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A bacterium growing in a salt solution containing a single type of carbon source, such as glucose, must carry out a large number of chemical reactions. Not only must it derive from the glucose the chemical energy needed for many vital processes, it must also use the carbon atoms of glucose to synthesize every type of organic molecule that the cell requires. These reactions are catalyzed by hundreds of enzymes working in reaction "chains" so that the product of one reaction is the substrate for the next; such enzymatic chains, called metabolic pathways, will be discussed in the following chapter.
Originally, when life began on earth, there was probably little need for such elaborate metabolic reactions. Cells with relatively simple chemistry could survive and grow on the molecules in their surroundings. But as evolution proceeded, competition for these limited natural resources would have become more intense. Organisms that had developed enzymes to manufacture useful organic molecules more efficiently and in new ways would have had a strong selective advantage. In this way the complement of enzymes possessed by cells is thought to have gradually increased, generating the metabolic pathways of present organisms. Two plausible ways in which a metabolic pathway could arise in evolution are illustrated in Figure 1-14.
If metabolic pathways evolved by the sequential addition of new enzymatic reactions to existing ones, the most ancient reactions should, like the oldest rings in a tree trunk, be closest to the center of the "metabolic tree," where the most fundamental of the basic molecular building blocks are synthesized. This position in metabolism is firmly occupied by the chemical processes that involve sugar phosphates, among which the most central of all is probably the sequence of reactions known as glycolysis, by which glucose can be degraded in the absence of oxygen (that is, anaerobically). The oldest metabolic pathways would have had to be anaerobic because there was no free oxygen in the atmosphere of the primitive earth. Glycolysis occurs in virtually every living cell and drives the formation of the compound adenosine triphosphate, or ATP, which is used by all cells as a versatile source of chemical energy. Certain thioester compounds play a fundamental role in the energy-transfer reactions of glycolysis and in a host of other basic biochemical processes in which two organic molecules (a thiol and a carboxylic acid) are joined by a high-energy bond involving sulfur (Figure 1-15). It has been argued that this simple but powerful chemical device is a relic of prebiotic processes, reflecting the reactions that occurred in the sulfurous, volcanic environment of the early earth, before even RNA had begun to evolve.
Linked to the core reactions of glycolysis are hundreds of other chemical processes. Some of these are responsible for the synthesis of small molecules, many of which in turn are utilized in further reactions to make the large polymers specific to the organism. Other reactions are used to degrade complex molecules, taken in as food, into simpler chemical units. One of the most striking features of these metabolic reactions is that they take place similarly in all kinds of organisms, suggesting an extremely ancient origin.