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PROTOCELL

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An Intermediary between the in-organic and the organic world

Introduction

a few days ago, I posted an articcle"Origin of life" where I explained how science has been able to explain how Life could have evolved from the primordial non-organic molecules and the energy conditions that could have catalyzed the reaction. On this topic, I recieved a comment from @scienceblocks who made mention of how scientists have made extensive efforts in creating an artificial life. wow! a great contribution. However, this comment prompted the idea of sharing this today. Science has been tireless in trying to close the gap between the non-living and the living. it is well known that all living organisms are made of tiny molecules that are not living, this give a reason for a connection between the inorganic world and the organic world. however, in order to connect these worlds science has been trying to create a model that is made of inorganic molecules and at the same time mimic the biological cell in structure, morphology and function. In order to produce an artificial model that would mimic the living cell, it is desirable that the model satisfy the criteria for a biological cell before it can be declared a living system. Biological cells and their unique properties ----- A biological cell is the basic structural and functional unit of all living organism. It serve as the building block for every living system by possessing the key characteristics and functionalities of Life. From a biological view, a living cell can be seen to be a system with a definite boundary containing reproducible hereditary instructions in form of nucleic acid molecules, and that metabolize by controlling the rate of chemical reactions using the proteinaceous catalyst called Enzymes. Although, defining a cell in this way is not bad, but a living cell possess a unique quality, “Autopoiesis”. This is ability to self maintain, self-organize, self-reproduce, evolve and die.

Biological cells possess three main components for performing the essential functions of life: >- A stable, semi-permeable membrane that encloses cell constituents protecting them from being damaged by the external environment while allowing selective material and energy exchanges. >- Biomacromolecules (DNA or RNA) that carry the genetic information, control the dynamics of the cell, and endow it with the capability of evolution. >- A series of metabolic pathways used for providing energy to cells, to make them self-maintain and self-renew, as well as self-process information. ncbi The Protocell --- To begin with, it was suggested that the biological cell we have today would have evolved from a simpler precursor that is simple enough to carry out life activities. This model was called a Protocell. This structure being a precursor for the first cell serves as an intermediary that closes the gap between the inorganic and the organic world. In order to serve this function, it has to support the two sides (inorganic and organic); on the first side, the protocell must be made of inorganic primordial molecules that supports life and which can be formed by elements present on the early Earth, while on the other side it has to possess basic functionalities of a living cell. This primordial structure must posses unique features that allow it to grow, replicate and evolve.

The protocell was proposed to have two major structures : - A membrane : a bilayer structure that encapsulate the structure, separating it from the environment and this also create a medium for transmissive communication (material exchange) - An RNA molecule that houses, and interpret genetic codes, this genetic code organizes and controls metabolic pathways, and maintain biological function. This RNA is responsible for cellular replication. The plasma membrane of a biological cell serves as a protective sheath separating the body from the surrounding environment. The plasma membrane is composed of a complex amphiphilic phospholipid structure that possesses an hydrophilic head and an Hydrophobic tail.

This amphiphilic property gives it the ability to form a bilayer with selective permeability. The phospholipid in plasma membrane is too complex to have existed in the early earth, however a simpler structure that could mimic its function has to be considered for the Protocell. The fatty acid, a single-chained amphiphilic molecule can act just like the phospholipid as it possess similar property; It can form a bilayer, vescicles and micelles in water. the primitive membrane of fatty acid does not just serve as an enclosing sheet but also support metabolic exchange of materials by diffusion, protein synthesis, growth, and division. The fatty acid membrane grow by incorporation fatty acid monomer, micelle or vesicle in the chain thereby creating a spontaneous increase in the bulk of the membrane. When the size get to a critical value, the vesicle divides into daughter protocells due to thermodynamic imbalance. There is a random division of RNA replicases between the two daughter cells.

Another major characteristic of a living cell is the ability to evolve by natural selection. This is the ability of a living organism to produce an offspring with predominant potentials that favors survival in the environment. The theory Evolution by natural selection was proposed by Charles Darwin and Alfred Wallace. In this theory, it is stated that natural selection gradually leads to predominance of genes that favors survival and reproduction in that particular environment. That is, traits (either innate or mutated), found in parents, that are most favorable for survival and reproduction in their environment have a greater chance of passing on to succeeding generations. This phenomenon gives rise to a stronger species that are better suited to the particular environment and lifestyles. During the replication of a protocell, there are mistakes at times causing a mutation in in the RNA of the daughter cells. This mutation can give rise to a poorer or better species thereby providing a room for competing for survival. Antonymous pathways engineering an artificial cell. --- The artificial cell (protocell) that could exhibit features of a true cell can be created by two pathways – the bottom – up pathways and the top – down pathway. These two pathways are different and opposite views of engineering a protocell.

The Top – down pathway involves simplifying the genome of a living cell to a minimal cell. the minimal cell is a cell with a less number of genomes as a result of knocking out of non-essential genes. In this course, there is knocking out or replacement of genes that are not very essential in carrying out life processes. That is the cell is left with genes needed to survive and not to perform special function. This procedure tends to provide a knowledge of the basic life processes and its requirements for creating an artificial life. The Bottom – up pathway involves building an artificial cell by combining simple molecules that were proposed to have existed in the early earth. With the availability of the right catalyst and energy supply, these molecules tends to aggregate in the right proportion to form complex biomolecules. The Miller-Urey experiment prove how prebiotic gases (methane, ammonia and hydrogen) with water and lightening could have formed amino-acids which are building blocks for living cell. Benefits/advantages of the artificial Bio-engineering ----- The subsequent experiment being carried out in order to synthesize a protocell/artificial cell does not just follow a closed pathway that ends in creating an artificial life. Although being that a true cell has not yet been engineered from a protocell; there has been a significant application of this knowledge in different scientific sectors like medicine, biotechnology, cell engineering and bioenergy. It has helped in the establishment of facts and theories that explains how cellular life could have evolved, providing knowledge that bridge the gap between the non-living and the living. it has also helped in synthetic biology where new cells are being synthesized with new features that are absent in biological cells. Drug distribution and delivery to target organ; biomedical applications such as replacement or supplement of deficient cells to improve health are also significant advantages.

Source: Steemstem Project

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