A cycle is defined as any complete round or series of occurrences that repeats or is repeated in the same order and at the same intervals. The natural existence of cycles is highly important without cycles resources would run out and organisms would die. The amount of carbon nitrogen and oxygen on the earth is fixed, they can exists in different forms but no more may be added. So that these don’t run out they are cycles so they may be reused. Photosynthesis and respiration are the reverse of each other, and you couldn’t have one without the other.
Carbon is an essential component of all organic substances, necessary in nucleic acids, proteins, carbohydrates. The only way that can enter ecosystems is when it’s used for photosynthesis. Carbon dioxide diffuses into the plants stomata and through the Calvin cycle is combined with other molecules to make glucose. This may then be used in lipids, carbohydrates and proteins, incorporating carbon into the plants biomass e. g. cellulose cell wall and used for respiration. When a plant respires it releases some of this carbon back into the atmosphere as carbon dioxide.
Moreover if deforestation or slash and burn occurs it releases carbon dioxide into the atmosphere during the combustion process. When the primary producer is eaten by the primary consumer it passes its biomass and carbon along too. This happens through all the trophic levels. The consumers will leave detritus either urine, faeces or the carcass, or in the case of producers leaf litter. Decomposers known as Saprophytic bacteria then break down the detritus using enzymes. As they do so they respire again releasing carbon dioxide into the atmosphere.
If plants or animals die in situations were there are no decomposers for instance deep oceans, the carbon in them may turn into fossil fuels over millions of years by the process of fossilisation. Alternatively vast amount of the carbon is used by marine zooplankton to make calcium carbonate shells. These are not eaten by consumers and cannot easily be decomposed, so turn into carboniferous rocks -chalk, limestone, coral, 99% of the Earth’s carbon is in this form. The oceans also absorb far more carbon (than terrestrial plants) using microscopic marine producers dissolving it in the oceans.
In recent year human activity has influenced the amount of carbon dioxide in the atmosphere this is partially due combustion of fossil fuels and deforestation. Nitrogen is an unreactive gas that is essential for all animals and plants, but they are unable to use it in this form. Nitrogen is needed by all organisms fro amino acids, protein and nucleic acids. For plants to absorb the nitrogen it must be converted into non toxic soluble nitrates. The process by which this is done is nitrification.
Detritus (dead matter, urine and faeces) which contains organic nitrogenous compounds is decomposed by saprophytic bacteria to give ammonium ions or ammonia. Next in the process is the use of nitrifying bacteria, which are chemoautotroph’s, meaning they produce food for themselves using chemicals. The nitrifying bacteria use an oxidation reaction to convert the ammonium ions to nitrites, these are toxic to plants so another oxidation reaction occurs to produce nitrates. Nitrates are non toxic to the plants and very soluble so are easily absorbed by plants root hairs.
Either when plants die and produce detritus or when they are consumed and their consumer produces detritus the cycle will begin again. Nitrogen may also be made available to plants by nitrogen fixation. Some bacteria (cyanobacteria) are capable of using atmospheric nitrogen and convert it into organic substances. These bacteria are found in two places either free in the soil or in root nodules of legumes. Examples of leguminous plants are clover and peas. The bacteria that live in the root nodules have a symbiotic mutualistic relationship with the plant i. e. acteria receive carbohydrates from the plants photosynthesis, and the plant receives nitrogenous compounds for amino acids etc. The bacteria use the enzyme nitrogenase to make this conversion, it works best in low concentrations of oxygen hence it is often found in roots nodules. Another process by which nitrates are increase nitrate concentration in the soil is by the addition of artificial fertilisers or by lightening. The electric energy in lightening combines nitrogen gas and oxygen to produce various nitrogenous compounds, the rain washes these compounds into the soil.
To produce the nitrogen for fertilisers the harbour process is used, combining nitrogen and hydrogen to form ammonia. The process by which nitrogen is converted from nitrates to atmospheric nitrogen gas is denitrifying. Anaerobic bacteria often found in water logged soils convert the nitrates and ammonium ions back to nitrogen gas, where nitrogen fixing bacteria then convert it back. The Calvin or light independent cycle doesn’t use light energy (as the name suggests, however the cannot take place without the products from the light dependent reactions) occurs in the stroma of the chloroplast.
Carbon dioxide diffuses into the leaf through the stomata as a result the carbon dioxide enters the stroma. A five carbon compound ribulose bisphosphate RuBP combines with the carbon dioxide to form two molecules of glycerate 3-phosphate GP each consisting of three carbons. The next step in the cycle is the conversion from GP to triose phosphate TP (each of which are 3 carbons). This requires ATP to be converted to ADP + Pi and NADP to be converted to reduced NADP. This is a reduction reaction, in which rNADP provides the hydrogen and ATP supplies the energy.
Some of the TP is converted to carbohydrates used for elsewhere in the plant. The rest of the TP is used to resynthesise RuBP using the Pi– phosphate form earlier in the cycle. The Krebs cycle is the third stage of aerobic respiration, it is a series of reactions that occur in the mitrochondrial matrix. It happens once for each pyruvate molecule made in glycolysis, and it goes round twice fro every glucose molecule that enters the respiration pathway. Acetylcoenzyme A produced in the link reaction enters the cycle where it combines with a four carbon compound(oxaloacetate), which produces a six carbon compound (citrate).
For each complete turn of the krebs cycle one molecule of ATP (by substrate-level Phosphorylation) is produced. The reactions used involve oxidation and hydrogens given off are used to reduce the coenzymes. As a consequence the krebs cycle produces three molecules of rNAD, one molecule of rFAD, one ATP and two molecules of carbon dioxide. The importance of the krebs cycle is the production of the coenzymes which are passed onto the electron transport chain, where is converts these to ATP.