Month: March 2008

Energy in Motion: How the nanomachines of life harvest randomness to do the cells’ work

TAMING CHANCE. This molecule acts like the microscopic demons James Clerk Maxwell envisaged in the 19th century. Thermal or Brownian motion moves a ring-shaped molecule (blue) from one side to another of a dumbbell-shaped molecule (yellow). But a “gate” molecule (green) is designed to lock the ring molecule to just one side of the dumbbell. Brownian motion provides energy to move the ring, but the gate molecule steers it. Credit: Stephen Goldup/Univ. of Edinburgh
This article was first published in Science News magazine, February 23rd, 2008; Vol.173 #8.

Occasionally, scientists stumble upon what seems to be a free lunch. But they’re not concerned about possibly violating the laws of economics. It would be much more shocking to break the laws of physics.

To physicists, the no-free-lunch rule is precious. One form of it is the first law of thermodynamics, which says that energy cannot be created from nothing. The second law of thermodynamics goes even further, declaring not only that lunches are never free but also that they come at some minimum price.

Nonetheless, some natural phenomena seem, at first glance, to violate the spirit, if not the letter, of those laws. Take living cells. In recent years, scientists have found that some molecular machines—proteins that perform crucial tasks of life, from shuttling molecules through membranes to reading information off of DNA—seem to move spontaneously. These machines are likely powered by the random motion of water molecules in their environment, the “thermal noise” that thermodynamics insists is not available for doing work.

While some researchers debate how such machines work without breaking physical laws, other scientists have begun to exploit similar phenomena to create artificial molecular motors—nanomachines that imitate nature by putting randomness to work. “The idea is, let’s take advantage of thermal noise, rather than fight against it,” says Dean Astumian, a theoretical chemist at the University of Maine in Orono.

Researchers have just begun to build artificial nanomachines that perform simple tasks, such as moving molecules, by steering random motion in one direction rather than another. In the Feb. 13 Journal of the American Chemical Society, a team led by David Leigh, a chemist at the University of Edinburgh in Scotland, describes the first molecule designed to use chemical energy to open or close a gate and allow one of its parts to randomly cross the gate in one direction, but not the other.

It’s very much like the task assigned to a hypothetical “demon” by the 19th-century Scottish physicist James Clerk Maxwell. His thought experiment was an early attempt to show how the second law defines group behavior and thus applies only to large numbers of particles.

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