1. 1
   A vertical farm grows lettuce indoors under light-emitting diodes (LEDs). Batches of seedlings were grown for 30 days under LEDs of a single colour, each giving the same number of photons per second. Mean dry mass per plant was: red LEDs (660 nm) 12.4 g; blue LEDs (450 nm) 10.8 g; green LEDs (530 nm) 4.1 g.

   (a)

   Which statement best explains the low dry mass under green LEDs?

   [1 mark]

   • AGreen light has too little energy to be absorbed by any molecule
   • BGreen light is absorbed strongly but destroys chlorophyll
   • CChlorophyll absorbs little green light, reflecting or transmitting most of it
   • DGreen light stops the plant taking up carbon dioxide

   (b)

   What happens when a chlorophyll molecule absorbs a photon of red light?

   [1 mark]

   • AAn electron in the pigment is raised to a higher energy level
   • BA proton is released from the chlorophyll molecule
   • CThe chlorophyll molecule is split to release oxygen
   • DThe light is converted directly into glucose

   (c)

   Distinguish between an absorption spectrum and an action spectrum.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Sprigs of the pondweed Cabomba were placed in two concentrations of sodium hydrogencarbonate solution, 0.01 mol dm⁻³ and 0.05 mol dm⁻³, at 25 °C, and the bubbles released per minute were counted at five relative light intensities. At 0.01 mol dm⁻³ the rates at relative light intensities 1, 2, 4, 8 and 16 were 5, 10, 18, 20 and 20 bubbles per minute. At 0.05 mol dm⁻³ the rates were 5, 10, 20, 34 and 38 bubbles per minute.

   (a)

   What is the dependent variable in this investigation?

   [1 mark]

   • ALight intensity
   • BRate of bubble production
   • CSodium hydrogencarbonate concentration
   • DTemperature

   (b)

   Which factor limits the rate at relative light intensity 16 in 0.01 mol dm⁻³ sodium hydrogencarbonate?

   [1 mark]

   • ALight intensity
   • BNumber of chloroplasts formed during the experiment
   • COxygen concentration
   • DCarbon dioxide concentration

   (c)

   If the carbon dioxide supply were removed completely while the light stayed on, photosystem II would eventually stop functioning. Explain why.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A suspension of Chlorella was kept in bright light with 1% carbon dioxide until the concentrations of Calvin cycle intermediates were steady: glycerate 3-phosphate (GP) was 2.0 and ribulose bisphosphate (RuBP) was 1.5 arbitrary units. The light was then switched off. Within 30 seconds GP had risen to 3.4 units and RuBP had fallen to 0.3 units. In a second experiment in continuous light, the carbon dioxide concentration was suddenly reduced from 1% to 0.003%.

   (a)

   Explain the changes in the concentrations of GP and RuBP when the light was switched off.

   [3 marks]

   (b)

   Predict, with reasons, the changes in GP and RuBP in the second experiment, and suggest why chloroplasts need a very high concentration of Rubisco.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Cyanobacteria were the first organisms to split water in photosynthesis, using photosystems located in internal membranes. Rock evidence suggests that oxygen began to accumulate in the oceans and atmosphere about 2.4 billion years ago. Between about 2.5 and 1.8 billion years ago, huge layers of iron oxide known as banded iron formations were deposited on the sea floor, and later the atmospheric oxygen concentration rose to levels that allowed an ozone layer to form.

   (a)

   Explain how light energy absorbed by the photosystems in cyanobacteria is used to produce oxygen, ATP and reduced NADP.

   [6 marks]

   (b)

   Discuss the consequences of the evolution of oxygen generation by photolysis for living organisms and for geological processes on Earth.

   [6 marks]

   Total for question 4: 12 marks

End of questions