Metabolic Pathways

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Metabolic pathway

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A metabolic pathway is a series of chemical reactions that occur within the organism.

Linear metabolic pathways

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Metabolic pathways may be linear in which a series of chemical reactions going from a starting material to some product, much like a traditional assembly line in a factory. A good example is glycolysis, the first metabolic pathway involved in cellular respiration. The starting material for glycolysis is glucose and in a series of steps glucose is first converted to fructose 1,6 biphosphate. This in turn is split into two molecules of PGAL and in turn these are converted in a series of steps to a three carbon compound called pyruvate.

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Nonlinear metabolic pathways

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Nonlinear metabolic pathways typically involve a series of chemical pathways leading to the recycling of some critical starting material.

A good example is the Calvin cycle from the light independent reactions of photosynthesis. In this cycle, six molecules of a five carbon compound called RuBP is used to pick up six carbon dioxide and in a series of chemical reactions produce 12 molecules of a three carbon compound called PGAL. Two of these PGAL are harvested as a product of photosynthesis and the other 10 PGAL are chemically rearranged to effectively recycle the RuBP.

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Electron Transport Systems

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Some metabolic pathways involve a series of proteins which form a so called electron transport system (ETS). These proteins along with a series of electron acceptors carry electrons and as the electrons are passed along the electron transport system, energy from the electrons is used to do work, often to make ATP.

Typically electron transport systems are found in plasma membranes of the cell, for instance the inner membrane of the mitochondrion as shown here. The yellow arrows represent a flow of electrons, and the green structures represent either proteins in the electron transport system or associated proteins using energy from the electrons to make a ATP from ADP + Phosphate as part of a process called electron transport phosphorylation.

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pgd created 6/28/02