Energy Transfer in Metabolism.

• Introduction
• ATP-ADP cycle
  • Catabolism
  • Anabolism
• Electron Transport
  • Oxidation, reduction
  • Redox reactions

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Introduction

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In metabolism the cell must harvest energy and use energy to do work. Energy harvesting and transport in metabolism takes two basic forms: the ATP ADP cycle and electron transport.

ATP-ADP cycle

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The ATP-ADP cycle, shown here involved the harvesting of chemical energy by breaking down complex organic molecules into simpler molecules, using the energy from this process to join a phosphate group to an ADP molecule to make ATP. The resulting phosphate bond between the second and third phosphate of the ATP has a small amount of net energy which can be released to do work.

Catabolism

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Catabolism is the general name given to those steps in metabolism where complex organic compounds are broken down into simpler compounds, for example the taking glucose and breaking it down into carbon dioxide and water.

Anabolism

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Anabolism is the general name for those steps in metabolism in which simple compounds are used to make more complex compounds, for example taking amino acids and combining them together to make proteins.

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Electron Transport

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In electron transport, electrons are harvested from energy rich organic molecules and transferred to other molecules, usually along with hydrogen ions.

• Oxidation and Reduction
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  In electron transport, the energy rich organic molecules that are loosing electrons are said to be oxidized, since a chemical is said to be oxidized when it looses electrons to another chemical. The chemical that gains electrons in a chemical reaction is said to be reduced. The terminology comes from the fact that oxygen has a strong tendency to grab electrons from many other chemicals.
• Redox Reaction
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  A redox reaction is a reaction that involves a chemical in the reaction donating an electron to another chemical. The chemical accepting the electron is said to be reduced, the chemical donating the electron is said to be oxidized. In biological systems since reduction and oxidation reactions typically are coupled together we call these reactions redox reactions.

An example of electron transport and a redox reaction is this particular step in photosynthesis where electrons are donated from a compound called NADPH to a compound called PGA. The result is an energy rich compound called G3P.

Note that the NADPH actually donates two electrons and a hydrogen ion to the PGA. This is a redox reaction since the PGA is reduced and the NADPH is oxidized resulting in an ion called NADP+ .

Actually the electrons in this case are not transferred directly to the PGA. Observe that the PGA first is rearranged using some energy donated by ATP. This rearrangement produces an intermediate compound which is then reduced by the NADPH.

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pgd. Created 07/02/02