Metabolism 3: CAC and ETC

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The Citric Acid Cycle

The citric acid cycle, also called the Krebs cycle, occurs in the matrix of the mitochondria. It involves a series of 8 enzyme-catalysed reactions. Many of these reactions involve oxidation of organic molecules derived from pyruvate. In summary, acetyl-CoA enters the cycle and carbon from this exits as carbon dioxide. The citric acid cycle generates NADH and FADH2 and goes around twice, because for each glucose molecule, two pyruvate molecules are produced.

The overall aim of the citric acid cycle is to remove electrons (which can later be used for energy) in the form of NADH and FADH2 from the organic molecules that enter the chain. However, very little energy is made during the cycle itself (1 ATP), even less than in glycolysis which yields 2 ATP.

The Electron Transport Chain

The electron transport chain occurs in a series of 6 steps:
  1. NADH and FADH2 enter the electron transport chain.
  2. High energy electrons from these carriers are passed along the chain, gradually losing energy with each step.
  3. Hydrogen ions are released with each step. These are sent to coenzymes embedded in the inner membrane of a mitochondrion.
  4. When electrons reach the end of the transport chain, they have lost their energy and are now low energy electrons.
  5. Oxygen accepts these electrons and combines with the hydrogen ions that were released along the way to form water.
  6. The energy from the electrons is released along the way.
There are some molecules involved in these processes, that can be found along the chain:
  • FMN or flavin mono nucleotides; prosthetic group of complex 1 or NAD dehydrogenase
  • FAD or flavin adenine dinucleotide; complex 2 or succinate dehydrogenase
  • Q or ubiquione; not a protein, small, mobile, hydrophobic molecule
  • b or complex 3, cytochrome bc1 complex
  • Cytochrome C; small, mobile protein
  • a & a3 or complex 4 cytochrome c oxidase

Chemiosmosis

Chemiosmosis involves the movement of hydrogen ions, or protons, through the protein channel. When H+ moves through, the channel undergoes a 'conformational change'. This change activates ATP synthase, which is the enzyme required for phosphorylating ADP to ATP. This occurs in the mitochondrial matrix.

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