Counterintuitive physics property found to be widespread in living organisms
Since the end of the 19th century, physicists have known a counter-intuitive property of certain electrical circuits called negative resistance. Generally, the increase in voltage in a circuit also leads to an increase in electrical current. But under certain conditions, the increase in voltage can lead to a decrease in current instead. This essentially means that pushing harder on electrical charges actually slows them down.
Because of the relationship between current, voltage and resistance, in these situations, resistance produces energy rather than consuming it, resulting in "negative resistance". Today, negative resistance devices have a wide variety of applications, such as fluorescent lamps and Gunn diodes, which are used in radar guns and automatic door openers, among other devices.
The best known examples of negative resistance occur in devices designed by humans rather than in nature. However, in a new study published in the New Journal of Physics, Gianmaria Falasco and co-authors from the University of Luxembourg have shown that a similar property called a negative differential response is in fact a widespread phenomenon found in many biochemical reactions that occur in living organisms. They identify the property in several vital biochemical processes, such as enzymatic activity, DNA replication and ATP production. It seems that nature has used this property to optimize these processes and make the functioning of living beings more efficient at the molecular level.
"This counter-intuitive phenomenon, although common, was discovered in a multitude of physical systems after its first discovery in low-temperature semiconductors," the researchers wrote in their article. "We have shown that a negative differential response is a widespread phenomenon in chemistry with major consequences on the effectiveness of biological and artificial processes."
As the researchers explained, a negative differential response can occur in biochemical systems that are in contact with several biochemical reservoirs. Each tank tries to pull the system to a different equilibrium point (such as a equilibrium point), so that the system is constantly exposed to competing thermodynamic forces.
When a system is in equilibrium with its environment, any small disturbance, or noise, affecting the reservoirs will generally lead to an increase in the production rate of certain products, in accordance with positive entropy. The production rate of a product can be considered as a chemical flow. From this point of view, the increase in noise that causes an increase in chemical current is similar to the "normal" case of electrical circuits in which an increase in voltage causes an increase in electrical current.
But when a system in contact with several tanks becomes unbalanced, it may react differently to noise. In an unbalanced system, other factors come into play, so that an increase in noise reduces the chemical current. This negative differential response is similar to the case where the electrical circuits have a negative resistance.
In their work, the researchers have identified several biological processes that have negative differential responses. Substrate inhibition, for example, is a process used by enzymes to regulate their ability to catalyse chemical reactions. When a single substrate molecule binds to an enzyme, the resulting enzyme-substrate complex breaks down into a product, generating a chemical current. On the other hand, when the substrate concentration is high, two molecules of the substrate can bind to an enzyme, and this double binding prevents the enzyme from producing more product. Since an increase in the molecular concentration of the substrate results in a decrease in the chemical current, this is a negative differential response.
As a second example, the researchers showed that a negative differential response also occurs in autocatalytic reactions - "auto-catalytic" reactions or reactions that produce products that catalyze the reaction itself. Autocatalytic reactions occur throughout the body, such as DNA replication and ATP production during glycolysis. The researchers have shown that negative differential responses can occur when two autocatalytic reactions occur simultaneously in the presence of two different chemical concentrations (reservoirs) in an out-of-balance system.
The researchers also identified negative differential responses in dissipative self-assembly, a process in which energy is needed for a system to self-assemble, making it far from being in equilibrium. Dissipative self-assembly occurs, for example, in ATP-driven self-assembly of actin filaments - the long, thin microstructures of the cell cytoplasm that give cells their structure.
Nature does everything for a reason, and the presence of a negative differential response in living organisms is no exception. The researchers have shown that this property has advantages for biochemical processes, mainly in terms of energy efficiency. In substrate inhibition, for example, it allows a system to achieve homeostasis with less energy than would otherwise be required. In dissipative self-assembly, the negative differential response allows the system to obtain an almost optimal signal-to-noise ratio, thus increasing the efficiency of the self-assembly process.
