3.5.3 Energy and ecosystems

Describe how biomass is formed in plants. (3 marks)

- During photosynthesis, plants produce organic carbon compounds from atmospheric or aquatic carbon dioxide.

- Most sugars synthesised are used by the plant as respiratory substrates.

- The remaining sugars are used to make other biological molecules such as carbohydrates, lipids, and proteins to form biomass.

Define how biomass can be measured. (1 mark)

Biomass can be measured as the mass of carbon or dry mass of tissue per given area.

Describe how the dry mass of tissue can be measured. (3 marks)

1. Dry the sample in an oven at about 100°C to avoid combustion.

2. Weigh and reheat the sample at regular intervals.

3. Continue until the mass remains constant, showing that all water has been removed.

Explain why dry mass is more representative than fresh mass. (1 mark)

Water volume in fresh samples can vary, but it does not affect dry mass.

Describe how the chemical energy stored in dry biomass can be estimated. (4 marks)

1. By calorimetry.

2. A known mass of dry biomass is fully combusted.

3. The heat energy released warms a known volume of water.

4. The temperature increase in the water is used to calculate the chemical energy of the biomass.

Explain how features of a calorimeter enable valid measurement of heat energy released. (3 marks)

- A stirrer evenly distributes heat in the water.

- Air insulation reduces heat loss and gain to and from the surroundings.

- Water has a high specific heat capacity.

Define gross primary production (GPP) and net primary production (NPP). (2 marks)

- GPP is the chemical energy stored in plant biomass in a given area or volume over a given time.

- NPP is the chemical energy stored in plant biomass after respiratory losses to the environment have been subtracted.

State the formula for NPP. (2 marks)

- NPP = GPP − R.

- Where R is respiratory losses to the environment.

Explain the importance of NPP in ecosystems. (2 marks)

- NPP provides energy for plant growth and reproduction.

- NPP is also available to other trophic levels in the ecosystem, such as herbivores and decomposers.

Define primary or secondary productivity and state the units. (2 marks)

- The rate of primary or secondary production, respectively.

- kJ ha⁻¹ year⁻¹.

Explain why these units for primary or secondary productivity are used. (4 marks)

- Per unit area standardises results to allow comparison between environments of different sizes.

- Per year accounts for seasonal variation in biomass production.

- This makes measurements more representative.

- This also enables comparison between environments.

Explain why most light falling on producers is not used in photosynthesis. (3 marks)

- Light is reflected or is of the wrong wavelength.

- Light may miss chlorophyll, chloroplasts, or photosynthetic tissue.

- Carbon dioxide concentration or temperature may be a limiting factor.

State the formula for net production of consumers (N). (2 marks)

- N = I − (F + R).

- Where I = the chemical energy store in ingested food, F = the chemical energy lost to the environment in faeces and urine and R = respiratory losses to the environment.

State the formula for efficiency of energy transfer. (1 mark)

Energy or biomass available after transfer ÷ energy or biomass available before transfer × 100.

Explain why energy transfer between trophic levels is inefficient. (4 marks)

- Heat energy is lost via respiration.

- Energy is lost via parts of the organism that are not eaten, such as bones.

- Energy is lost via food that is not digested and is lost as faeces.

- Energy is lost via excretion, for example in the form of urea in urine.

Explain how crop farming practices increase energy transfer efficiency. (5 marks)

- Simplifying food webs reduces energy or biomass losses to non-human food chains.

- Herbicides kill weeds, reducing competition (e.g. for light) so more energy is available to create biomass.

- Pesticides kill insects (pests), reducing biomass loss from crops.

- Fungicides reduce fungal infections, allowing more energy to be used to create biomass.

- Fertilisers, such as nitrates, prevent poor growth due to lack of nutrients.

Explain how livestock farming practices increase energy transfer efficiency. (5 marks)

- Reducing respiratory losses within a human food chain increases energy available for biomass production.

- Restricting movement and keeping animals warm reduces energy lost as heat from respiration.

- Slaughtering animals while still growing or young ensures most energy is used for growth.

- Treating animals with antibiotics prevents energy loss due to pathogens.

- Selective breeding produces breeds with higher growth rates.