Energy is released from glucose molecules during cellular respiration through a series of biochemical reactions that convert the energy stored in glucose into a usable form, primarily adenosine triphosphate (ATP). Here’s a step-by-step overview of the process:
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Glycolysis: This is the first step of cellular respiration and occurs in the cytoplasm. During glycolysis, one molecule of glucose (a six-carbon sugar) is broken down into two molecules of pyruvate (three-carbon compounds). In this process, a small amount of energy is released, which is used to convert ADP and inorganic phosphate into ATP. Additionally, high-energy electron carriers, such as NADH, are produced.
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Pyruvate Oxidation: If oxygen is present, the pyruvate produced in glycolysis is transported into the mitochondria, where it undergoes a conversion to acetyl-CoA. This process produces CO2 as a waste product and generates another molecule of NADH.
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Citric Acid Cycle (Krebs Cycle): The acetyl-CoA then enters the citric acid cycle, which takes place in the mitochondrial matrix. Here, it undergoes a series of reactions that result in the production of ATP, NADH, and FADH2, another high-energy electron carrier. During this cycle, CO2 is released as a waste product.
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Oxidative Phosphorylation: The NADH and FADH2 produced in the previous steps carry high-energy electrons to the electron transport chain, located in the inner mitochondrial membrane. As electrons are transferred through a series of protein complexes, energy is released, which is used to pump protons (H+ ions) from the mitochondrial matrix to the intermembrane space, creating a proton gradient.
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Chemiosmosis: The stored energy in this proton gradient is used by ATP synthase, an enzyme that facilitates the synthesis of ATP from ADP and inorganic phosphate as protons flow back into the matrix through the enzyme.
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Final Electron Acceptors: At the end of the electron transport chain, the electrons are transferred to molecular oxygen, which combines with protons to form water. Oxygen is essential for this process, as it is the final electron acceptor and allows for the continuous flow of electrons through the chain.
Through these combined processes, cellular respiration efficiently converts the chemical energy stored in glucose into ATP, which can then be utilized by the cell for various energy-requiring processes. Overall, the complete oxidation of one glucose molecule can yield up to about 30-32 ATP molecules, depending on the efficiency of the processes involved.