Lisa Zyga, writing for Phys.Org: Ever since the late 19th century, physicists have known about a counterintuitive property of some electric circuits called negative resistance. Typically, increasing the voltage in a circuit causes the electric current to increase as well. But under some conditions, increasing the voltage can cause the current to decrease instead. This basically means that pushing harder on the electric charges actually slows them down. Due to the relationship between current, voltage, and resistance, in these situations the resistance produces power rather than consuming it, resulting in a “negative resistance.” Today, negative resistance devices have a wide variety of applications, such as in fluorescent lights and Gunn diodes, which are used in radar guns and automatic door openers, among other devices.
Most known examples of negative resistance occur in human-engineered devices rather than in nature. However, in a new study published in the New Journal of Physics, Gianmaria Falasco and coauthors from the University of Luxembourg have shown that an analogous property called negative differential response is actually a widespread phenomenon that is found in many biochemical reactions that occur in living organisms. They identify the property in several vital biochemical processes, such as enzyme activity, DNA replication, and ATP production. It seems that nature has used this property to optimize these processes and make living things operate more efficiently at the molecular scale. The researchers provided two examples of biological processes that have negative differential responses. The first example is substrate inhibition, which is a process used by enzymes to regulate their ability to catalyze chemical reactions: “When a single substrate molecule binds to an enzyme, the resulting enzyme-substrate complex decays into a product, generating a chemical current,” writes Zyga. “On the other hand, when the substrate concentration is high, two substrate molecules may bind to an enzyme, and this double binding prevents the enzyme from producing more product. As an increase in substrate molecule concentration causes a decrease in the chemical current, this is a negative differential response.”
The second example has to do with autocatalytic reactions — “self-catalyzing” reactions, or reactions that produce products that catalyze the reaction itself: “Autocatalytic reactions occur throughout the body, such as in DNA replication and ATP production during glycolysis,” writes Zyga. “The researchers showed that negative differential responses can arise when two autocatalytic reactions occur simultaneously in the presence of two different chemical concentrations (reservoirs) in an out-of-equilibrium system.”
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