Section 5 ( Energy Transfers in between Organisms ), Topics 13
Energy constantly enters ecosystems via sunlight and is lost as heat. Heat cannot be
recycled, and the flow of energy is therefore in one direction. Provided that the sun
continues to supply energy to the Earth, the ecosystems are always stable. However,
nutrients do not have an extraterrestrial source, and there is limited availability of nutrient
ions in a usable form. It is therefore crucial that elements such as carbon, nitrogen and
phosphorus are recycled, and this flow is cyclic.
All nutrient cycles have one simple general sequence at their heart:
Nutrients are taken up by producers ( plants ) as simple inorganic molecules.
Producers incorporate the nutrient into complex organic molecules.
When the producer is eaten, the nutrient passes into consumers ( animals )
Nutrients pass through the consumers.
When consumers and producers die, their complex molecules are broken down by
saprobiontic microorganisms ( decomposers ) that release enzymes ( extracellular
digestion ), releasing the nutrients in its original inorganic form.
o The role of these saprobionts in nutrient cycles is to ensure that nutrients are
released for reuse in their inorganic form. Without them, nutrients would
become locked up in organic form and cannot be taken up by plants again.
Nitrogen Cycle
Living organisms require a source of nitrogen from which to manufacture biological
molecules such as Proteins ( Amino Acids ), Nucleic Acids ( Nucleotides, DNA ) and other
nitrogen containing compounds. Although 78% of the Earth's atmosphere contains
Nitrogen, they are pretty much useless as it is a diatomic triple bonded molecule. Plants take
up most of the nitrogen in the form of Nitrates ( NO3-) from the soil. These ions are absorbed
using active transport by the root cells. This is where plants obtain Nitrates, and animals
obtain such compounds by absorbing and digesting plants. Nitrate ions are very soluble and
easily go beyond the reach of plant roots. In ecosystems, these nitrates are restored by the
recycling of nitrogen containing compounds. In agriculture, fertilisers containing nitrogen
are used to increase the amount of Nitrates. When plants and animals die, the process of
decomposition happens, by which microorganisms replenish the nitrates concentration in
the soil.
There are 4 main processes in the Nitrogen Cycle:
Ammonification
Nitrification
Nitrogen Fixation
Denitrification
Assimilation ( absorption )
All of these require saprobiontic molecules.
, Nitrogen Fixation ( Nitrogen Fixing Bacteria )
Atmosphere nitrogen is converted to Ammonia by nitrogen fixing bacteria in the soil. These
bacteria form a mutualistic relationship with plants and inhabit their roots, increasing its
surface area for a quicker uptake of ions. The bacteria provide a source of ammonium ions
for the plants and the plant provides sugars from photosynthesis.
Nitrogen fixation happens during lightning storms, but the vast majority of this reaction
occurs within two types of nitrogen fixing bacteria.
Free Living Nitrogen Fixation Bacteria
o These bacteria reduce nitrogen to ammonia, which is then used to
manufacture amino acids.
Mutualistic Nitrogen Fixing Bacteria
o These bacteria live in nodules on the roots of plants. They obtain
carbohydrates from the plant, and the plant obtains amino acids from the
bacteria.
Ammonification ( Saprobiontic Organisms )
Ammonification is the production of Ammonia from organic nitrogen containing
compounds. In nature, these compounds include urea ( from the breakdown of excess
amino acids ) and proteins, nucleic acids, vitamins ( found in faeces and dead organisms ).
Saprobiontic microorganisms mainly fungi and bacteria, feed on faces and dead organisms,
