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Cellular respiration is the process by which cells convert glucose and oxygen into carbon dioxide, water, and energy in the form of adenosine triphosphate (ATP). It is a vital process that occurs in the mitochondria of eukaryotic cells and is essential for the survival and functioning of all living organisms.
The process of cellular respiration can be divided into three main stages: glycolysis, the citric acid cycle (also known as the Krebs cycle), and oxidative phosphorylation (also known as the electron transport chain).
1. Glycolysis: This is the first stage of cellular respiration and takes place in the cytoplasm of the cell. It involves the breakdown of one molecule of glucose into two molecules of pyruvate. Glycolysis occurs in ten steps and does not require oxygen. It produces a small amount of ATP and NADH (nicotinamide adenine dinucleotide), which is an electron carrier.
2. Citric Acid Cycle: After glycolysis, the pyruvate molecules produced enter the mitochondria. In the presence of oxygen, each pyruvate molecule is converted into acetyl-CoA, which enters the citric acid cycle. This cycle occurs in the mitochondrial matrix and involves a series of chemical reactions that break down acetyl-CoA into carbon dioxide. The citric acid cycle generates more ATP, NADH, and FADH2 (flavin adenine dinucleotide), which is another electron carrier.
3. Oxidative Phosphorylation: The final stage of cellular respiration takes place in the inner mitochondrial membrane. The NADH and FADH2 molecules produced in the previous stages donate their electrons to the electron transport chain. This chain consists of a series of protein complexes that transfer electrons from one molecule to another, releasing energy in the process. As the electrons move through the chain, protons are pumped across the inner mitochondrial membrane, creating an electrochemical gradient. This gradient drives the synthesis of ATP through a process called chemiosmosis. Oxygen acts as the final electron acceptor, combining with protons to form water.
Overall, cellular respiration is an efficient process that produces a net gain of 36-38 ATP molecules per glucose molecule. It is important to note that cellular respiration is not only limited to glucose metabolism but can also utilize other molecules, such as fatty acids and amino acids, as energy sources.