The main elements that enter in organic matter composition of all organisms are carbon (C), hydrogen (H), oxygen (O) and nitrogen (N). Water and other minerals, like phosphorus (P), sulfur (S), calcium (Ca), sodium (Na), potassium (K) and magnesium (Mg), are also essential.
These elements pass through the non-living compartment - water, air and soil - and reach living organisms, coming back to non-living compartment after organisms death, what describes a cycle, known as Matter Cycle or Biogeochemical Cycle. This process is deeply related to food chains.
Unlike energy transfer, that flows to one direction, matter is continuously cycling: chemical elements are removed from environment, used by organisms and returned to environment.
Although we know that all nutrients are recycled, just cycles of main organic matter components will be presented.
Water Cycle
In nature water is found in three physical states: solid, liquid and gaseous. Most is found in liquid form on Earth surface, in seas, rivers and ponds, while a small part is present inside organisms and soil. Water cycle takes place under a rigorous balance, related to many factors, as the amount of vegetation that covers the soil, wind regime and presence of water bodies, like rivers and lakes.
Water is an essential element to metabolic activities, assuming fundamental paper in organic and inorganic substances transport, in plants structural maintenance, and in animal thermal regulation. Fresh water is found mainly in solid form, 75%, whereas 24% is in liquid form and the remaining as vapor.
Water vapor can be originated by water evaporation. In this case water from rivers and lakes of the forest, and in the surface of the soil, is transformed to vapor by solar radiation action. Water vapor colds and condenses forming clouds, returning to terrestrial surface in rain form. This water can reaches ground water and became available. Plants absorb water through their roots and the consumers absorb it directly, by drinking, or indirectly, through the consumption of plants and animals.
The vapor of water can also be formed through respiration, excretion and perspiration processes. After organism death, water comes back to environment through decomposers action.
Carbon Cycle
Carbon constitutes carbon dioxide molecules (CO2) present in air and water. It is absorbed during the photosynthesis, and is incorporated to organic matter by chemical reactions, being transported from one trophic level to another by food chain.
It returns to environment through respiration of producers, consumers and decomposers in CO2 form.
Fuels like coal and petroleum are important sources of carbon, because during its burning or combustion consumption occurs CO2 liberation.
Oxygen Cycle
Oxygen atoms in organic matter can have different origins, one of them through CO2 incorporation during photosynthesis.
In free form, oxygen (O2) is indispensable for aerobic respiration. In this case, it is removed from physical middle - air or water - and is used in the respiration process. Its replacement to physical middle occurs during photosynthesis, when the oxygen is liberated. Liberated photosynthesis oxygen comes from water molecules used in this process.
Carbon and oxygen cycles are deeply related because they are directly associated to photosynthesis respiration process.
Nitrogen Cycle
Nitrogen is a very important element for organisms, because it is essential in protein and nucleic acid construction.
It is very abundant in air (79% of air is constituted by nitrogen) but just few microorganisms are capable to absorb it in a process called nitrogen fixation. Bacteria, cyanobacteria and fungi, are responsible to fix air nitrogen in ammonia (NH3). Ammonia can be produced by two groups, one associated to plant roots, like bean and soy, and other that lives free in soil.
Nitrogen fixers associated with plant are more efficient than life free, because they are oxygen protected inside the nodules where they live and receive part of nutrients absorbed by plants. By this process, plant receives a free nitrogen supply, in ammonia form, that can be directly incorporated to proteins.
Free living fixers transform ammonia to nitrite through Nitrosomonas bacteria activity, after ammonia formation. Nitrite is transformed to nitrate by Nitrobacter bacteria activity. These bacteria are called nitrificants, because they use the energy liberated in nitrification process to synthesize its organic substances. Nitrate can be absorbed by plants, that use the nitrogen in proteins and nucleic acid formation.
Through food relationships, the nitrogen is transferred from one organism to another. Animals, therefore, only have access to nitrogen through the ingestion of plants or animals that, direct or indirectly, fed on plants.
Nitrogen returns to environment by excretion and decomposition. In both cases, nitrogen composites are transformed in ammonia. Ammonia can returns to the cycle by the action of nitrifying bacteria or can be transformed in gaseous nitrogen by denitrifying bacteria, coming back to the air, closing the cycle.
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