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Active droplets center internal particles

Active droplets are a class of active matter, where the material forming the droplets is turned over by chemical reactions. We here show that this turnover can be used to center solid particles inside the droplets. Such control over the droplet morphology explains the structure of centrosomes, which are biomolecular condensatein cells. More generally, driving droplets with chemical reactions might allow biological cells to control droplet formation to structure their interior.

 

Spontaneously dividing active droplets as a model for protocells

The first lifeforms on earth must have been simple enough to emerge spontaneously, but they also needed to divide and propagate, such that natural selection and evolution could lead to more complex lifeforms. So far, the physical properties of such early cells are unclear, but it has been proposed almost a century ago that they could have been liquid-like droplets. We showed that such simple liquid droplets can indeed divide spontaneously if the chemical reaction that builds the droplet material from precursors is driven by an external energy input, like sun light or heat from thermal vents. Here, the chemical reaction of these active droplets plays the role of a prebiotic metabolism.

 

Stable emulsions

Controlling the formation and stabilizing droplets is important in many fields ranging from the food industry to cosmetics and medicine. Furthermore, there is more and more evidence that droplets also play an important role to organize the interior of biological cells. Indeed, we propose that centrosomes are liquid droplets. One problem with liquid droplets is that they try to combine to form on large droplet, which is energetically more favorable.

The video on the left shows that chemical reactions influencing the physical properties of the droplet material can prevent this droplet coarsening. We study generic physical models of droplet formation under the influence of chemical reactions to identify the necessary conditions where multiple droplets are stable. This improves our understanding of droplet formation inside cells and might also benefit technical applications.

 
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Centrosomes described as active droplets

Centrosomes are small organelles present in all animal cells. They are important for organizing the mitotic spindle, which segregates the DNA during cell division. Cancer cells often contain more than two centrosomes, which impairs normal cell division. It is thus important to understand the assembly of centrosomes in order to tackle failures in their formation and function.

The centrosome is a dynamic aggregate, which forms and dissolves in synchrony with the cell cycle. Beside reactions between and diffusion of centrosome components, aspects of non-equilibrium thermodynamics have to be considered to describe the observed behavior. In fact, we developed a physical description of centrosomes as autocatalytic droplets, whose formation is controlled by chemical reactions. This project was carried out in close collaboration with the group of Tony Hyman at the MPI of Molecular Cell Biology and Genetics.