Respiration in Detail

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Respiration is often summarised as: ‘glucose + oxygen react to form carbon dioxide + water (+ energy)’. Criticise this summary and give a more detailed account of the respiratory process. The first point to be noted is that this is only a summary (accurate or otherwise) of aerobic respiration. A similar summary for anaerobic respiration could read: Glucose is broken down to form carbon dioxide + Ethanol (in plants) or Lactate (in animals) (+ energy) Aerobic respiration is in fact a highly complex metabolic process comprising of at least 30 steps. There are 3 main processes that comprise respiration: glycolysis; the Krebs cycle; and the respiratory chain. Glucose enters the cell by facilitated diffusion, and glycoloysis then takes place in the cytoplasm. Glucose is phosphorylated (using a phosphate molecule from the breakdown of adenosine triphosphate (ATP)) to form glucose phosphate, a compound which can also be used for the synthesis of glycogen elsewhere in the cell. Glucose phosphate is phosphorylated again, and splits into two triose phosphate sugars. Each of these is converted in a number of steps to pyruvate (another 3-carbon sugar), a process that releases some energy, and a hydrogen atom, which is transferred for use in the respiratory chain by the co-enzyme NAD. The Pyruvate molecules then diffuse into the mitochondrial matrices within the cell. They are then converted to acetyl co-enzyme A (a two-carbon compound) with a Carbon Dioxide (CO2) molecule and a Hydrogen atom (taken up by co-enzyme NAD again) given off. Acetyl Co-enzyme A then enters the Krebs cycle. In this metabolic cycle the 2-carbon acetyl compound dissociates from the co-enzyme A and is converted into a 4-carbon oxaloacetate compound, which in turn is converted into a 6-carbon citrate compound. This is then gradually broken down back into the 4-carbon

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