A-Level Biology AQA Notes

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Biomass

Plants synthesise organic compounds from atmospheric, or aquatic, carbon dioxide.
Most of the sugars synthesised by plants are used as respiratory substrates. The rest are used to make other groups of biological molecules, forming the biomass of the plant.
Biomass is the total mass of living material in a specific area at a given time
Dry biomass shows the chemical energy store in an organism and can be measured by the process of calorimetry. A dry sample is weighed and burnt in pure oxygen within a sealed chamber, the temperature increase of the fixed volume of water is used to calculate the energy released.

Production & Productivity

Gross primary production (GPP) is the total quantity of chemical energy stored in plant biomass, in a given area or volume.
Net primary production (NPP) is the chemical energy store in plant biomass after respiratory losses to the environment have been taken into account
- NPP = GPP – R
- Where R represents respiratory loses to the environment
- NPP, GPP & R use units of (kJ m^-2 yr^-1)
The NPP is available for plant growth and reproduction. It is also available for consumers in the food chain such as herbivores and decomposers.

Net production (N) is the total chemical energy consumers store after energy losses to faeces, urine and respiration have been taken away from the chemical energy store of the ingested plant food
- N = I - (F + R)
- Where N is net production, I represents the total chemical energy store in ingested food, F is the energy lost in faeces and urine, and R is energy lost to respiration. All use units (kJ m^-2 yr^-1)
Primary and secondary productivity is the rate of primary or secondary production, respectively. It is measured as biomass in a given area in a given time e.g. kJ ha^–1 year^–1
The percentage efficiency of energy transfer from one tropic level to another can be calculated as

Farming practices increase the efficiency of energy transfer to increase yields by:
- Reducing respiratory loses in a human food chain e.g. reduce movement of animals
- Simplifying food chains to reduce energy loss to non-human food chains e.g. killing weeds and pest using herbicides and insecticides

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