Introduction
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is the sum total of the energy using and energy releasing chemical reactions that take place in the organism. For example, the energy rich molecules in your body that result from digestion can be broken apart and the electrons used to provide energy for the cell. Conversely, one of the things the cell can use energy for is to build up complex organic molecules from simpler molecules, for instance using the amino acids that result from digestion of proteins to make new protiens that the body needs.
One way to view metabolism is in terms of the types of chemical activity that goes on in the body.
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Anabolism refers to those reactions that build complex molecules out of simpler building blocks, such as making proteins by linking amino acids together.
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Catabolism is the reverse of anabolism, namely the tearing apart of complex molecules to make simpler molecules either to harvest energy from the molecules or to use the building blocks for some other purpose.
Catabolism and anabolism as sets of chemical reactions are typically coupled or connected together as energy harvesting reactions and then energy using reactions as shown here as part of the ATP-ADP cycle.
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Five Types of Metabolic Reactions
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The metabolic reactions in your body fall into five broad types and understanding these will make it much easier for you to understand the important metabolic reactions that happen in organisms.
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A common type of reaction is a functional group transfer from one compound to another. For example in your muscles, during muscle activity creatine phosphatewill give up its phoshate to ADP(Adenosine di-phosphate) resulting in Creatine and ATP (Adenosine tri-phophate).
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Electrons carry energy and this energy is released
by transfering electrons from one substance to another, for example from
an electron carrier to some type of protein system that can transport
electrons and use the electron's energy to do work. For instance, the
illustration shows NADH an electron carrier being broken down into NAD+
and H+ along with 2 electrons. These electrons are picked up by a system
of proteins in the plasma membrane of certain organelles. These proteins
then pass the electrons along from one to the other resulting in energy
being released to do work. Often this work involves making ATP but in
the case of bacteria the work may be to move the organism around
its environment. 
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Many times a molecule's chemical structure will be rearranged into another
molecule that has the same empirical formula(i.e., same number of atoms
of each element). The result of such a rearrangement is a molecule with
approriate physical and chemical properties for some other set of reactions.
For instance, during cellular respiration glucose is combined with
phosphates, transferred from ATP and rearranged to form fructose 1-6 bi-phosphate.
Note that the rearrangement involves an input of energy which later on
allows more energy to be harvested. 
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Cells, and organisms in general, take large molecules and break them down into smaller molecules. For instance, when you eat a steak and then digest the proteins, the proteins are broken down into amino acids. The most common form of cleavage is called hydrolysis because it involves essentially adding water to the bounds joining the small subunits of larger molecules, breaking those bonds.
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The small molecules resulting from cleavage can then be used by the cell to make just the complex molecules it needs. For example, the amino acids from a steak or other protein source are recombined in protein synthesis into just the prooteins the cell needs and these may be quite different in function from the original muscle proteins in the steak. The common type of condensation reactions in cells is called dehydration synthesis because it involved removing a hydroxyl group from one molecule and a hydrogen from the other molecule when the two molecules are joined resulting in water as a by-product.
pgd created 1/29/00 revised 4/26/05
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