The light-dependent reactionAQA A-Level Biology: Subtopic test

1. 1
   In the thylakoid membranes of a chloroplast, light energy absorbed by chlorophyll causes the chlorophyll to release electrons. The chlorophyll must then be supplied with replacement electrons so that it can go on absorbing light.

   (a)

   What is the release of electrons from chlorophyll by light called?

   [1 mark]

   • APhotolysis
   • BPhosphorylation
   • CPhotoionisation
   • DHydrolysis

   (b)

   Which products of the light-dependent reaction are used in the light-independent reaction?

   [1 mark]

   • AATP and reduced NADP
   • BOxygen and ATP
   • CProtons and oxygen
   • DATP and carbon dioxide

   (c)

   Explain why chlorophyll needs a supply of replacement electrons and state where they come from.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Isolated chloroplasts were illuminated in a solution that contained no carbon dioxide but did contain an artificial electron acceptor. Oxygen was released while the chloroplasts were in the light.

   (a)

   What is the source of the oxygen released?

   [1 mark]

   • ACarbon dioxide
   • BGlucose
   • CATP
   • DWater

   (b)

   What is the most likely role of the artificial electron acceptor?

   [1 mark]

   • AIt replaces carbon dioxide in the light-independent reaction
   • BIt accepts electrons in place of NADP
   • CIt replaces chlorophyll
   • DIt provides the protons for ATP synthase

   (c)

   Explain why oxygen release would stop if the chloroplasts were placed in the dark.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student extracted chloroplasts from leaves in ice-cold isotonic buffer. She added a sample to a solution of DCPIP, a blue dye that becomes colourless when it is reduced, and measured the absorbance of the mixture. In a tube in the light, the absorbance fell from 0.80 to 0.20 over 12 minutes. In a tube kept in the dark it fell from 0.80 to 0.78, and in a tube containing boiled chloroplasts in the light it fell from 0.80 to 0.79.

   (a)

   Calculate the rate of change of absorbance in the tube in the light. Give your answer in absorbance units per minute.

   [3 marks]

   (b)

   Explain the results for the three tubes, and why the chloroplasts were extracted in ice-cold isotonic buffer.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A herbicide binds to a protein in the electron transfer chain of chloroplast thylakoid membranes and stops electrons passing along the chain. Plants sprayed with the herbicide stop photosynthesising and die.

   (a)

   Describe how ATP and reduced NADP are produced in the light-dependent reaction of an untreated chloroplast.

   [6 marks]

   (b)

   Explain why plants treated with the herbicide cannot photosynthesise and die.

   [6 marks]

   Total for question 4: 12 marks

End of questions