Encyclopedia of Evolution.pdf - Online Reading Center

dioxide, which diffuses into the environment as a waste product.
Products from the Krebs cycle produce electric currents
in membranes. From both the cycle and the currents, large
amounts of ATP are produced. However, in the electric currents,
the electricity has to have someplace to go. It goes into
oxygen molecules, converting them (along with hydrogen
ions) into water molecules.
The primitive anaerobic bacteria had only glycolysis and
fermentation. The more advanced bacteria, and mitochondria,
have both glycolysis and aerobic respiration. The more
advanced reactions appear to have been added onto the more
primitive reactions, perhaps by horizontal gene transfer or
by an ancient genetic merger between two kinds of bacteria,
one with glycolysis (like the anaerobic bacteria still extant)
and one with aerobic respiration (now extinct). Respiration
respiration, evolution of
occurs in the mitochondria of many protists and almost all
eukaryotic cells. The set of glycolysis reactions in the cytoplasm,
and of aerobic respiration reactions in mitochondria,
are nearly identical to the corresponding reactions in bacteria.
This is not coincidence. Mitochondria are the evolutionary
descendants of bacteria that moved into and formed a
mutualistic association with primitive eukaryotic cells (see
symbiogenesis).
Further Reading
Raymond, Jason, and Daniel Segrè. “The effect of oxygen on biochemical
networks and the evolution of complex life.” Science 311
(2006): 1,764–1,767. Summary by Falkowski, Paul G., “Tracing
oxygen’s imprint on Earth’s metabolic evolution.” Science 311
(2006): 1,724–1,725.