Metabolism is a process of energy acquisition and conversion. It is necessary because organisms are constantly undergoing cellular changes--they are not in a state of equilibrium. Metabolism is an attempt to regulate cellular conditions by making internal changes to maintain a steady cellular state. As a general rule, nature's tendency is towards conditions of disorder. This means that disorderly conditions are energetically favorable--they release energy. Highly ordered and organized conditions are not energetically favorable and require energy to occur. As a result, the thousands of reactions that constantly occur inside us to maintain cellular organization need energy. The body produces this needed energy by breaking down ATP, and then using this energy to promote energetically unfavorable, but biologically necessary reactions.
In order to begin any of these processes, cells need an external energy source. The breakdown of the external source can provide the energy that can couple to drive other reactions. Cells acquire this external energy in one of two ways. Phototrophs get their energy from the sun through photosynthesis. Plants are phototrophs. Plants use light energy to convert carbon dioxide and water into carbohydrates and oxygen. Chemotrophs, such as humans, derive energy from the breakdown of organic compounds such as carbohydrates, lipids, and proteins. Our focus in discussing cell respiration and metabolism will be on this second, chemical type of energy acquisition. The relationship between phototrophs and chemotrophs is complimetary: chemotrophs require oxygen and expire carbon dioxide while phototrophs require carbon dioxide and expire oxygen. Additionally, many of the carbohydrates ingested by chemotrophs derive from the metabolic carbohydrate products of phototrophs.
Among chemotrophs, there are two major categories of metabolic pathways. The distinction between the two is that one involves degradation reactions while the other involves synthesis reactions. Catabolic pathways involve the breakdown of ingested food molecules. Anabolic pathways involve the synthesis of essential biomolecules. Along each of these pathways, a number of enzymes work in combination to help drive the reactions. The catabolic pathways are involved in breaking down carbohydrates and proteins into their polysaccharide, or sugar, and amino acid subunits. These reactions release energy needed by the cell (this is why food, the source of carbohydrates and proteins, is essential for survival). Anabolic pathways take the simple products of catabolic degradation--ATP, for example--and use energy from their degradation to synthesize complex biomolecules.
As we have mentioned, the breakdown of ATP is an energetically favorable reaction. This is true because it involves splitting one larger, more organized molecule into two smaller ones. The energy that is released in this process can be used to drive other, less favorable reactions forward. In this way, ATP acts as a major energy source for cells.
As one can imagine, there are many different anabolic and catabolic reactions going on at any second in our bodies. As a result, metabolic pathways must be highly regulated as to ensure that the proper enzymes for synthesis and degradation are active at the appropriate times. Some of this regulation is made possible by different metabolic processes occurring in distinct parts of the cell.
There are a number of different types of metabolic reactions that typically take place. One class of reactions that will be mentioned a lot in this guide are oxidation and reduction reactions. These reactions involve the gain and loss of electrons and often also involve the cleavage of carbon-hydrogen bonds. When they are favorable, such reactions yield a large amount of free energy. In order to understand the specifics of what occurs in these reactions, a strong chemistry background is necessary. Here, it will suffice to understand that an oxidation reaction involves the loss of electrons (which corresponds to the breaking of bonds) and that a reduction reaction involves a gain of electrons (corresponding to a making of bonds).