Photosynthesis overview and pigmentsAQA A-Level Biology: Subtopic test

1. 1
   Cyanobacteria, algae, mosses and flowering plants all carry out photosynthesis. In every group, light is absorbed by chlorophyll, and the energy is used to make ATP by a very similar set of reactions involving membrane-bound proteins.

   (a)

   Which of the following describes a photoautotroph?

   [1 mark]

   • AAn organism that makes organic molecules from inorganic molecules using light energy
   • BAn organism that obtains organic molecules by eating other organisms
   • CAn organism that makes organic molecules from inorganic molecules using energy from chemical reactions
   • DAn organism that absorbs organic molecules from dead organisms using light energy

   (b)

   The shared features of photosynthesis are described as indirect evidence for evolution. Why is the evidence indirect?

   [1 mark]

   • AChlorophyll is not found inside cells
   • BIt shows that photosynthesis arose separately in each group
   • CEvolution cannot be tested in any way
   • DIt is inferred from similarities between living organisms rather than seen as changes over time

   (c)

   Explain how the similarities in photosynthesis between these groups suggest that photoautotrophs share a common ancestor.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Isolated chloroplasts were kept in the dark in a buffer at pH 4 until the space inside their thylakoids also reached pH 4. They were then transferred to a buffer at pH 8 containing ADP and inorganic phosphate, still in the dark. ATP was made for a short time.

   (a)

   What do these results show?

   [1 mark]

   • AATP synthesis needs light to be absorbed directly by the thylakoid
   • BA proton gradient across the thylakoid membrane can drive ATP synthesis
   • CChloroplasts make ATP only at pH 8
   • DATP is made when protons move against their concentration gradient

   (b)

   Through which structure do protons move back across the thylakoid membrane to make ATP?

   [1 mark]

   • AChlorophyll molecules
   • BThe phospholipid bilayer directly
   • CATP synthase
   • DA molecule of NADP

   (c)

   Explain why ATP was made in this experiment even though there was no light.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student separated the photosynthetic pigments from the leaves of two plants by thin-layer chromatography. The solvent front travelled 9.0 cm from the origin. The extract from a plant grown in full sun gave spots at 8.1 cm (carotene), 6.3 cm (chlorophyll a) and 5.4 cm (chlorophyll b) from the origin. The extract from a shade-tolerant plant gave the same three spots, but its chlorophyll b spot was much larger and darker.

   (a)

   Calculate the Rf value of chlorophyll a in the extract from the plant grown in full sun. Show your working.

   [3 marks]

   (b)

   Explain how chromatography separates the pigments, and suggest an advantage to the shade-tolerant plant of having more chlorophyll b.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A greenhouse grower measured the light energy falling on a tomato crop and found that only about 2% of it was stored as chemical energy in the plants' biomass. The grower wants to understand why photosynthesis is so inefficient, and how the plants use the light they do absorb to make ATP.

   (a)

   Explain why photosynthesis is not 100% efficient.

   [6 marks]

   (b)

   Explain how the absorption of light by chlorophyll leads to the production of ATP in a chloroplast.

   [6 marks]

   Total for question 4: 12 marks

End of questions