3.5.3 energy and ecosystems
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what do plants do in any ecosystem?
synthesise organic compounds from atmospheric or aquatic carbon dioxide
what are the sugars synthesised by plants used for?
• most are used by the plants as respiratory substrates • the rest are used to make other groups of biological molecules which form the biomass of the plants
how can biomass be measured?
in terms of mass of carbon or dry mass of tissue per given area
how can the chemical energy store in dry biomass be estimated?
using calorimetry: 1. water is removed from the sample 2. the biomass is burned 3. the energy released is used to heat a known volume of water 4. the change in temperature is used to calculate the chemical energy in the biomass
what is gross primary production (GPP)?
the chemical energy store in plant biomass in a given area or volume (total energy from photosynthesis)
what is net primary production (NPP)?
the chemical energy store in plant biomass after respiratory losses to the environment have been taken into account (available to plants to create new biomass and pass to next trophic level)
what do GPP and NPP do?
quantify how productive an ecosystem is
what does the productivity of an ecosystem depend on?
• abiotic and biotic factors - more water, light, warmth and green plants maximise rates of photosynthesis • this means more carbohydrates are produced in plants
how do you calculate net primary production?
NPP = GPP - R where GPP = gross production and R = respiratory losses to the environment
what is net primary production available for?
• plant growth and reproduction • also available to other trophic levels in the eocsystem eg. herbivores and decomposers
how can the net production (N) of consumers like animals be calculated?
N = I - (F + R) where I = chemical energy store in ingested food, F = chemical energy lost to the environment in faeces and urine and R = respiratory losses to the environment
what is primary and secondary productivity and how is it measured?
• the rate of primary or secondary production, respectively • measured as biomass in a given area in a given time eg. kJ ha^-1 year^-1
why do the units for primary and secondary productivity have 3 components?
• kJ - unit for energy • per unit area - standardises results so environments can be compared (vary in size) • per year - seasons affect abiotic factors like heat and light, so annual average allows comparison between environments
what are the efficiencies of energy transfer through a food chain?
• sun to producer = 2% • producer to primary consumer = 5-10% • primary consumer to secondary consumer = 10-15% • secondary consumer to tertiary consumer = 10-15%
what are the reasons for the low efficiency of energy transfer between the sun and producers?
• not all wavelengths are absorbed by pigments • some light is reflected • not all of the light falls on photosynthetic parts of the plant
what are the reasons for the low efficiency of energy transfer between trophic levels?
• some parts of the organism are not eaten • some food is not digested eg. faeces
why is the efficiency of energy transfer less between the producer and primary consumer?
there is more indigestible biomass in producers