Biomass and productivityAQA A-Level Biology: Subtopic test

1. 1
   An ecologist estimates the biomass of a meadow. She cuts all the vegetation from several 1.0 m² quadrats, dries it in an oven at 80 °C and reweighs it at intervals until the mass stops changing. The mean dry mass of vegetation per quadrat is 240 g.

   (a)

   Why is dry mass preferred to fresh mass as a measure of the biomass of the meadow?

   [1 mark]

   • ADry mass includes the mass of water, which stores chemical energy
   • BDrying increases the mass of carbon in the sample
   • CThe water content of plants varies, so dry mass is a more reliable measure of the organic material
   • DDry mass can be measured without killing the plants

   (b)

   Why does the ecologist reweigh the sample until the mass stops changing?

   [1 mark]

   • ATo show that all the water has been removed
   • BTo show that the organic compounds have been burned off
   • CTo show that the plants have stopped respiring
   • DTo allow the mean dry mass to be calculated

   (c)

   The ecologist's quadrats are 1.0 m² and 1 ha = 10 000 m². Calculate the mean dry mass of the meadow vegetation in kg ha⁻¹.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student measures the chemical energy stored in dried meadow plants using calorimetry. She burns 0.50 g of dried, ground plant material beneath a boiling tube containing 100 g of water. The temperature of the water rises by 19.0 °C. The specific heat capacity of water is 4.2 J g⁻¹ °C⁻¹.

   (a)

   What does the rise in temperature of the water measure?

   [1 mark]

   • AThe mass of carbon in the plant material
   • BThe percentage of water in the plant material
   • CThe rate of respiration of the plant material
   • DThe chemical energy released when the dry biomass is burned

   (b)

   Which would make the student's calorimetry value lower than the true energy store?

   [1 mark]

   • ABurning the sample completely
   • BHeat lost to the surroundings instead of being transferred to the water
   • CStirring the water during heating
   • DReading the thermometer to 0.1 °C

   (c)

   Calculate the energy released per gram of dried plant material, in kJ g⁻¹.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A conifer plantation of area 1 ha has a gross primary production (GPP) of 21 000 kJ m⁻² year⁻¹. The trees lose 8 400 kJ m⁻² year⁻¹ to the environment as a result of respiration. A second plantation (B) of the same area has the same GPP but a net primary production (NPP) of 15 750 kJ m⁻² year⁻¹.

   (a)

   Calculate the net primary production (NPP) of the first plantation in kJ m⁻² year⁻¹, and state what this energy is available for.

   [3 marks]

   (b)

   Calculate the respiratory loss in plantation B and the percentage of its GPP that this represents. Suggest two reasons why a smaller percentage of GPP is lost in respiration in plantation B than in the first plantation.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A research team measures the productivity of a crop of sugar beet over one growing season. They harvest plants from 1 m² plots at intervals, dry them to constant mass and use calorimetry to estimate the chemical energy store in the dried biomass.

   (a)

   Explain how the sugar beet plants produce biomass, and how their net primary production is related to their gross primary production.

   [6 marks]

   (b)

   Describe how the team could estimate the chemical energy store in the sugar beet in kJ m⁻² year⁻¹, and explain why the value from simple calorimetry is likely to be an underestimate.

   [6 marks]

   Total for question 4: 12 marks

End of questions