1. 1
   A gardener sets up three identical compost bins in her garden, each containing the same mass of chopped vegetable waste. Bin 1 is kept warm and moist, Bin 2 is kept cold and moist, and Bin 3 is kept warm but very dry. After four weeks she checks how much the vegetable waste in each bin has broken down.

   (a)

   Which type of organism is mainly responsible for breaking down (digesting) dead plant material during decay?

   [1 mark]

   • AMicroorganisms
   • BGreen plants
   • CHerbivores
   • DCarnivores

   (b)

   Based on the conditions described, which bin would you expect to show the fastest rate of decay?

   [1 mark]

   • AAll three bins would decay at the same rate
   • BBin 2 (cold and moist)
   • CBin 3 (warm and dry)
   • DBin 1 (warm and moist)

   (c)

   State one condition, other than temperature and moisture, that increases the activity of decay microorganisms, and explain why the gardener would want the compost to decay quickly.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A diagram-free description of a woodland carbon cycle is given to a class: trees absorb carbon dioxide from the air; some of this carbon becomes part of the trees' fats and proteins; a deer eats leaves from the trees and some of the carbon becomes part of the deer's body; when the deer dies, its body is broken down by microorganisms in the soil; a nearby factory also burns wood from fallen branches for fuel.

   (a)

   By which process is carbon dioxide removed from the atmosphere by green plants and algae in the carbon cycle?

   [1 mark]

   • ARespiration
   • BPhotosynthesis
   • CCombustion
   • DDecay

   (b)

   When the deer's body is broken down by microorganisms after it dies, how is carbon released back into the atmosphere?

   [1 mark]

   • AAs carbon dioxide, through photosynthesis by the microorganisms
   • BAs oxygen, through the death of the deer's cells
   • CAs carbon dioxide, through respiration by the microorganisms
   • DCarbon is not released back into the atmosphere during decay

   (c)

   Describe how the factory burning wood from fallen branches affects the carbon cycle, and name this process.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Scientists studying a stable grassland community measure the total mass of carbon stored across all the plants, animals and soil in the area every year for ten years. They find that although individual organisms are constantly growing, reproducing, dying and decaying, the total carbon stored in the community stays roughly the same from year to year.

   (a)

   Scientists studying a stable grassland community find that the total amount of carbon stored in the plants, animals and soil stays roughly constant from year to year, even though individual plants and animals are constantly growing, dying and decaying. Explain, in terms of the carbon cycle, why the total amount of carbon in this community stays roughly the same over time.

   [3 marks]

   (b)

   The scientists then find a nearby grassland where a housing development has removed most of the plants and animals, and observe that the soil there now contains far less carbon than before. Suggest an explanation for this decrease, and predict what might happen to the amount of carbon dioxide in the atmosphere directly above this cleared area compared with the stable grassland.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A farmer stores two identical piles of grass cuttings of the same starting mass. Pile A is left loosely spread out in the open air, exposed to rain and air. Pile B is compacted tightly and covered with a waterproof sheet, excluding most air. After several weeks, Pile A has visibly shrunk and rotted down into dark, crumbly material, while Pile B is still largely intact and has a strong, unpleasant smell.

   (a)

   A farmer stores two identical piles of grass cuttings. Pile A is left loosely spread out in the open air, exposed to rain and air. Pile B is compacted tightly and covered with a waterproof sheet, excluding most air. After several weeks, Pile A has visibly shrunk and rotted down into dark, crumbly material, while Pile B is still largely intact and has a strong, unpleasant smell. Using your knowledge of decay and the carbon cycle, explain the difference between the two piles, including the role of microorganisms and the conditions needed for decay.

   [6 marks]

   (b)

   The farmer wants to find out how much faster Pile A decays compared with Pile B. Describe an investigation he could carry out to measure and compare the rate of decay of the two piles over time, including what he would measure and how he would make the comparison valid.

   [6 marks]

   Total for question 4: 12 marks

End of questions