The Earliest Cells Probably Produced ATP by Fermentation
As explained in Chapter 1, the first living cells on Earth are thought to have arisen more than 3.5 × 109 years ago, when the Earth was not more than about 109 years old. The environment lacked oxygen but was presumably rich in geochemically produced organic molecules, and some of the earliest metabolic pathways for producing ATP may have resembled present-day forms of fermentation.
In the process of fermentation, ATP is made by a phosphorylation event that harnesses the energy released when a hydrogen-rich organic molecule, such as glucose, is partly oxidized (see Figure 2-72). The electrons lost from the oxidized organic molecules are transferred (via NADH or NADPH) to a different organic molecule (or to a different part of the same molecule), which thereby becomes more reduced. At the end of the fermentation process, one or more of the organic molecules produced are excreted into the medium as metabolic waste products; others, such as pyruvate, are retained by the cell for biosynthesis.
The excreted end-products are different in different organisms, but they tend to be organic acids (carbon compounds that carry a COOH group). Among the most important of such products in bacterial cells are lactic acid (which also accumulates in anaerobic mammalian glycolysis) and formic, acetic, propionic, butyric, and succinic acids.