A group of American and German scientists described the mechanism by which the protocells, which were the predecessors of the first living organisms on our planet, acquired the ability to grow and divide.
Since ancient times, people have been interested in the question of the origin of life. In the course of history, several hypotheses have emerged, of which only the theory of the primordial soup is probably of scientific value. All the others turned out to be untenable. Creationism, or the theory of divine creation, which dates back to the late Neolithic period, is considered unscientific; the hypothesis of the eternal existence of life completely contradicts paleontological and astronomical data; the hypothesis of bringing life to our planet from the outside (the concept of panspermia), in principle, does not solve the problem and, on the contrary, provokes the question of how life could arise in another world.
For the first time, the version that small droplets in the early stages of the origin of life could be formed due to the separation of molecules in complex mixtures due to phase separation in a coacervate (the so-called primary broth) was expressed by the Soviet biologist Alexander Oparin, a little later - by the British scientist John Haldane. According to the hypothesis, these droplets provided the formation of reactive chemical centers, but at the same time, it remains unclear how they grew and multiplied.
As part of the new study, scientists have observed the behavior of droplets in systems maintained by an external source of energy in a state far from thermodynamic equilibrium. In such systems, droplet growth is carried out by adding droplet material that is produced during chemical reactions. It was found that the growth of a drop, which occurs as a result of chemical processes, entails an instability of the drop shape and provokes its division into two smaller droplets.
Thus, the chemically active droplets showed growth and division cycles resembling the proliferation of tissue in a living organism due to cell multiplication by division (proliferation). The researchers hypothesize that dividing active droplets could serve as a model for prebiotic protocells, in which chemical reactions in the droplet promote prebiotic metabolism.
Liquid droplets are self-organizing structures that can coexist with the surrounding liquid. The surface that divides two adjacent phases gives the droplets a certain shape, due to surface tension - spherical. In addition, some substances have the ability to penetrate the surface of coacervate droplets. Dividing the medium into droplets accumulates a limited volume of material and leads to certain chemical reactions.
Scientists have established the thermodynamics of the birth of a drop, but at the same time they still do not understand how it grows and multiplies, that is, it has the main features that are inherent in a living organism. It is generally accepted that the growth of droplets occurs due to the absorption of a material from a supersaturated medium or the process of recondensation - the transfer of a solute from small particles to large ones by means of dissolution (this process is called Ostwald ripening). In this case, small droplets disappear, only large ones remain. In addition, scientists admit that small droplets can combine and form large ones. Over time, all these processes lead to an increase in the size of droplets and a decrease in their number, although the protocell, upon reaching a certain size, must divide into two.
The researchers hypothesize that coacervate droplets that are maintained far from thermodynamic equilibrium with a chemical fuel may have unusual features, for example, Ostwald ripening in the presence of chemical reactions can be suppressed, whereby several droplets can exist stably with a certain size, which is given by the speed reactions. In this case, spherical droplets, which are subject to chemical reactions, are randomly divided into two smaller droplets of the same size. Scientists suggest that in this way, chemically active droplets can grow and divide, and therefore multiply, using the incoming material as fuel. Therefore, in the presence of chemical reactions that are triggered from external sources, the droplets behave like cells. Such active droplets can be models for the growth and division of protocells with primitive metabolism, which is a simple chemical reaction supported by external fuel.
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These droplets are a kind of reservoir for the spatial organization of certain chemical reactions. For the appearance of drops, it is necessary to separate the phases into two liquid phases of different composition, which exist side by side. The phases are divided due to molecular action, in which similar molecules lower their own energy, being in close proximity to each other. A liquid is capable of stratification if the decrease in energy associated with molecular action due to mixing overcomes the effect of increasing chaos. If such interactions are strong enough, a surface is formed that separates the coexisting phases. If the surface material is formed and destroyed by chemical reactions, the droplets can become reactive.
So, for example, if we consider the model of a simple drop, we can see that it has a minimum number of necessary conditions for the formation and multiplication of a coacervate drop: a phase interface, two phases, as well as an external energy source that keeps the system away from the state of thermodynamic equilibrium … Droplet formation is due to the D-droplet material generated inside the droplet from a high-energy material N, which acts as a nutrient. The droplet material is capable of decomposing into lower energy components W (waste), which, as a result of diffusion, leave the droplet. A drop can survive when there is a continuous supply of N and a constant removal of W. This can be achieved through the recirculation of N using an external energy source, in particular,sunlight or certain fuels.
The study authors believe that the physics of active droplets is quite simple. It is easiest to understand by the example of a model with two components A and B. When the phase of the material of the droplet B separates from the solvent, it can be randomly transformed due to the chemical reaction BA into type A molecules, which are soluble in the background liquid. A drop remains. The reverse reaction A-B is no longer spontaneous, since B has a higher energy than A. New droplet material B can be obtained by the reaction A + C-B + C associated with fuel. In this case, C is a low-energy reaction product of fuel molecules. The fuel provides a chemical potential difference, which makes it possible to reach state B with high energy from a lower energy state A. The potential difference can be constant ifif the concentrations of C in them are given by an external reservoir. In this case, the system is kept far from the state of thermodynamic equilibrium.
Scientists have studied the combination of phase separation and unbalanced chemical reactions also in a continuous model. Researchers have found that chemically active spherical droplets can be unstable and divide into two smaller droplets. Initially, the drop grows until it reaches a stationary size. After that, it lengthens, forming a dumbbell shape. This dumbbell is then divided into two smaller droplets of the same size. Ultimately, the smaller droplets begin to grow again until a new division.
As the scientists note, the phenomena that they modeled can be directly observed in the experiment. According to the researchers, the instability of droplets, which is triggered by an external influx of energy and which leads to droplet fission, can be compared to the Mullins-Sekerki instability, which is often discussed in the context of crystal growth. However, in contrast to it, instability of the droplet shape can also occur in the presence of a motionless non-growing droplet.
Modern cells have some chemical structures that are not separated from the cellular cytoplasm by a membrane. They are formed by phase separation from the cytoplasm. Most of them are liquid and consist of RNA-binding proteins and RNA molecules. According to the hypothesis of the RNA world, in the early periods of life, RNA was both a carrier of genetic information and played the role of a ribozyme. It is likely that the combination of RNA with simple peptides was sufficient to form coacervate droplets.
As the authors of the study note, the transformation of chemically active droplets in a cell that is dividing for the first time is a big problem for understanding the early evolutionary process. Unlike the external and internal droplet media, the interface between these media is amphiphilic. Those lipids that have no affinity for the internal and external environment of the droplet could accumulate on the amphiphilic surface, provided they are present in the external environment of coacervate droplets. According to experts, membranes in coacervates could appear much earlier than the first division of protocells took place.