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Overview of Metabolism
Carbohydrate Catabolism
Metabolism of Lipids
Photosynthesis
There are three basic steps involved with carbohydrate metabolism: Glycolysis, the Citric Acid Cycle, and Oxidative Phosphorylation at the Electron Transport Chain.
Step 1: Glycolysis
Glycolysis is the pathway for the catabolism of glucose that leads to pyruvate. A net of two molecules of ATP per molecule of glucose are produced by substrate level phosphorylation. (Phosphate transfers from organic compounds to ADP, forming ATP). Two molecules of ATP are consumed in the conversion of glucose to fructose-1,6-biphosphate. The first substrate-level phosphorylation of glycolysis is a phosphoryl group transfer from 1,3-biphosphoglycerate to ADP. The second is a phosphoryl group transfer from phosphoenolpyruvate to ADP. NAD+ is also reduced to NADH as glyceraldehyde-3-phosphate is oxidized. The reactions of glycolysis and an interactive glycolytic pathway are available.
Step 2:Citric Acid Cycle
Pyruvate from glycolysis enters into the Citric Acid Cycle . This cycle occurs in the mitochondria of the cell in aerobic conditions.
The pyruvate loses a carbon dioxide group, forming acetyl-CoA, the compound which forms a link for many other pathways and helps build other compounds. The citric acid cycle pathway consists of eight reactions that process incoming molecules of Acetyl CoA. The carbon atoms leave the cycle in the form of molecules of carbon dioxide. The hydrogen atoms and electrons leave the cycle in the form of reduced coenzymes NADH and FADH2. The cycle is regulated by three allosteric enzymes in response to cellular levels of ATP. One Acetyl CoA molecule entering the citric acid cycle produces three molecules of NADH, one of FADH2, and one of GTP. Click here to see the citric acid cycle.
Step 3: Oxidative Phosphorylation/Electron Transport Chain
The electron transport chain is the primary site for ATP synthesis, and occurs in the mitochondria. This pathway involves a series of reactions that pass electrons from NADH and FADH to molecular oxygen. Each carrier in the series has an increasing affinity for electrons. Four of the carriers, known as cytochromes, contain iron, which accepts and then transfers the electrons.
As NADH and FADH2 release their hydrogen atoms and electrons, NAD+ and FAD are regenerated for return to the citric acid cycle. There are three sites within the electron transport chain where the decrease in free energy is sufficient to convert ADP to ATP. Oxidative phosphorylation is the process by which NADH and FADH2 are oxidized, with concomitant production of ATP. Two molecules of ATP are produced when FADH2 is oxidized, and 3 molecules of ATP are produced when NADH is oxidized. The synthesis of ATP occurs because of a flow of protons across the inner mitochondrial membrane. The complete oxidation of one glucose molecule by the citric acid cycle and oxidative phosphorylation yields 36 molecules of ATP, vs. two molecules of ATP by glycolysis. For an excellent tutorial on oxidative phosphorylation, click here.
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Related Links:
Biochemistry on the Web:Practical Applications of Glycolysis
