March 5, 2013
Glycolysis and the Citric Acid Cycle
Glycolysis occurs in the cytoplasm outside the mitochondria, it is a metabolic pathway that is the breakdown of glucose (C6) and ends with 2 pyruvate (C3) molecules. Energy is invested to activate the glucose, 2 ATP are gained, and oxidation results in NADH, which will be used later for additional ATP production. Glycolysis is divided into (1) the energy-investment step, when ATP is used; and (2) the energy-harvesting steps, when NADH and ATP are produced. A net gain of 2 ATP can be calculated by subtracting those expended during the energy-investment step from those produced during the energy-harvesting steps. During the energy-investment step, 2 ATP transfer phosphate groups into substrates, and 2 ADP + P result. The ATP has been broken down however; the phosphate groups activate the substrates, so they can undergo reactions. During the energy-harvesting steps, substrates are oxidized by the removal of hydrogen atoms, and 2 NADH result. Oxidation produces substrates with energized phosphate groups, which are used to synthesize 4 ATP. As a phosphate group is transferred to ADP, ATP results. The inputs of Glycolysis are glucose, 2 NAD+, 2 ATP, and 4 ADP+P. The outputs of Glycolysis are 2 pyruvate, 2 NADH, 2 ADP, 4 ATP that equals a net gain of 2 ATP.
The Citric acid cycle also known as the Krebs cycle, to honor the scientist who first studied it, is a cyclical metabolic pathway located in the matrix of the mitochondria. At the start of the citric acid cycle, the C2 acetyl group carried by CoA joins with a C4 molecule, and a C6 citrate molecule results. The CoA returns to the preparatory reaction to be used again. During the citric acid cycle, the acetyl group is oxidized, and the rest of the CO2 we breathe out per glucose molecule is released, both NAD+ and FAD accept hydrogen atoms, resulting in NADH and FADH2, and the substrate-level ATP synthesis occurs, as ATP...
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