Unlike an uncontrolled reaction, the electron transport chain passes electrons in a series of steps instead of one explosive reaction • O 2 pulls electrons down the chain in an energy-yielding tumble • The energy yielded is used to regenerate ATP The Stages of Cellular Respiration: A Preview • Harvesting of energy from glucose has three stages – Glycolysis (breaks down glucose into two molecules of pyruvate) – The citric acid cycle (completes the breakdown of glucose) – Oxidative phosphorylation (accounts for most of the ATP synthesis) • The process that generates most of the ATP is called oxidative phosphorylation because it is powered by redox reactions • Oxidative phosphorylation accounts for almost 90% of the ATP generated by cellular respiration
• A smaller amount of ATP is formed in glycolysis and the citric acid cycle by substrate- level phosphorylation • For each molecule of glucose degraded to CO 2 and water by respiration, the cell makes up to 32 molecules of ATP Concept 9.2: Glycolysis harvests chemical energy by oxidizing glucose to pyruvate • Glycolysis (“splitting of sugar”) breaks down glucose into two molecules of pyruvate • Glycolysis occurs in the cytoplasm and has two major phases – Energy investment phase – Energy payoff phase • Glycolysis occurs whether or not O 2 is present Concept 9.3: After pyruvate is oxidized, the citric acid cycle completes the energy-yielding oxidation of organic molecules • In the presence of O 2 , pyruvate enters the mitochondrion (in eukaryotic cells) where the oxidation of glucose is completed Oxidation of Pyruvate to Acetyl CoA • Before the citric acid cycle can begin, pyruvate must be converted to acetyl Coenzyme A ( acetyl CoA ), which links glycolysis to the citric acid cycle • This step is carried out by a multienzyme complex that catalyses three reactions The Citric Acid Cycle • The citric acid cycle, also called the Krebs cycle, completes the break down of pyruvate to CO 2 • The cycle oxidizes organic fuel derived from pyruvate, generating 1 ATP, 3 NADH, and 1 FADH 2 per turn • The citric acid cycle has eight steps, each catalyzed by a specific enzyme • The acetyl group of acetyl CoA joins the cycle by combining with oxaloacetate, forming citrate • The next seven steps decompose the citrate back to oxaloacetate, making the process a cycle • The NADH and FADH 2 produced by the cycle relay electrons extracted from food to the electron transport chain
Concept 9.4: During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis • Following glycolysis and the citric acid cycle, NADH and FADH 2 account for most of the energy extracted from food • These two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation
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