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Summary Notes on Energy and ecosystems - AQA A Level Biology
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3.5.3 Energy transfer in ecosystems – AQA A Level Biology Summary NotesIn any ecosystem, plants synthesise organic compounds from the atmospheric, or aquatic, carbon
dioxide
Most of the sugars synthesised by plants are used as respiratory substrates. The rest are sued to
make other groups of biological molecules. These biological molecules form the biomass of the plant
Food chains tend to start with plants = Producers (Autotrophs)
- Autotrophs are organisms which can make organic compounds (ie carbon containing
compounds – carbs, lipids and proteins) from carbon dioxide
- So basically they make their own food
- Usually green plants or algae because they can photosynthesise
Plants “fix” carbon by carrying out photosynthesis to produce sugars such as glucose.
This glucose can be used as a respiratory substrate so plants can create ATP
Remaining glucose is used to make other biological molecules which ultimately allows a plant to
grow and gain mass
**ESSAY – can always link respiration to ATP – growth AND photosynthesis to coverting TP into
many biological molecules eg amino acids, RNA, DNA, surcrose….
Incident energy = amount of energy absorbed for photosynthesis
Biomass can be measured in terms of mass of carbon or dry mass of tissue per given area
A plants growth can be tracked by measuring changes in its biomass
Biomass can be defined in two different ways:
1. Mass of carbon contained in a given area
2. Dry mass of tissue contained in a given area
“Given area” may refer to either an area of land the plants are grown in or per section of a plant
Some ways which biomass may be recorded
1. 50 (This means the plant contains 50 grams per meter squared it is grown in)
2. 10g leaf (This means each of the plants leaves has a mass of 10g)
Scientists measure DRY mass of tissue, rather than simply mass of tissue BECAUSE water content
varies constantly which would affect fresh mass. Therefore, removing water as a factor allows us to
make a valid comparison between mass of areas as changing water content will not affect dry mass.
An increase in dry mass only shows the increase in organic material
Method for finding the dry mass of a biological sample:
1. Hear the sample to a high temperature that causes water to evaporate, but doesn’t burn the
sample (eg 100 degrees)
2. Weigh the sample and heat again
3. Repeat the process until the weight remains constant
, The chemical energy store in dry biomass can be estimated using calorimetry
Burning crisps practical from GCSE. This is the method:
1. Measure the mass of a fixed volume of water and place in a bomb calorimeter
2. Burn the organic material in a bomb calorimeter until it is fully combusted. The chemcial
energy stored in dry biomass is used to raise the temperature of the water
3. Measure the water temperature before and after heating and calculate the change in
temperature
4. Calculate the energy released from the organic matter using this equation:
Energy released (J) = Mass of water (g) x Temperatrure rise ( ) x 4.2( )
Energy released per gram of organic material = above calculation/dry mass of sample burned
Why this experiment may not give accurate results:
1. Not all the organic material being burned (eg due to lack of oxygen)
2. Not all chemcial energy stored in dry biomass being converted to heat energy (eg some
converted to light)
3. Not all heat energy released being transferred to the water (eg some lost to surroundings
due to lack of insulation)
The use of a bomb calorimeter helps to ensure accurate results:
1. They are fully insulated to prevent heat loss to the surroundings, so all energy from the food
is used to heat the water
2. There is an oxygen inlet to ensure complete combustion of the food sample
3. A stirrer is present to ensure heat is evenly distributed in the water that is having its
temperature measured
Could also gain more accurate results by
4. Using a large mass of water. This would result in less uncertainty whilst measuring the mass
of water
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