"Energy is the currency of biology. By harvesting electrons from a stunning range of starting materials, Earth’s organisms produce adenosine triphosphate (ATP), which powers biological reactions. In the case of mammals and most eukaryotes, sugars and other organic molecules are common electron sources, the oxidation of which drives ATP production. Bacteria and archaea can use a range of other chemicals, from sulfide to iron to ammonium. Cells take up these electron-rich molecules and capture their electrons, which jump down an electron transport chain in the mitochondrial or cell membrane. As electrons move along the membrane toward a final electron acceptor, protons are pumped from the cell’s interior to the exterior, setting up a chemical gradient. Finally, protons stream back into the cell, releasing the chemical pressure and generating ATP. With each energy-requiring reaction, from flagella construction to cell division and growth, cells draw upon their ATP bank. The wide variety of biochemical modes of existence reflects billions of years of evolution, adaptation, and niche differentiation rather than a standardized characterization of biological fortitude. This elegant, multistep process is a pervasive feature of life as we know it, but energetic challenges are ever-present. If the electrical potentials of electron donor and acceptor are too closely aligned, for example, it won’t be possible to squeeze much energy from their coupling. The concentrations of the reactants and the speed at which enzymes can mobilize them are also key factors. These two components—the magnitude of energy available from a particular pairing and the rate of such reactions—determine how much energy a cell can produce." - Todd Hoff