Metabolism refers to all the chemical reactions which occur in life.
These reactions are:
degradative reactions which:
a. Convert food into twelve, key, low molecular weight intermediates (which can be converted in anabolic reactions into low molecular
precursors of proteins, polysaccharides, lipids, and polynucleotides).
b. Oxidize food, generating NADH + H+, which transfers electrons to the electron transport chain, with resulting ATP generation.
c. Convert food into low molecular weight compounds which can serve to generate ATP by substrate level phosphorylation
2. Anabolic -
biosynthetic reactions which generate amino acids, fatty acids, monosaccharides, and mononucleotides and polymerize them
into proteins, lipids, polysaccharides, and polynucleotides.
Sets of reactions in which the product of one reaction serves as the substrate for the next reaction are called pathways. For example, compound
A might be converted into compound E by four, successive reactions.
Enzymes are necessary to catalyze most biochemical reactions so that the reaction reaches equilibrium within a time scale useful for life.
Enzymes usually are proteins; they usually catalyze one, specific reaction; usually every reaction requires catalysis by one specific enzyme.
Although a given reaction, such as A + B = C + D, might come to equilibrium, in the test tube, with only slightly more C + D formed than the
amount of A + B remaining, in living organisms, this reaction usually goes to completion. This is possible because C or D is removed by, for
example, conversion to some other product -if C or D is not removed, the reaction comes to equilibrium.
Glycolysis is the conversion of glucose to pyruvate (also called the Embden-Meyerhoff Pathway):
NAD+ is converted into NADH+ H+
ATP is generated by substrate level phosphorylation
Respiration converts pyruvate to CO
(Kreb’s cycle, citric acid cycle, Tricarboxylic acid cycle) and in the process:
NADH + H+ are formed, FAD is converted into FADH2, GDP is converted into GTP.