25 mark essay. cycles in biology PDF

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Sudeera Fernando 25-mark Synoptic Essay: Cycles in Biology A cycle can be defined by a series of events or steps that may be repeated in order to form an end product that is the same as the starting product. In biology, there are many cycles which occur inside an organism such as the cardiac cycle and krebs cycle. There are also biological cycles that may occur exterior to an organism such as the carbon, nitrogen and phosphorous cycles. In the cardiac cycle, there are a series of contractions (systoles) and relaxations (diastoles) of the cardiac muscles that control the flow of blood around the body of organisms. The heart consists of four chambers; these are the left and right atria and the left and the right ventricles. In between, the atria and the ventricles there are valves that prevent the back flow of blood. Between the left atria and the left ventricle is the bicuspid valve. Between the right atrium and right ventricle there is the tricuspid valve. The first occurrence in this cycle is the diastole period. In this phase, all chambers of the heart are relaxed and blood from the vena cava fills the right atrium whilst the pulmonary vein fills in the left atrium. Progressively the volume of the atrium increases as they fill with blood. The second phase of the cardiac cycle results in the contraction of the atria. Electrical impulses cause the contraction of the atria in which results in blood being forced into ventricles. As the pressure in the ventricles increase it forces shuts the bicuspid and tricuspid valves. Once the ventricles are filled with blood, ventricular systole occurs in which blood is forced into the aorta from the left ventricle and blood is forced into the pulmonary artery from the right ventricle. Semi lunar valves in the aorta prevent blood flowing back into the ventricle. The cycle is then complete, and all chambers of the heart relax (diastole). This cycle is vital for organisms as a key component of the human body is blood as it acts as an efficient delivery system of molecules such as oxygen to respiring tissue along with glucose. Furthermore, it transports white blood cells and other assets of the immune system to fight infection in the needed area. Excretory products such as urea and carbon dioxide must also be transported away from tissues and excreted so that they do not cause adverse effects after staying in the body for long periods of time. The heart is a vital organ as it is the sole component which aids in the transport of blood inside the organism. The Calvin cycle is a cycle that occurs in stroma of chroloplasts of plants and is the second step process of photosynthesis. Carbon dioxide from the atmospheric air diffuses into the leaf via the stomata located on the underside of the leaf. The carbon dioxide then dissolved in the water surrounding the mesophyll cells and then further diffuses through the cell-surface membrane, cytoplasm and the chloroplast membranes into the stroma of the chloroplasts. Inside the chloroplasts, carbon dioxide reacts with ribulose bisphosphate (RuBP) which is a 5- carbon compound. This reaction is catalysed by the enzyme rubisco. The reaction between RuBP produces two molecules of the 3- glycerate 3-phosphate (GP). The reduced NADP from the light-dependant reaction is used to reduce glycerate 3-phosphate to triose phosphate (TP) using the energy release from the hydrolysis of ATP. The NADP is re-formed and goes back to the light-dependant reaction to be reduced again by accepting more H+ ions. Some of the TP molecules are converted into organic substances that the plant requires such as starch, cellulose, lipids, glucose, amino acids, and nucleotides. Most of the TP molecules are used to regenerate ribulose bisphosphate using ATP from the light-dependent

Sudeera Fernando reaction. This cycle is important because every living thing on earth depends on it in order to survive. Plants and algae and other photosynthesising organims rely on it as it converts carbon dioxide from the earth along with water into sugars as a food source for the plant. This is then important for herbivores as they depend on plants to feed on. Hence, the importance of this process for carnivores and omnivores as they will feed on herbivores. Another cycle in biology is the Krebs cycle which involves a sequence of oxidation-reduction reactions which take place in the matrix of the mitochondria. This cycle occurs by the 2carbon acetylcoenzyme A from the link reaction combines with a 4-carbon molecule to produce a 6-carbon molecule. Within a series of reactions this 6-carbon molecule loses carbon dioxide and hydrogen to give a 4- carbon molecule and a single molecule of ATP produced as a result of substrate-level phosphorylation. The 4-carbon molecule that was produced can combine with another molecule of aetylcoenzyme A to repeat the cycle once again. Two molecules of pyruvate are produced from one single molecule of glucose. The link reaction and Krebs cycle operate and produce reduced coenzymes NAD and FAD which have the potential to provide the energy required to produce molecules of ATP via oxidative phosphorylation. They also form 2 molecules of ATP and 6 molecules of carbon dioxide. This cycle is vital for organisms as this is required to release energy from aerobic respiration in order for the organism to carry out biological processes in order for the organism to survive. The Krebs cycle is significant as it breaks down large macromolecules into smaller ones. Furthermore, it produces hydrogen atoms that are carried by NAD to the electron transfer chain which in turn provides energy for oxidative phosphorylation. An organic cycle that occurs is the nitrogen cycle. In this cycle, nitrogen from the air is fixated by Rhizobium bacteria. There are two types, free living and mutualistic bacteria. Mutualistic bacteria live in legume plant root nodules and receive carbonates in exchange for amino acids. The organic nitrogen from plants move into the soil when the plants die. Alternatively, organic nitrogen is consumed by animals which in turn gets released back into the soil during excretory processes. Ammonium ions are converted into NO2- ions by the bacterium nitrosomonas. These ions are then further oxidised into NO3- ions by nitrobacter bacteria. These two processes are called nitrification. Another type of bacteria called denitrifying bacteria convert these ions back to nitrogen gas. These bacteria require water logged anaerobic conditions. This cycle is essential for life as it is a building block of amino acids, proteins, RNA, DNA and chlorophyll. Furthermore, it is required for plant growth which is in turn vital for herbivores and further consequent consumers....