THE REQUIREMENTS OF LIFE
Energy is the first essential component for living systems. It is available as;
- radiation from the sun
- geothermal energy (eg volcanoes, hot springs)
- energy in the bond between oxygen and hydrogen in water
- found in biological materials
Energy, unlike nutrients, is not recycled in an ecosystem.
(Refer Fig 44.1 pg 979 Knox et al)
Carbon is the second requirement of living things. It enters the ecosystem through photosynthesis. So photosynthesis carries out the dual role of capturing radiant energy and then using that energy to assemble sugars from carbon dioxide and water.
H20 + CO2 + PHOTOSYNTHESIS = CH2O (+02)
Carbon is not in short supply, and when in association with other molecules, it is very soluble in water. Ecosystems do not suffer from a lack of carbon.
Nitrogen is one of the components of protein. Organisms are made up of proteins.
Although nitrogen gas is abundant in the atmosphere, most organisms which carry out photosynthesis do not have the ability to incorporate this nitrogen. Conversion of atmospheric nitrogen into a biologically accessible form is called Nitrogen Fixation, and is carried out by a relatively small number of micro-organisms, mostly bacteria.
In some ecosystems, nitrogen will be in short supply, and will influence the distribution of organisms.
Phosphorus is an important component of ATP (the compound used to convey energy in cells) and nucleic acids (DNA and RNA).
Phosphorus is not in rich supply and is not easily dissolved in water. Plants can have difficulty obtaining it, and it can be a limiting factor in some ecosystems.
Water has many uses in living organisms including;
- nutrient uptake
- absorption and release of respiratory gases
- excretion of some compounds (eg nitrogen)
- providing hydrogen for photosynthesis
97 % of the Earth's water is oceanic. It can be in very low supply on land.
A food chain is a simple representation of the movement of nutrients in an ecosystem. In most ecosystems, food chains are short because the number of links or levels in a food chain depends on the productivity of the system.
(Refer Fig 44.9 pg 984 Knox et al)
Food chains begin with producers (autotrophs), which support a community of herbivorous consumers, which in turn may support a layer of carnivorous consumers (both heterotrophs).
An ecological pyramid is another representation of a food chain. It can also be used to represent biomass or energy in an ecosystem. Energy pyramids will always have a true "pyramid" shape because energy does not cycle through ecosystems. It is eventually converted to heat and leaves the system.
(Refer Fig 44.2a, 44.3b, 44.4a pgs 980 81 Knox et al)
Food webs are a series of interlinked food chains. They reflect more of the complexity of interactions in an ecosystem. For example, omnivorous animals can be seen as both primary and secondary consumers, or the importance of decomposers in recycling material can be more clearly illustrated.