Nutrient cyclesIB MYP Biology: Subtopic test

1. 1
   A pine forest in Finland takes in carbon dioxide from the air all summer. In autumn the needles and branches fall to the forest floor, where fungi and bacteria slowly break them down. Not far away, a power station burns coal to generate electricity.

   (a)

   Which process removes carbon dioxide from the air in the forest?

   [1 mark]

   • ARespiration by the pine trees
   • BCombustion of the coal
   • CDecomposition of the pine needles
   • DPhotosynthesis by the pine trees

   (b)

   Which process returns carbon dioxide to the air when the fallen needles are broken down?

   [1 mark]

   • APhotosynthesis by fungi and bacteria
   • BRespiration by fungi and bacteria
   • CCombustion of the needles by fungi
   • DFossilisation of the needles within a year

   (c)

   Explain how carbon in the carbon dioxide of the air can become part of a deer that feeds on the plants in the forest.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A farmer in Kenya grows maize for several years in the same field, and the yields fall each year because the soil runs short of nitrates. A neighbour grows beans (a legume) in the field every third year. Bean roots have small swellings called nodules that contain bacteria.

   (a)

   What do the bacteria in the nodules of the bean roots do?

   [1 mark]

   • AThey convert nitrates in the soil into nitrogen gas
   • BThey break down dead plants and release carbon dioxide
   • CThey convert nitrogen gas from the air into nitrogen compounds that plants can use
   • DThey convert water into oxygen for the roots

   (b)

   In waterlogged soil, denitrifying bacteria are common. What do they convert nitrates into?

   [1 mark]

   • ANitrogen gas
   • BMore nitrates
   • CAmmonia
   • DProteins

   (c)

   Explain why growing beans in the field every third year helps the maize crop that follows.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student investigates how temperature affects the speed at which decomposers break down dead leaves. She places 20 g of dried leaf litter and 100 g of moist soil in each of four sealed jars. She keeps the jars at 5 °C, 15 °C, 25 °C and 35 °C for four weeks, then measures the mass of leaf litter that remains. The percentage of the leaf mass lost was 2% at 5 °C, 9% at 15 °C, 21% at 25 °C and 12% at 35 °C.

   (a)

   Identify the independent variable, the dependent variable and two control variables in this investigation.

   [3 marks]

   (b)

   Describe the trend in the results and explain it. Evaluate how well the method allows the best temperature for decomposition to be found.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A government in a tropical country plans to clear a large area of rainforest to create farmland and grazing pasture for cattle. The forest is part of a region where most of the rain falls on the same land from which water has evaporated or been released by the trees. Farmers and workers in the area depend on the rain, and a river runs through the region.

   (a)

   Explain how clearing large areas of rainforest could reduce the rainfall in the region and increase the risk of flooding in the river.

   [6 marks]

   (b)

   Discuss and evaluate the government's plan to clear the rainforest for farmland and pasture.

   [6 marks]

   Total for question 4: 12 marks

End of questions