ORIGIN OF THE CELL: MAIN THEORIES (PROKARYOTIC AND EUKARYOTIC) - BIOLOGY - 2024

The origin of the cell dates back more than 3.5 billion years old. The way in which these functional units originated has aroused the curiosity of scientists for several centuries.

The origin of life per se was accompanied by the origin of cells. In a primitive environment, the environmental conditions were very different from what we observe today. The oxygen concentration was practically zero, and the atmosphere was dominated by another composition of gases.

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Different experiences in the laboratory have shown that under the initial environmental conditions of the Earth, the polymerization of various biomolecules characteristic of organic systems is possible, namely: amino acids, sugars, etc.

A molecule with catalytic capacity and to replicate itself (potentially, an RNA) could be enclosed in a phospholipid membrane, forming the first primitive prokaryotic cells, which evolved following Darwinian principles.

Likewise, the origin of the eukaryotic cell is usually explained using the endosymbiotic theory. This idea supports that a large bacterium engulfed a smaller one and with the passage of time originated the organelles that we know today (chloroplasts and mitochondria).

The cell theory

Cell is a term that comes from the Latin root cellula, which means hollow. These are the functional and structural units of living things. The term was first used in the 17th century by researcher Robert Hooke, when he was examining a sheet of cork under the light of a microscope and observed a kind of cells.

With this discovery, more scientists - notably the contributions of Theodor Schwann and Matthias Schleiden - became interested in the microscopic structure of living matter. In this way, one of the most important pillars of biology was born: cell theory.

The theory holds that: (a) all organic beings are made up of cells; (b) cells are the unit of life; (c) the chemical reactions that sustain life occur within the limits of the cell and (d) all life comes from pre-existing life.

This last postulate is summarized in the famous phrase of Rudolf Virchow: "omnis cellula e cellula" - all cells are derived from other already existing cells. But where did the first cell come from? Next we will describe the main theories that seek to explain the origin of the first cellular structures.

Evolution of the prokaryotic cell

The origin of life is a phenomenon closely linked to the origin of cells. On earth, there are two cellular forms of life: prokaryotes and eukaryotes.

Both lineages differ basically in terms of their complexity and structure, with eukaryotes being larger and more complex organisms. This is not to say that prokaryotes are simple - a single prokaryotic organism is an organized and intricate agglomeration of various molecular complexes.

The evolution of both branches of life is one of the most exciting questions in the world of biology.

Chronologically, life is estimated to be 3.5 to 3.8 billion years old. This appeared approximately 750 million years after the formation of the Earth.

Evolution of early life forms: Miller's experiments

In the early 1920s the idea that organic macromolecules could polymerize spontaneously under the environmental conditions of a primitive atmosphere - with low oxygen concentrations and high concentrations of CO ₂ and N ₂, as well as a series of of gases such as H ₂, H ₂ S, and CO.

It is assumed that the hypothetical primitive atmosphere provided a reducing environment, which together with an energy source (such as sunlight or electric discharges), created the conditions conducive to the polymerization of organic molecules.

This theory was confirmed experimentally in 1950 by researcher Stanley Miller during his graduate studies.

The need for a molecule with self-replicating and catalytic properties: the world of RNA

After specifying the necessary conditions for the formation of the molecules that we find in all living beings, it is necessary to propose a primitive molecule with the ability to store information and replicate itself - current cells store genetic information under a language of four nucleotides in the DNA molecule.

To date, the best candidate for this molecule is RNA. It was not until 1980 that researchers Sid Altman and Tom Cech discovered the catalytic capabilities of this nucleic acid, including the polymerization of nucleotides - a critical step in the evolution of life and cells.

For these reasons, it is believed that life began using RNA as genetic material, and not DNA as the vast majority of current forms do.

Limiting the barriers of life: phospholipids

Once the macromolecules and the molecule capable of storing information and replicating itself have been obtained, the existence of a biological membrane is necessary to determine the boundaries between the living and the extracellular environment. Evolutionarily, this step marked the origin of the first cells.

The first cell is believed to have arisen from an RNA molecule that was enclosed by a membrane made up of phospholipids. The latter are amphipathic molecules, which means that one portion is hydrophilic (soluble in water) and the other portion is hydrophobic (not soluble in water).

When phospholipids are dissolved in water, they have the ability to spontaneously aggregate and form a lipid bilayer. The polar heads are grouped facing the aqueous environment and the hydrophobic tails inside, in contact with each other.

This barrier is thermodynamically stable and creates a compartment that allows the cell to be separated from the extracellular environment.

With the passage of time, the RNA enclosed within the lipid membrane continued its evolutionary path following Darwinian mechanisms - until it presented complex processes such as protein synthesis.

Evolution of metabolism

Once these primitive cells were formed, the development of the metabolic pathways that we know today began. The most plausible scenario for the origin of the first cells is the ocean, so the first cells were able to obtain food and energy directly from the environment.

When food became scarce, certain cell variants had to appear with alternative methods of obtaining food and generating energy that allowed them to continue their replication.

The generation and control of cell metabolism are essential for its continuity. In fact, the main metabolic pathways are widely conserved among current organisms. For example, both a bacterium and a mammal carry out glycolysis.

It has been proposed that energy generation evolved in three stages, beginning with glycolysis, followed by photosynthesis, and ending with oxidative metabolism.

Since the primitive environment lacked oxygen, it is plausible that early metabolic reactions did without it.

Evolution of the eukaryotic cell

Cells were uniquely prokaryotic until around 1.5 billion years ago. At this stage, the first cells with a true nucleus and organelles themselves appeared. The most prominent theory in the literature that explains the evolution of organelles is the endosymbiotic theory (endo means internal).

Organisms are not isolated in their environment. Biological communities present multiple interactions, both antagonistic and synergistic. An umbrella term used for different interactions is symbiosis - formerly used only for mutualistic relationships between two species.

Interactions between organisms have important evolutionary consequences, and the most dramatic example of this is the endosymbiotic theory, which was initially proposed by American researcher Lynn Margulis in the 1980s.

Postulates of endosymbiotic theory

According to this theory, some eukaryotic organelles - such as chloroplasts and mitochondria - were initially free-living prokaryotic organisms. At some point in evolution, a prokaryote was swallowed by a larger one, but was not digested. Instead, he survived and was trapped within the larger organism.

In addition to survival, the reproduction times between the two organisms were synchronized, managing to pass to successive generations.

In the case of chloroplasts, the engulfed organism exhibited all the enzymatic machinery to carry out photosynthesis, supplying the larger organism with the products of these chemical reactions: monosaccharides. In the case of mitochondria, it is postulated that the engulfed prokaryote could be an ancestral α-proteobacteria.

However, the potential identity of the larger host organism is an open question in the literature.

The engulfed prokaryotic organism lost its cell wall, and throughout evolution underwent the pertinent modifications that originated modern organelles. This is, in essence, the endosymbiotic theory.

Evidences for the endosymbiotic theory

Currently there are multiple facts that support the theory of endosymbiosis, namely: (a) the size of current mitochondria and chloroplasts is similar to that of prokaryotes; (b) these organelles have their own genetic material and synthesize part of the proteins, although they are not completely independent of the nucleus and (c) there are multiple biochemical similarities between both biological entities.

Advantages of being eukaryotic

The evolution of eukaryotic cells is associated with a series of advantages over prokaryotes. The increase in size, complexity and compartmentalization allowed the rapid evolution of novel biochemical functions.

After the arrival of the eukaryotic cell, came multicellularity. If a cell "wants" to enjoy the benefits of a larger size, it cannot simply grow, since the cell surface must be large in relation to its volume.

Thus, organisms with more than one cell were able to increase their size and distribute the tasks among the multiple cells that compose them.

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