Understanding Cellular Respiration: AQA Topic 5

The Krebs Cycle, also known as the Citric Acid Cycle, is a key metabolic pathway in cellular respiration.It takes place in the matrix of mitochondria, the powerhouse of the cell.The cycle begins after glycolysis, when pyruvate is converted to acetyl-CoA through a process called pyruvate decarboxylation.This conversion releases carbon dioxide and produces NADH, an important electron carrier.For each glucose molecule that enters glycolysis, the Krebs cycle runs twice, once for each pyruvate molecule.Let's trace the path of the citric acid cycle and see how it generates energy carriers.The cycle begins when acetyl-CoA combines with oxaloacetate to form citrate, a six-carbon molecule.The final step regenerates oxaloacetate, which allows the cycle to begin again with a new acetyl-CoA molecule.To summarize, for each glucose molecule, the Krebs cycle runs twice and generates a total of two ATP, six NADH, and two FADH₂ molecules.The NADH and FADH₂ molecules are high-energy electron carriers that will fuel the final stage of cellular respiration, the electron transport chain.Oxidative phosphorylation is the final stage of cellular respiration.This process occurs in the inner mitochondrial membrane, which is highly folded to maximize surface area.The electron transport chain consists of a series of protein complexes embedded in the inner mitochondrial membrane.ATP synthase, sometimes called Complex Five, is a remarkable enzyme that acts like a rotary motor to produce ATP.The electron transport chain accepts electrons from NADH and FADH₂, which were produced during glycolysis and the Krebs cycle.As electrons move through these complexes, they release energy. This energy is used to pump hydrogen ions, or protons, across the membrane.This creates a high concentration of protons in the intermembrane space, forming an electrochemical gradient.This proton gradient drives ATP synthase, which allows protons to flow back into the mitochondrial matrix through a channel in the enzyme.This process is called chemiosmosis, where the energy of an electrochemical gradient is converted to chemical energy in the form of ATP.Oxidative phosphorylation produces the majority of ATP in cellular respiration. Each NADH yields approximately two point five ATP, while each FADH₂ yields about one point five ATP. In total, this stage produces approximately twenty-eight ATP molecules per glucose.This completes our exploration of oxidative phosphorylation and the electron transport chain.