Protocells: at the interface of life and non-life

Synthesis of Phospholipids Under Plausible Prebiotic Conditions and Analogies with Phospholipid Biochemistry for Origin of Life Studies

Phospholipids are essential components of biological membranes and are involved in cell signalization, in several enzymatic reactions, and in energy metabolism. In addition, phospholipids represent an evolutionary and non-negligible step in life emergence. Progress in the past decades has led to a deeper understanding of these unique hydrophobic molecules and their most pertinent functions in cell biology. Today, a growing interest in "prebiotic lipidomics" calls for a new assessment of these relevant biomolecules.

The optimal size of protocells from simple entropic considerations

Potential constraints on protocell size are developed from simple entropic considerations. To do that, two new different indexes as measures of their structural and dynamic order were developed and applied to an elemental model of the heterotrophic protocell. According to our results, cell size should be a key factor determining the potential of these primitive systems to evolve and consequently to support life. Our analyses also suggest that the size of the optimal vesicles could be constrained to have radii in the interval [Formula: see text], where the two extreme limits [Formula: see text] and [Formula: see text] represent the states of maximum structural order (largest accumulation of substrate inside the vesicle) and the maximum flux of entropy production, respectively. According to the above criteria, the size of the optimum vesicles falls, approximately, in the same spatial range estimated for biological living cells assuming plausible values for the second-order rate constant involved in the catabolic process. Furthermore, the existence of very small vesicles could be seriously affected by the limited efficiency, far from the theoretical limits, with which these catabolic processes may proceed in a prebiotic system.

The Conception of Synthetic Entities from a Personalist Perspective

Synthetic biology opens up the possibility of producing new entities not found in nature, whose classification as organisms or machines has been debated. In this paper we are focusing on the delimitation of the moral value of synthetic products, in order to establish the ethically right way to behave towards them. In order to do so, we use personalism as our ethical framework. First, we examine how we can distinguish between organisms and machines. Next, we discuss whether the products of synthetic biology can be considered organisms at all and assess what their moral value is and how should we behave towards them. Finally, we discuss the hypothetical case of synthetic humans.

Constraining the Prebiotic Cell Size Limits in Extremely Hostile Environments: A Dynamical Perspective

The ability to support a replicator population in an extremely hostile environment is considered in a simple model of a prebiotic cell. We explore from a classical approach how the replicator viability changes as a function of the cell radius. The model includes the interaction between two different species: a substrate that flows from the exterior and a replicator that feeds on the substrate and is readily destroyed in the environment outside the cell. According to our results, replicators in the cell only exist when the radius exceeds some critical value [Formula: see text] being, in general, a function of the substrate concentration, the diffusion constant of the replicator species, and the reproduction rate coefficient. Additionally, the influence of other parameters on the replicator population is also considered. The viability of chemical replicators under such drastic conditions could be crucial in understanding the origin of the first primitive cells and the ulterior development of life on our planet. Key Words: Prebiotic cell-Chemical replicator-Environment-Reproduction rate. Astrobiology 18, 403-411.

What Does "the RNA World" Mean to "the Origin of Life"?

Corresponding to life’s two distinct aspects: Darwinian evolution and self-sustainment, the origin of life should also split into two issues: the origin of Darwinian evolution and the arising of self-sustainment. Because the “self-sustainment” we concern about life should be the self-sustainment of a relevant system that is “defined” by its genetic information, the self-sustainment could not have arisen before the origin of Darwinian evolution, which was just marked by the emergence of genetic information. The logic behind the idea of the RNA world is not as tenable as it has been believed. That is, genetic molecules and functional molecules, even though not being the same material, could have emerged together in the beginning and launched the evolution—provided that the genetic molecules can “simply” code the functional molecules. However, due to these or those reasons, alternative scenarios are generally much less convincing than the RNA world. In particular, when considering the accumulating experimental evidence that is supporting a de novo origin of the RNA world, it seems now quite reasonable to believe that such a world may have just stood at the very beginning of life on the Earth. Therewith, we acquire a concrete scenario for our attempts to appreciate those fundamental issues that are involved in the origin of life. In the light of those possible scenes included in this scenario, Darwinian evolution may have originated at the molecular level, realized upon a functional RNA. When two or more functional RNAs emerged, for their efficient cooperation, there should have been a selective pressure for the emergence of protocells. But it was not until the appearance of the “unitary-protocell”, which had all of its RNA genes linked into a chromosome, that Darwinian evolution made its full step towards the cellular level—no longer severely constrained by the low-grade evolution at the molecular level. Self-sustainment did not make sense before protocells emerged. The selection pressure that was favoring the exploration of more and more fundamental raw materials resulted in an evolutionary tendency of life to become more and more self-sustained. New functions for the entities to adapt to environments, including those that are involved in the self-sustainment per se, would bring new burdens to the self-sustainment—the advantage of these functions must overweigh the corresponding disadvantage.

The essence of life

Abstract

Although biology has achieved great successes in recent years, we have not got a clear idea on “what is life?” Actually, as explained here, the main reason for this situation is that there are two completely distinct aspects for “life”, which are usually talked about together. Indeed, in respect to these two aspects: Darwinian evolution and self-sustaining, we must split the concept of life correspondingly, for example, by defining “life form” and “living entity”, separately. For life’s implementation (related to the two aspects) in nature, three mechanisms are crucial: the replication of DNA/RNA-like polymers by residue-pairing, the sequence-dependent folding of RNA/protein-like polymers engendering special functions, and the assembly of phospholipid-like amphiphiles forming vesicles. The notion “information” is significant for us to comprehend life phenomenon: the life form of a living entity can just be defined by its genetic information; Darwinian evolution is essentially an evolution of such information, transferred across generations. The in-depth analysis concerning the essence of life would improve our cognition in the whole field of biology, and may have a direct influence on its subfields like the origin of life, artificial life and astrobiology.

Reviewers

This article was reviewed by Anthony Poole and Thomas Dandekar.