12.2 The Krebs Cycle

• The Krebs Cycle involves a series of Redox reactions that take place in the matrix of the mitochondria.
• (1) The 2-carbon acetylcoeznyme A from the Link Reaction combines with a 4-carbon molecule to form a 6-carbon molecule.
• (2) In a series of Redox reactions, the 6-carbon molecule loses carbon dioxide and hydrogen to form a 4-carbon molecule and 1 molecule of ATP. This is a result of substrate-level phosphorylation.
• (3) The 4-carbon molecule can then be recycled in the process, combining with a new acetyl CoA.
• For each pyruvate molecule, reduced coenzymes (NAD and FAD) are produced, as well as one ATP molecule and three molecules of CO2. However, since 2 pyruvate molecules are produced in glycolysis initially, the yield is double this.
• Coenzymes are not enzymes. They are molecules that some enzymes require to function, and play a major role in photosynthesis and respiration.
• NAD is important in respiration.
• FAD is important in the Krebs Cycle.
• NADP is important in photosynthesis.
• One reason why the Krebs Cycle is important is because it breaks down macromolecules into smaller molecules (e.g. pyruvate into carbon dioxide).
• One reason why the Krebs Cycle is important is because it produces hydrogen atoms that are carried by NAD to the electron transport chain and provides energy for oxidative phosphorylation. This leads to the production of ATP for metabolic energy for the cell.
• One reason why the Krebs Cycle is important is because it regenerates the 4-carbon molecule that combines with acetylcoenzyme A, which would otherwise accumulate.
• One reason why the Krebs Cycle is important is because it is a source of intermediate compounds used by cells to manufacture other important substances (e.g. fatty acids and chlorophyll).