Biological Cycles

Another crucial biological cycle is the carbon cycle. All organisms need carbon to make essential compounds, therefore it is important that it is continuously used and returned back to the atmosphere. The cycle itself can be summarised in several stages. Firstly, plants absorb carbon dioxide from the atmosphere or areas of water for photosynthesis. The carbon-containing products of photosynthesis become incorporated into the tissues of these plants. The carbon is then passed on to other organisms if the plant is eaten by a primary consumer, then to a secondry consumer and so on. Once a plant or animal dies, microorganisms known as decomposers will break down the dead organic material using enzymes and absorb the carbon compounds by diffusion. Carbon is returned to the atmosphere in the form of carbon dioxide when organisms respire. This can also happen through the combustion of fossil fuels. If the dead organic matter ends up in a place where there arent any decomposers, e.g the bottom of the ocean, the carbon compounds can be turned into fossil fuels by heat and pressure.

Another very important biological cycle is the Krebs cycle. This cycle is a series of oxidation reduction reactions which take place in the matrix of the mitochondria. Acetyl coenzyme A binds to the oxaloacetate in order to convert it to citrate. The six carbon citrate molecule is converted to a five carbon compound through decarboxylation, where carbon dioxide is removed. A hydrogen is also removed and is then used to produce reduced NAD from NAD. The five carbon molecule is then further changed into a four carbon molecule through decarboxylation and dehydrogenation. This produces reduced FAD and two molecules of reduced NAD. ATP is also produced through the combining af ADP and a phosphate group from another compound. Citrate has now been converted to the four carbon compound oxalocetate, which is ready to be used to start the whole process over again. This cycle happens twice for every molecule of glucose.

Finally, the light independent reaction in photosynthesis is known as the calvin cycle. This cycle takes place in the stroma of the chloroplasts and is used to make triose phosphate from carbon ddioxide and ribulose biosphosphate. In the first stage of the cycle carbon dioxide is combined with RuBP to form a six carbon compound called glycerate-3-phosphate(GP). This rection is catalysed by rubisco. Then ATP from the light dependant reaction provides energy to turn GP into triose phosphate (TP). This reaction also requires a hydrogen ion which comes from reduced NADP. TP is then used to help produce useful organic compounds for the body such as glucose. However, Five out of every six molecules of TP formed are used to regenerate RuBP and start the cycle again.