Basics of Metabolism
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.
Oxidation and Reduction Reactions
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).
