Proteins in the cell membranes of most organisms act like the hypothetical “demon” imagined by James Clerk Maxwell in 1867, which was thought to break the laws of physics.
A hypothetical being described in a 155-year-old thought experiment, at the time thought to break the laws of thermodynamics, actually evolved billions of years ago in the form of proteins used by almost every living organism.
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| James Clerk Maxwell imagined a demon that operates a door between two boxes of gas, allowing one side to become hotter than the other Alpha Historica / Alamy /Science Photo Library |
In 1867, physicist James Clerk Maxwell was pondering possible exceptions to the second law of thermodynamics, which says things must always flow from hot to cold unless there is some energy source to counter this tendency. He imagined a weightless door between two boxes of gas, operated by a tiny demon. If the demon only lets faster particles move into one chamber and slower ones into the other chamber, one box would heat up and the other would cool down. This would mean you could create energy from nothing by exploiting the temperature difference of the boxes, violating the second law.
This apparent paradox was only solved in the 20th century, when information theorists like Rolf Landauer realised that the demon would have to measure information about each gas particle, store it in its memory and erase its memory for more measurements. This would require more energy than the demon could create by keeping the boxes at different temperatures.
In nature, there are many “non-equilibrium systems” similar to the hot and cold boxes, such as the different concentrations of various molecules inside and outside living cells. Physicists have long suspected that something like Maxwell’s demon might be at play with these — but they couldn’t mathematically prove it.
Now, Paolo De Los Rios at the Swiss Federal Institute of Technology in Lausanne, Switzerland, and his colleagues have shown that ABC transporters – tiny proteins that can shuttle molecules across a cell membrane – act exactly like the demons proposed in Maxwell’s original paradox.
“Nature already understood the rules billions of years ago,” says De Los Rios. “ABC transporters are present in all bacteria. They are really, really ancient. They go back to the last universal common ancestor of all life on Earth.”
De Los Rios and his team first wrote down simple equations describing how ABC transporters keep different concentrations of molecules inside and outside of a cell. They considered simple facts about how they work – such as using ATP, a molecular source of energy, to transport molecules across the membrane, and the different shapes the transporters can take when facing inside or outside the cell.
To play the role of Maxwell’s demon without breaking the laws of thermodynamics, Landauer theorised that an entity needs to consume energy, make recorded measurements and operate the door based on these measurements. De Los Rios and his colleagues found that the solution to their equations had three parts that matched these three conditions. “When you try to understand what these different terms mean, you actually recognise the building blocks of a Maxwell’s demon,” says De Los Rios.
There are some simplifications that they made to their model, such as assuming that each ABC transporter only uses one molecule of ATP at a time, but De Los Rios says that more complex models should also work as Maxwell’s demons. He also says that, given the similar functions and roles that many molecular machines play, it is likely that Maxwell’s demon is widespread in nature.
“They make very concrete connections between the rigorous idea of Maxwell’s demon as it is now understood in statistical physics and the way these ABC transporters work,” says Nahuel Freitas at the University of Buenos Aires in Argentina. “They go beyond the level of a metaphor.”
This connection also means that an ABC transporter can be thought of as a simple computational device, performing the same logical “AND” operation that silicon computer chips do, says Freitas.
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