorbs this light directly

   (c)

   Suggest why the action spectrum of the leaves does not match the absorption spectrum of chlorophyll a alone.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student extracted pigments from spinach leaves by grinding them in propanone, then separated them by thin-layer chromatography. After the run the solvent front had moved 8.0 cm from the origin. Four pigment spots were visible, at these distances from the origin: carotene 7.6 cm, xanthophyll 5.6 cm, chlorophyll a 4.4 cm and chlorophyll b 3.6 cm.

   (a)

   Calculate the Rf value of chlorophyll a and of xanthophyll.

   [3 marks]

   (b)

   Explain why the origin line was drawn in pencil and not in ink, why the solvent level had to be below the origin, and why carotene travelled furthest.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Seedlings of a woodland plant grow on the forest floor beneath a dense canopy of tall trees. Light reaching the seedlings has had much of its red and blue-violet light absorbed by the leaves above, so it is relatively rich in green light.

   (a)

   Explain the advantages to these seedlings of having a variety of photosynthetic pigments.

   [6 marks]

   (b)

   Describe how chromatography could be used to compare the pigments in a leaf from one of these shade seedlings with those in a leaf from a plant growing in full sun.

   [6 marks]

   Total for question 4: 12 marks

End of questions