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Cellular respiration is the process in which the chemical bonds of energy-rich molecules such as glucose are converted into energy usable for life processes. Oxidation of organic material—in a bonfire, for example—is an exothermic reaction that releases a large amount of energy rather quickly. The equation for the oxidation of glucose is: C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy released (2830 kJ mol−1) In a fire there is a massive uncontrolled release of energy as light and heat. Cellular respiration is the same process but it occurs in gradual steps that result in the conversion of the energy stored in glucose to usable chemical energy in the form of ATP. ATP is known as the universal currency because when the phosphoanhydride bonds in ATP are hydrolyzed in an exergonic reaction, the energy yield is 30kJ per mole under standard conditions. Waste products (CO2 + H2O) are released through exhaled air, sweat and urine.
Aerobic respiration
Glycolysis Glycolysis is a metabolic pathway that is found in the cytoplasm of cells in all living organisms and does not require oxygen. The process converts one molecule of glucose into two molecules of pyruvate, and makes energy in the form of two net molecules of ATP. Four molecules of ATP per glucose are actually produced but two are consumed for the preparatory phase. The initial phosphorylation of glucose is required to destabilize the molecule for cleavage into two triose sugars. During the pay-off phase of glycolysis four phosphate groups are transferred to ADP by substrate-level phosphorylation to make four ATP and two NADH are produced when the triose sugars are oxidized. Glycolysis takes place in the cytoplasm of the cell. The overall reaction can be expressed this way: Glucose + 2 ATP + 2 NAD+ + 2 Pi + 4 ADP → 2 pyruvate + 2 ADP + 2 NADH + 4 ATP + 2 H2O + 4 H+ Oxidative decarboxylation of pyruvate Produces acetyl-CoA from pyruvate inside the mitochondrial matrix. This oxidation reaction also releases carbon dioxide as a product. In the process one molecule of NADH is formed per pyruvate oxidized. Krebs cycle/Citric Acid cycle When oxygen is present, acetyl-CoA enters the citric acid cycle inside the mitochondrial matrix, and gets oxidized to CO2 while at the same time reducing NAD to NADH. NADH can be used by the electron transport chain to create further ATP as part of oxidative phosphorylation. To fully oxidize the equivalent of one glucose molecule two acetyl-CoA must be metabolized by the Krebs cycle. Two waste products, H2O and CO2 are created during this cycle. Oxidative phosphorylation In eukaryotes, oxidative phosphorylation occurs in the mitochondrial cristae. It comprises of the electron transport chain that establishes a proton gradient (chemiosmotic potential) across the inner membrane by oxidizing the NADH produced from the Krebs cycle. ATP is synthesised by the ATP synthase enzyme when the chemiosmotic gradient is used to drive the phosphorylation of ADP. Theoretical yields The yields in the table below are preceeded by one glucose molecule being fully oxidized into carbon dioxide. It is assumed that all the reduced coenzymes are oxidized by the electron transport chain and used for oxidative phosphorylation. Although there is a theoretical yield of 36 ATP molecules per glucose during cellular respiration, such conditions are generally not realized due to losses such as the cost of moving pyruvate (from glycolysis), phosphate and ADP (substrates for ATP syhthesis) into the mitochondria. All are actively transported using carriers that utilise the stored energy in the proton electrochemical gradient. The outcome of these transport processes using the proton electrochemical gradient is that more than 3 H+ are needed to make 1 ATP. Obviously this reduces the theoretical efficiency of the whole process. Other factors may also dissipate the proton gradient creating an apparently leaky mitochondria. An uncoupling protein known as thermogenin is expressed in some cell types and is a channel that can transport protons. When this protein is active in the inner membrane it short circuits the coupling between the electron transport chain and ATP synthesis. The potential energy from the proton gradient is not used to make ATP but generates heat. This is particularly important in a babies brown fat, for thermogenesis, and hibernating animals. Anaerobic respiration Main article: Anaerobic Respiration In the absence of oxygen, pyruvate is not metabolized by cellular respiration but undergoes a process of fermentation. The pyruvate is not transported into the mitochondrion, but remains in the cytoplasm, where it is converted to waste products that may be removed from the cell. This waste product can vary depending on the organism. In muscles, the waste product is lactate, or lactic acid. In yeast, the waste product is ethanol and carbon dioxide. 2 ATP are produced during anaerobic respiration per glucose, compared to the 36 ATP per glucose produced by aerobic respiration. See also | ||||||||||
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