Light-dependent reactionsEdexcel International A Level Biology: Subtopic test

1. 1
   The light-dependent reactions of photosynthesis take place in the thylakoid membranes of chloroplasts, where molecules of chlorophyll are arranged in groups called photosystems.

   (a)

   What happens to a chlorophyll molecule when it absorbs light?

   [1 mark]

   • AAn electron is raised to a higher energy level and leaves the molecule
   • BThe molecule is split into two smaller molecules
   • CA proton is released from the molecule
   • DThe molecule is reduced by hydrogen

   (b)

   Which substance is the final electron acceptor in non-cyclic photophosphorylation?

   [1 mark]

   • AOxygen
   • BADP
   • CNADP
   • DCarbon dioxide

   (c)

   Describe how the energy of excited electrons is used to make ATP.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In non-cyclic photophosphorylation, electrons that leave chlorophyll in photosystem II are replaced by electrons that come from water. Isolated chloroplasts that are illuminated in a suspension release oxygen.

   (a)

   Which equation represents photolysis?

   [1 mark]

   • AH₂O → H₂ + O₂
   • B2H₂O → 2H₂ + O₂
   • CCO₂ → C + O₂
   • D2H₂O → 4H⁺ + 4e⁻ + O₂

   (b)

   Why does cyclic photophosphorylation not release oxygen or form reduced NADP?

   [1 mark]

   • APhotosystem II is the only photosystem that is involved
   • BThe electrons return to photosystem I, so water is not split and NADP does not accept electrons
   • CNo ATP is made, so photolysis cannot take place
   • DIt takes place only in darkness

   (c)

   Explain why photosystem II needs electrons from water for non-cyclic photophosphorylation to continue.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   DCMU is a chemical that blocks the movement of electrons from photosystem II to the electron transport chain. Far-red light, with a wavelength greater than 700 nm, is absorbed only by photosystem I. A researcher illuminated suspensions of isolated chloroplasts, with and without DCMU.

   (a)

   Explain why oxygen is no longer released when DCMU is added to chloroplasts in white light.

   [3 marks]

   (b)

   When the chloroplasts with DCMU were illuminated with far-red light only, ATP was made, but no reduced NADP was formed. Explain these observations.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   The chloroplasts in a leaf carry out both cyclic and non-cyclic photophosphorylation. The reactions that make glucose from carbon dioxide need more ATP than reduced NADP.

   (a)

   Describe how non-cyclic photophosphorylation produces ATP, reduced NADP and oxygen.

   [6 marks]

   (b)

   Evaluate the importance of cyclic photophosphorylation to the plant.

   [6 marks]

   Total for question 4: 12 marks

End of questions