Unit 5 - Energy transfers in and between organisms
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AQA A Level Biology - Energy and Ecosystems (2021-22)
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Unit 5 - Energy transfers in and between organisms
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AQA
An in-depth informative booklet on Energy and Ecosystems including nutrient cycles, such as the nitrogen and the phosphorous cycle and succession. Also includes useful summary and exam style questions and the specification related to this topic.
Unit 5 - Energy transfers in and between organisms
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Year 2
ENERGY AND ECOSYSTEMS
NUTRIENT CYCLES
SUCCESSION
(3.5.3, 3.5.4, 3.7.4)
Name:
Teacher:
,3.5.2 ENERGY AND ECOSYSTEMS
Producers: Plants and algae photosynthesise. This means that they can synthesise glucose (a
complex organic molecule) from water and carbon dioxide. Light energy is needed to make this
reaction happen. The light energy is ultimately stored in the bonds of the glucose molecules as
chemical potential energy.
6CO2 + 6H20 C6H12O6 + 6O2
CO2 + H20 and
waste heat energy
Photosynthesis
Biomass:
Definition: Simply, Biomass is all the biological molecules made by the plant and are not
immediately used for respiration. This will include cellulose, starch, proteins, lipids etc. Even
more simply, it represents the growth of the plant
How is it measured?
Explanation for Units:
As Mass of Carbon (in the biological molecules)
So that we can fairly compare the
As Dry mass (kgm-2yr-1) mass of new plant - water
biomass produced in different
As an Energy value (kJm-2yr-1) – the molecules that make habitats we need to take account
up the biomass contain stored energy in their bonds. This of:
energy can be measured.
the size of the area it has
Practical Estimation of dry mass come from (m-2)
and also the time period
1 Heat in a low temperature oven to evaporate water in the in which it was produced
tissues (yr-1) – remember that
2 Until there is no further change in mass. growth rates of plants
3 Calculate dry biomass in kgm-2y-1 vary with time of the year
4 Carbon content estimated as 50% of dry mass so it is useful to take
account of a whole year’s
Q: Why use dry biomass rather than wet biomass? growth.
A: Hence kgm-2yr-1 or kJm-2yr-1
,Practice Calculation
Time /hr Mass of plant 1. How long did it take to remove the water from the plant
tissue /g tissues?
2. Why is this only an estimate?
0 2300
1 2000 3. What was the final dry biomass?
The plant tissue (grass clippings) was removed from an area of
2 1600
3.5 m2 over the course of 8 months
3 1100 4. Calculate the dry biomass in kgm-2y-1
4 500
5. Calculate the mass of carbon in kgm-2y-1
5 200
6 200 6. What assumption do your answers to Qs 4 and 5 make?
Energy in Dry Biomass
Biological molecules store chemical potential energy. To find out how
much, we set it on fire!
Dried biomass from a known area is placed in a calorimeter
The dry biomass is then burnt (combustion)
The energy released is used to heat water
The change in water temperature is used to calculate the energy
released.
Energy per square metre per year is calculated (kJ m-2y-1)
Net Primary Production:
Gross Primary Production (GPP) - the total amount of light energy captured in photosynthesis
and stored in molecules as chemical potential energy.
Respiration (R) - the amount of energy lost from the plant in respiration. This returns to the
environment as waste heat
Net Primary Production (NPP) - this is the remaining amount of chemical potential energy
stored in the biomass of the plant. This can be used for growth and reproduction.
, Important equation: NPP= GPP - R
Untrapped light energy: Only about 1-5% of the light energy falling on a leaf is captured in
photosynthesis.. This is for several reasons:
Some light is reflected from the surface of the leaf.
Only certain wavelengths of light are absorbed by chlorophyll.
Light may not strike a chloroplast / chlorophyll and pass straight through the leaf.
Some other factor may be limiting e.g. carbon dioxide, meaning the light energy can’t be
used.
Some may be absorbed but transferred into thermal energy i.e. it simply warms the
plant.
NB: it is not entirely clear whether the section above (untrapped light energy) is on the
specification – it has in previous specifications so it’s here just in case.
Why is NPP important?
This is the energy in the new biomass made by the plant. It can be used to make new cells and
tissues for growth and for reproduction = seeds etc. It is also the part of the light energy
captured by the plant that, through the tissues of the plant being eaten, can be transferred to
consumers.
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