Energy transfer in ecosystems (5.10, 5.11)
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Gross primary productivity (GPP) is the rate at which an ecosystem converts energy from sunlight into chemical energy.
The units for GPP are usually units of energy per area per year, e.g.,
Some of the products of photosynthesis will be used for respiration to release energy required for cell processes, such as active transport. The other products of photosynthesis are converted into biomass in the form of molecules such as DNA, polysaccharides, phospholipids and proteins.
Net primary productivity (NPP) is the rate at which energy from sunlight is converted into chemical energy in the form of organic molecules, which make up the plant biomass.
NPP = GPP – respiration
The units for NPP are usually the same as the units for GPP.
Biomass (the total mass of living material in an area) decreases at each trophic level of a food chain. As a result, less biomass is available to support organisms at higher trophic levels. This is why most food chains only have four or five trophic levels.
Biomass is lost between trophic levels through:
- Respiration: energy is released and lost as heat.
- Excretion: waste products are not passed to the next trophic level.
- Heat loss: energy is lost to the surroundings during metabolic processes.
- Uneaten or indigestible material (bones, roots, etc.).
The producer to primary consumer energy transfer is very inefficient. This is because not all food will be eaten, undigested food will be excreted in waste and a large proportion of the energy absorbed by the consumer is used in respiration.
The energy transfer between primary and secondary consumers is often more efficient. This is because the majority of the primary consumer is eaten and the protein-rich diet is more easily digested, meaning less biomass will be lost as waste.
The efficiency of the energy transfer between trophic levels can be measured using the following equation:
This can also be written as:
Primary productivity is the rate at which energy is incorporated into organic molecules by the producers in an ecosystem.
Autotrophs (producers), synthesise their own organic compounds from inorganic substances. Most autotrophs do this by photosynthesis (using light energy). Some autotrophs are chemosynthetic autotrophs, which obtain energy from chemical reactions involving inorganic compounds.
Gross Primary Productivity (GPP) is the rate at which energy is incorporated into organic molecules by an ecosystem.
The units of GPP are energy per unit area per year, e.g.,
The percentage efficiency of photosynthesis can be calculated:
Net Primary Production (NPP) is the chemical energy stored in plant biomass after subtracting energy lost in respiration.
It represents the energy available for growth, reproduction, and for consumers in the food chain. It is calculated using the formula:
Where:
- = total energy fixed in photosynthesis.
- = energy lost through respiration.