Life cycle of carbon and nitrogen in our environment

Decomposer are organism that breaks down dead organic matter and recycles it. Saprobiotic nutrition, used by organisms such as fungi to obtain nutrients from nonliving organic matter using. Extracellular digestion extracellular digestion organisms excrete enzymes to release nutrients from food and then absorb these nutrients through their cell walls.

Th main processes involved in cycling carbon through ecosystems are:

• photosynthesis – the process that fixes carbon atoms from an inorganic source (carbon dioxide) into organic compounds (for example, glucose)

• feeding and assimilation – feeding passes carbon atoms already in complex molecules to the next tropic level in the food chain where they are assimilated into (become part of) the body of that organism

• respiration – this releases inorganic carbon dioxide from organic compounds

• fossilization – sometimes living things do not decay fully when they die due to the conditions in the soil, and fossil fuels (for example, coal, oil and peat) are formed

• combustion – fossil fuels are burned, releasing carbon dioxide into the atmosphere

Cycling carbon is essential to the living world as all the organic molecules found in living organisms are based on carbon. We often talk about the Earth having ‘carbon-based life forms’.

What are the main stages in the nitrogen cycle?

Nitrogen is found in many biological compounds. It is present in proteins, amino acids, DNA, RNA (all kinds) and adenosine triphosphate (ATP) as well as ADP. Without nitrogen, organisms could not synthesise:

• their genetic material (DNA)

• their principal structural materials (proteins)

• their principal energy transfer molecule (ATP)

Th main processes in the cycle are: • plants absorb nitrates from the soil

• the nitrates are then used to form amino acids, which are used to synthesize proteins

• the plants are eaten by animals, the proteins digested and the amino acids absorbed and assimilated into animal proteins

• both plants and animals die, leaving a collection of dead materials (detritus) which contain the nitrogen still fixed in organic molecules; in addition, excretory products such as urea also contain nitrogen

• Decomposers decay the excretory products and detritus, releasing ammonium ions (NH4+) into the soil; this process is often referred to as ammonifiation

• Nitrifying bacteria oxidize the ammonium ions to nitrates (NO3–) (which are taken up by the plants) in a process called nitrification; in this process there is an intermediate product called nitrite (NO2–)

Remember redox reactions?
Th conversion of ammonium ions to nitrite ions is an oxidative process involving redox reactions, because the nitrogen atoms in the original ammonium ion:
• gain oxygen atoms and
• lose hydrogen ions
as the simple equation below shows

The hydrogen ions lost must go somewhere and the oxygen atoms must have come from somewhere, so something else is being reduced. In actual fact, it is a much more complex reaction as the next equation shows! You will not have to
remember this equation; it is just to illustrate how complex the reactions are and where the oxygen atoms come from and where the hydrogen ions go to.

The hydrogen ions reduce hydrogen carbonate ions to carbonic acid. The oxygen atoms that join the nitrogen atom are themselves reduced in the reaction.
Reduction and oxidation always occur together.

Denitrifying bacteria reduce nitrate to nitrogen gas that escapes from the soil. This decreases the total amount of nitrogen available to the plants, and, therefore, to all the other organisms also.

Nitrogen-fixing bacteria ‘fix’ nitrogen gas into ammonium ions. This happens in two main situations:

• Nitrogen-fixing bacteria free in the soil (belonging to the genera Azotobacter and Klebsiella) reduce nitrogen gas into ammonium ions in the soil. These ammonium ions can be oxidized immediately into nitrates by nitrifying bacteria, adding to the amount of nitrogen available to the plants and, therefore, the other organisms also.

• Nitrogen-fixing bacteria in nodules on the roots of legumes (belonging to the genus Rhizobium) form ammonium ions that are passed to the legumes and used by them to synthesize amino acids. Th extra nitrogen only becomes available to other organisms when the legumes die and are decomposed.

At the moment an immense amount of research into the genetics of nitrogen fixation is being carried out. Th aim is to isolate the genes that control nitrogen fixation and transfer them by genetic engineering into other cells. Or persuading bacteria like Rhizobium to form symbiotic associations with other species of crop plants. If all the cereal plants that are grown in the world had nitrogen-fixing bacteria in their roots or if their own cells could fi nitrogen, crop yields in countries with extreme food shortages would be hugely increased. And the plants would not need nitrogen fertilizer – they would make their own! Transferring this ability to other plants would have a huge impact on our ability to feed the planet.

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