Decomposition and carbon recyclingEdexcel International A Level Biology: Subtopic test

1. 1
   A gardener makes a compost heap from grass cuttings, vegetable peelings and fallen leaves. After eight weeks the material has become a dark, crumbly substance, and the heap has been noticeably warm throughout.

   (a)

   Which organisms are mainly responsible for the breakdown of the material in the compost heap?

   [1 mark]

   • AProducers
   • BPrimary consumers
   • CSaprobiotic bacteria and fungi
   • DPathogenic viruses

   (b)

   Why has the compost heap become warm?

   [1 mark]

   • APhotosynthesis in the microorganisms releases heat
   • BMicroorganisms release heat during respiration
   • CThe microorganisms are dividing by meiosis
   • DWater in the heap is condensing on the leaves

   (c)

   Explain how fungi in the heap digest the dead plant material.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student buries identical masses of dry leaves in mesh bags in soil kept equally moist at four different temperatures. After four weeks she measures the percentage loss in mass of the leaves: 4% at 5 degrees Celsius, 13% at 15 degrees Celsius, 32% at 25 degrees Celsius and 21% at 35 degrees Celsius.

   (a)

   What was the mean percentage loss of mass per week at 25 degrees Celsius?

   [1 mark]

   • A0.8%
   • B4.0%
   • C32%
   • D8.0%

   (b)

   Why was the loss of mass lower at 35 degrees Celsius than at 25 degrees Celsius?

   [1 mark]

   • AEnzymes of the decomposers begin to denature above their optimum temperature
   • BDecomposers do not respire at 35 degrees Celsius
   • CThe leaves contain more cellulose at 35 degrees Celsius
   • DEnzymes work faster at 35 degrees Celsius, so there is less substrate

   (c)

   Explain why the leaves lost mass during the investigation.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In a woodland, carbon dioxide from the atmosphere is incorporated into the leaves of trees. When the leaves fall, they are broken down in the soil by bacteria and fungi. In a waterlogged peat bog nearby, dead plant material has built up over thousands of years to form deep layers of peat.

   (a)

   Describe how carbon in atmospheric carbon dioxide is incorporated into a leaf and later returned to the atmosphere after the leaf has fallen.

   [3 marks]

   (b)

   Explain why dead plant material builds up as peat in the waterlogged bog, and why draining the bog would increase the amount of carbon dioxide in the atmosphere.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A farmer compares two fields after harvest. In field A the crop residues are ploughed into the soil. In field B the residues are removed and burned. After five years, the soil in field A has a higher content of organic matter and contains more bacteria and fungi, and yields remain high. The soil in field B has lost organic matter and needs more fertiliser to keep yields up.

   (a)

   Explain how microorganisms in field A recycle the carbon in the crop residues.

   [6 marks]

   (b)

   Evaluate the effect on the carbon cycle and on soil fertility of burning crop residues rather than ploughing them into the soil.

   [6 marks]

   Total for question 4: 12 marks

End of questions