In-depth summary for content covered in the Energy Transfers and Nutrient Cycles topic of A-level AQA Biology. This will still be applicable to other exam boards, but take caution when looking at key-words and the order of specific processes.
Organisms in ecosystems rely on a source of energy. The main source of this is sunlight – conserved
as chemical energy by plants via photosynthesis.
-> This chemical energy is most commonly in the form of biomass (bio-molecules such as glucose)
Food chains and Energy transfer:
Organisms can be divided into three groups
-> Based on how they obtain their nutrients
Producers / Autotrophs – are photosynthetic organisms that manufacture organic
substances using light energy, water, carbon dioxide and mineral ions
Consumers – are organisms that feed on other organisms to obtain their nutrients, split into
primary, secondary and tertiary
Primary consumers: Eat producers (plants)
Secondary consumers: Eat primary consumers
Tertiary consumers: Eat secondary consumers
Saprobionts (decomposers) – are a group of organisms that break down the complex
materials in dead organisms into simple ones; recycle nutrients (e.g. fungi or bacteria)
A food chain is a feeding relationship involving these organisms – with each stage in consumption
being described as a trophic level; with arrows showing energy flow
However, food webs show how food chains link together within a habitat – however
complexity is a limit to their usefulness
Biomass:
Biomass is the total mass of living material in a specific area
-> this is easy to obtain using samples but variance of water often affects results (wet mass)
Therefore carbon / dry mass is used for a more accurate or repeatable test
However, this involves killing the organisms, meaning a small sample must be taken for
ethical reasons – making calculation less representative
Biomass is therefore given the units gm -2 or gm-3
-> via Biomass = Dry mass / Area or Volume
The chemical energy stored within the dry mass can be estimated using calorimetry.
In bomb calorimetry a sample of the material is burnt in pure oxygen within a
sealed chamber
The heat of combustion causes the water around to heat up slightly
The energy transfer can then be calculated using specific heat capacity of
water (4.18 JKg-1)
Figure 1 – reproduced from [1]
Energy Transfer:
, Oliver Dyson
The sun is the source of energy for ecosystems – however as little as 1-3% of energy is actually
transferred to photosynthetic plants. This is due to:
Over 90% is reflected back into space by clouds and dust, or absorbed by gases in the
atmosphere
Not all wavelengths of light can be absorbed and used for photosynthesis
Light may not fall on a chlorophyll molecule
Factors such as low CO2 levels may limit the rate of photosynthesis
The total quantity of chemical energy stored by plants as biomass in a given area or volume, in a
given time, is called gross primary production (GPP). However, since 20-50% of this energy is used in
respiration, taking this into account gives a value called net primary productivity (NPP).
NPP = GPP – R (where R is respiratory losses)
NPP is therefore available for the other trophic levels to access, but usually only 10-20% is
transferred effectively;
Not all of the organism is consumed
Some parts are consumed but cannot be digested
Energy can be lost in excretory material such as urine of faeces
Some energy losses occur due to heat from respiration
The net production for consumers is given by:
N = I – (F+R)
Where I represents the chemical energy store of ingested food, F represents the loss through faeces
and urine and R is energy lost via respiration.
Food chains;
Energy flows can be observed in food chains via the arrows connected each organism.
Most food chains only have 4-5 trophic levels since insufficient energy for a breeding
population is supplied as they increase
The total mass of organisms is lower at higher trophic levels
The total amount of energy is less at each level
Some methods are used in farming to reduce these energy losses – such as restricting movement so
there will be less heat loss from respiration, heating the area for the same reason and optimally
feeding to reduce wastes.
Figure 2 – reproduced from [1]
(13.3) Nutrient Cycles
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