PROTOBIONTS: ORIGIN AND PROPERTIES - BIOLOGY - 2025

The protobionts are biological complexes according to some hypotheses concerning the origin of life, preceded the cells. According to Oparín, these are molecular aggregates surrounded by a semipermeable lipid membrane or a structure similar to it.

These biotic molecular aggregates could present a simple reproduction and a metabolism that managed to maintain the chemical composition of the interior of the membrane different from its external environment.

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Some experiments carried out in the laboratory by different researchers have revealed that protobionts could form spontaneously using organic compounds created from abiotic molecules as building blocks.

Examples of these experiments are the formation of liposomes, which are aggregations of small droplets surrounded by membranes. These can form when lipids are added to water. It also happens when other types of organic molecules are added.

It may happen that liposome-like droplets were formed in ponds of prebiotic times and these randomly incorporated some polymers of amino acids.

In the event that the polymers made certain organic molecules permeable to the membrane, it would be possible to selectively incorporate said molecules.

Properties and characteristics

The putative protobionts could be formed from hydrophobic molecules that were organized in the form of a bilayer (two layers) on the surface of a droplet, reminiscent of the lipid membranes present in today's cells.

By Mariana Ruiz Villarreal, LadyofHats, from Wikimedia Commons

Semi-permeable membranes

Since the structure is selectively permeable, the liposome can swell or deflate depending on the concentration of solutes in the medium.

That is, if the liposome is exposed to a hypotonic environment (the concentration inside the cell is higher), water enters the structure, swelling the liposome. In contrast, if the medium is hypertonic (the concentration of the cell is lower), the water moves towards the external medium.

This property is not unique to liposomes, it can also be applied to the actual cells of an organism. For example, if red blood cells are exposed to a hypotonic environment, they can explode.

Excitability

Liposomes can store energy in the form of a membrane potential, which is a voltage across the surface. The structure can discharge voltage in a way reminiscent of the process that occurs in neuronal cells of the nervous system.

Liposomes have several characteristics of living organisms. However, it is not the same as claiming that liposomes are alive.

Origin

There is a wide diversity of hypotheses that seek to explain the origin and evolution of life in a prebiotic environment. The most outstanding postulates that discuss the origin of protobionts will be described below:

Oparin and Haldane hypothesis

The hypothesis on biochemical evolution was proposed by Alexander Oparin in 1924 and by John DS Haldane in 1928.

This postulate assumes that the prebiotic atmosphere lacked oxygen, but was strongly reducing, with large amounts of hydrogen that led to the formation of organic compounds thanks to the presence of energy sources.

According to this hypothesis, as the earth cooled, steam from volcanic eruptions condensed, precipitating as heavy and constant rains. When the water fell, it carried mineral salts and other compounds, giving rise to the famous primal soup or nutritive broth.

In this hypothetical environment, large molecular complexes called prebiotic compounds could form, giving rise to increasingly complex cellular systems. Oparin called these structures protobionts.

As protobionts increased in complexity, they acquired new abilities to transmit genetic information, and Oparin gave the name eubionts to these more advanced forms.

Miller and Urey experiment

In 1953, after the Oparin postulates, researchers Stanley L. Miller and Harold C. Urey carried out a series of experiments to verify the formation of organic compounds starting from simple inorganic materials.

Miller and Urey managed to create an experimental design that simulated prebiotic environments with the conditions proposed by Oparin on a small scale, managing to obtain a series of compounds such as amino acids, fatty acids, formic acid, urea, among others.

Genetic material of protobionts

RNA world

According to the hypotheses of current molecular biologists, protobionts carried RNA molecules, instead of DNA molecules, which allowed them to replicate and store information.

In addition to having a fundamental role in protein synthesis, RNA can also behave as an enzyme and carry out catalysis reactions. Due to this characteristic, RNA is an indicated candidate to be the first genetic material in protobionts.

RNA molecules capable of catalysis are called ribozymes and can make copies with complementary sequences of short stretches of RNA and mediate the splicing process, eliminating stretches of the sequence.

A protobiont that had a catalytic RNA molecule inside it varied from its homologues that lacked this molecule.

In case the protobiont could grow, divide and transmit the RNA to its offspring, the processes of Darwinian natural selection can be applied to this system, and the protobionts with RNA molecules would increase their frequency in the population.

Although the appearance of this protobiont may be very unlikely, it is necessary to remember that millions of protobionts may have existed in the bodies of water of the early earth.

Appearance of DNA

DNA is a much more stable double-stranded molecule compared to the RNA molecule, which is fragile and replicates imprecisely. This property of accuracy in terms of replication became more necessary as the genomes of the protobionts increased in size.

At Princeton University, researcher Freeman Dyson proposes that DNA molecules could have been short structures, assisted in their replication by polymers of random amino acids with catalytic properties.

This early replication could occur inside protobionts that had stored high amounts of organic monomers.

After the appearance of the DNA molecule, RNA could begin to play its current roles as intermediaries for translation, thus creating the "world of DNA".

References

1. Altstein, AD (2015). The progene hypothesis: the nucleoprotein world and how life began. Biology Direct, 10, 67.
2. Audesirk, T., Audesirk, G., & Byers, BE (2003). Biology: Life on Earth. Pearson education.
3. Campbell, AN, & Reece, JB (2005). Biology. Editorial Médica Panamericana.
4. Gama, M. (2007). Biology 1: a Constructivist Approach. Pearson Education.
5. Schrum, JP, Zhu, TF, & Szostak, JW (2010). The origins of cellular life. Cold Spring Harbor perspectives in biology, a002212.
6. Stano, P., & Mavelli, F. (2015). Protocells Models in Origin of Life and Synthetic Biology. Life, 5 (4), 1700–1702.