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Active systems: Life is motion

May 03, 2016

Physicists from Ludwig-Maximilians-Universitaet (LMU) in Munich have developed a way to distinguish the random motions of particles in non-living molecular systems from the motility of active living matter. The method affords new insights into fundamental biological processes.

What are the salient physical and chemical properties that distinguish living organisms from non-living matter? This is a question that has fascinated scientists for centuries. One of the key differences between the two classes lies in the fact that living systems are maintained in a non-equilibrium state. To avoid the otherwise inevitable slide into thermodynamic equilibrium they must continuously expend energy -- to power active motions and keep the cell alive. The LMU physicist Professor Chase Broedersz, in collaboration with researchers based in Göttingen, Amsterdam, the Massachussetts Institute of Technology and Yale University, has now developed a method that can differentiate between the active motions characteristic of living cells and those driven by the random molecular movements that give rise to passive diffusion. The technique also provides deeper insights into fundamental processes that are specific to biological systems. The new findings appear in the latest issue of the leading journal Science.

"Strikingly, in the world of microscopically tiny particles, molecular motion as such does not necessarily imply that one is dealing with a thermodynamically non-equilibrium state or an actively driven process. Molecular motions can also result from the thermally driven bombardment of small particles by molecules in the surrounding medium," Broederzs points out. These thermal collisions with molecules alter the trajectory of minute particles and give rise thermal diffusion. At first sight, many actively driven processes in living cells appear to be equally random in nature. "So in order to understand cell functions, one must be able to distinguish them from equilibrium systems," says Broedersz.

Video analysis of organelle motion

Broedersz and his colleagues now describe a method which, for the first time, enables living systems to be conclusively and non-invasively identified to be out of equilibrium at microscopic scales. The procedure makes use of the principle of detailed balance, which states that, in systems that have attained equilibrium, the average rate of every elementary process is equal to that of its reverse -- forward and backward reactions effectively cancel out. If this principle does not hold, the system is by definition in a non-equilibrium state and must be driven by the input of energy from an external source. "Our new method relies on a video imaging system which allows us to visualize microscopic motions in real time. The resulting imaging data can then be analyzed to determine whether or not the system obeys the principle of detailed balance," says Broedersz.

In the study, the team analyzed the motions of two types of hair-like cell protrusions made up of proteinaceous filaments - the so-called flagella found on the unicellular green alga Chlamydomonas reinhardtii and the primary cilium found on many epithelial tissues in multicellular organisms. Flagella and primary cilia are quite similar in their basic structure, but their biological functions and modes of action differ. Flagella are used by microorganisms to swim through liquid media, while primary cilia act primarily as motile sensors on epithelial surfaces. "With the help of our imaging data," says Broedersz, "we were able to demonstrate that, instead of simply waving back and forth, both flagella and cilia on average carry out cycles of actively driven and distinct movements -- and in so doing they violate the principle of detailed balance."

Moreover, the two organelles differ with respect to the precise nature of the movements they exhibit: Flagella beat periodically, and their motions display relatively little random variability. Ciliary motions, on the other hand, are characterized by a much higher level of irregularity. In spite of these differences, however, the analyses showed that both systems contravene the principle of detailed balance.

"These findings are of interest not only in the context of biology, although they provide a means of recognizing non-equilibrium situations in biological systems and afford new insights into the complex processes that make life possible," says Broedersz. "They are also of great significance for the fields of statistical mechanics and biophysics, because they raise fundamental issues relating to the question of how active molecular processes drive large-scale non-equilibrium dynamics."
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Ludwig-Maximilians-Universität München

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