Chloroplasts, pigments and limiting factorsEdexcel International A Level Biology: Subtopic test

1. 1
   A student extracts pigments from spinach leaves using propanone and applies a concentrated spot to a pencil line near the bottom of a strip of chromatography paper. The paper is placed in a solvent, which runs 9.0 cm from the pencil line to the solvent front. Four pigment spots separate. Their centres are 8.5 cm (carotene), 7.2 cm (xanthophyll), 6.3 cm (chlorophyll a) and 5.4 cm (chlorophyll b) from the pencil line.

   (a)

   Calculate the Rf value of chlorophyll a.

   [1 mark]

   • A1.43
   • B0.70
   • C2.70
   • D0.63

   (b)

   Which statement best explains why the pigments travel different distances up the paper?

   [1 mark]

   • AThe pigments were applied to the paper in a different order
   • BEach pigment is carried by a different volume of solvent
   • CThe pigments that photosynthesise fastest travel furthest
   • DPigments that are more soluble in the solvent and less strongly attracted to the paper travel further

   (c)

   Explain why the pencil line must be above the level of the solvent in the container, and why the position of the solvent front must be marked as soon as the paper is removed.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A plant physiologist measures two spectra for a leaf. The absorption spectrum of extracted chlorophyll a shows strong absorption of blue light (about 430 nm) and red light (about 660 nm) and very little absorption of green light (about 550 nm). The action spectrum for photosynthesis in the leaf has peaks at similar wavelengths, but the rate of photosynthesis in green light is higher than the chlorophyll a absorption alone would predict.

   (a)

   What is plotted on an action spectrum for photosynthesis?

   [1 mark]

   • ARate of photosynthesis against wavelength of light
   • BAbsorbance of a pigment against wavelength of light
   • CLight intensity against rate of photosynthesis
   • DWavelength of light against time

   (b)

   Which statement best explains why the action spectrum shows more photosynthesis in green light than the chlorophyll a absorption spectrum predicts?

   [1 mark]

   • AGreen light is reflected by chloroplasts and used by the Calvin cycle
   • BGreen light changes chlorophyll a into chlorophyll b
   • CAccessory pigments absorb some green light and pass the energy to chlorophyll a
   • DThe Calvin cycle uses green light directly

   (c)

   Use the information to explain why a plant lit only with green light photosynthesises slowly but not at zero rate.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   An electron micrograph of a palisade mesophyll cell shows several chloroplasts. Each chloroplast is surrounded by a double membrane and contains stacks of flattened thylakoids joined by membranes called lamellae, a fluid stroma, small starch grains, and ribosomes and a loop of DNA.

   (a)

   Describe how the structure of a chloroplast is adapted to its role in photosynthesis.

   [3 marks]

   (b)

   Explain how the arrangement of the thylakoids and stroma, and the presence of starch grains, allow a chloroplast to carry out photosynthesis efficiently.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student investigates photosynthesis in the aquatic plant Cabomba. A shoot in sodium hydrogencarbonate solution is placed in a water bath at 25 °C and a lamp is placed at measured distances from it. The number of oxygen bubbles released in two minutes is counted. As the lamp is moved closer, the rate rises and then levels off at the shortest distances. In a second investigation the light intensity is kept constant and the temperature is raised from 10 °C to 50 °C: the rate rises to a maximum at about 35 °C and then falls rapidly.

   (a)

   Explain why the rate of oxygen production levelled off at the shortest lamp distances, and describe how the student could show which factor had become limiting.

   [6 marks]

   (b)

   Explain the effect of temperature on the rate of photosynthesis in the second investigation.

   [6 marks]

   Total for question 4: 12 marks

End of questions