A cell's metabolism is all of its complex chemical processes. These thousands of chemical reactions are linked in sequences called metabolic pathways.

Figure 1: In this diagram of a few of the chemical reactions in a cell each dot is a molecule. The lines are chemical reactions that transform the molecules.Taken from Biology 4th ed. Campbell

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Each of these reactions is cataylized by enzymes. Some cellular chemical reactions are exergonic (produce energy) and others are endergonic (require energy). An example of an exergonic reaction is cellular respiration in which sugar glucose is broken down into water and carbon dioxide releasing energy. An example of an endergonic reaction is the synthesis of a protein from amino acids.

All cell reactions require some energy to kick start the reaction. This energy is called activation energy.In a cell the Endergonic reactions obtain their energy from exergonic reactions. the reactions are said to be coupled reactions.
Typically the endergonic reaction is physical seperated from the exergonic reaction. So the senergy has to be transferred from one location in the body to another by energy-carrier molecules. The most common is adenosine triphosphate or ATP.The human bodies temperature is to low for most of the cellular chemical reactions to proceed fast enough. The body uses enzymes and other catalysts to speed up and slow down chemical reactions.

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Key terms: metabolism, metabolic pathways, activation energy, coupled reactions, energy-carrier molecules, ATP, enzymes

Basic facts: http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/M/Metabolism.html