BASIC CONCEPTS CONCERNING THE ORIGIN OF LIFE

CHAPTER THREE

CHAPTER SUMMARY

The cellular classification of life.

Life is a natural stage that comes about with the development of the universe. Although life is a very complex chemical system it is none-the-less a chemical system, and obeys physico-chemical laws. Psychic laws, related to mental processes, if they are ever found to exist will probably be related to wave theory and similarly be based upon physical and chemical principles.

The conditions necessary for life to develope on a planet (as opposed to in interstellar space) are probably fairly common in this universe. It has been estimated that about 1-5% of the stars in a galaxy might possess planets capable of supporting life. There are over 100,000,000 galaxies within the range of our telescopes and thus the number of planets that could possess living systems is approximately 10¹⁷ (1,000,000,000,000,000,000 planets). This figure is based upon observation and estimation using scientific reasoning and the laws of probability. Even if this figure is reduced 1 million or 1 billion times it still leaves us with the conclusion that life is scattered and even widespread in this universe.

If a stellar system has planets and some planets are at a distance from the star such that water occurs on the planets surface as a liquid, then such a planet is a good candidate to become a living-planet. Within the temperature range where liquid water exists the chemical reactions necessary for the development of living molecules can progress. At higher temperatures the chemical reactions slow down. Moreover, the stellar system must be at least a second generation star because it must have developed the heavier elements, particularly carbon. The density of the planet ideally should be similar to that of our inner planets and preferably have a size approaching that of planet earth. A smaller planet probably could not hold an adequate atmosphere and a larger planet would hold too dense an atmosphere and screen out radiation.

Although those life forms on earth are perfectly adapted to using our suns dominant radiation (visible spectrum) for seeing with, the variety of life processes that occur using sunlight use that part of the spectrum between 300-1,100 micrometers. Shorter wave lengths destroy large molecules, larger wave-lengths cannot activate photochemical reactions.

On planet earth the cell is the basic unit of living organisms. Any quest to understand the origin of life must end up showing how cells developed. From the chemical viewpoint cells are complex collections of organic molecules. These organic molecules are self regulating, and self organizing and exchange matter and energy with their environment by means of extremely efficient chemical reactions. The crucial final fact is that these complex chemical reactions take place within a spherical molecule: the cell membrane. The cell membrane protects the chemical reactions within from unstable conditions occurring outside. Allowing only certain chemical to pass through it the cell membrane is a semi-permeable membrane. Such a simple system exists today as a group of organisms called MICROPLASMA.

Living systems on earth actually show two kinds of cells. The simple cells consisting of a cell membrane containing chemicals and molecules is called the prokaryotic cell. Organisms with this kind of cell are the bacteria, and are grouped in the KINGDOM MONERA.

The second kind of cell shows within it other structures called organelles. Organelles are locations within the cell where very specific chemical reactions occur but these reacting centers are themselves encapsulated by a cell membrane. In one sense, these cells are cells containing cells! This presence of the cell membrane around the organelle actually makes these localized centers of chemical action more efficient: protecting them from other chemicals within the rest of the cell. These advanced cells are the eucaryotic cells. Organisms with this kind of cell are extremely diverse and include the KINGDOM PROTISTA, KINGDOM PLANTAE, KINGDOM FUNGI and KINGDOM ANIMALIA. These separate kingdoms developed as differing chemical modes of living.

Irrespective of whether or not a cell is procaryotic or eucaryotic chemically they are very much the same. The two cell types are differentiated on how they engage in chemical reactions not their basic chemical reactions and chemical composition.

In addition to the overall chemical structure of the cell one can classify life forms according to how they obtain their energy [photosynthetic or chemosynthetic] and carbon Organotrophic and Lithotrophic].

THE CHEMISTRY OF CELLS

The chemical elements required by cells are either:

It is significant that of the five most abundant chemical elements in the solar system all of them except helium play a very important part in the make-up of organic matter [H,O,C,N]. In addition, S and P are important in living matter and these are common elements in our solar system, (9th and 16th respectively). These six principal elements [H,O,C,N,S,P] found in organisms organize themselves into six major constituents. These are:

water: the major solvent;

fats: long term energy resources;

carbohydrates: intermediate energy resources;

Adenosine Phosphates: rapid energy transfer resources;

Proteins: forming structural units and enzymes;

Nucleic Acids: controlling protein synthesis.

Water ionizes H⁺ and OH^-. These ions are important in chemical reactions during metabolism. They also help to maintain the acidity of cell near neutral. The organic compounds are of increasing molecular weight and complexity: from carbohydrates, through fats, adenosine phosphates, proteins to nucleic acids. Most cells cannot use elemental material directly and must obtain compounds in the form of small organic molecules. Cells are capable of using only those organic compounds small enough to pass through their membrane. Such small organic compounds are called monomers. Monomers include such things as amino-acids, nucleotides and sugars. Within a cell these combine into macromolecules. Macromolecules are usually built from a single kind of monomer. Thus protein is formed from strings of amino acids. Nucleic acids are formed from strings of nucleotides, and carbohydrates are formed from sugars.

Bacteria and fungi are the major causes of breakup of macromolecules into monomers used by higher organisms. Within the living system we can place organisms into primitive and advanced according to whether or not they are dependent upon another organism to pre-form their basic molecules. Man for example, cannot manufacture vitamins but man needs vitamins so must obtain them from other organisms. Animals in general use bacteria in their gut to help them breakdown the macromolecules before animal cells use the organic compounds.

A general characteristic for early life forms must have been an ability to obtain monomers from outside the cell. Such a cell may have been procaryotic or eucaryotic. Although procaryotic systems are simpler to understand they are not necessarily more ancient than eucaryotic systems. There is one idea which suggests that both cell types originated at the same period of earth history.

At one time it was thought necessary to devise a mechanism whereby monomers could be built-up from the elements COHNPS under primitive earth conditions. Scientists spent many years showing that such could happen. The HALDANE-OPARIN THEORY on the biochemical origin of life, for example, states that initially inorganic matter gave raise to organic matter at the earth's surface, during the period of chemical evolution lasting some 1.5 BY. The monomers were formed and those with superior stability dominated. That this was possible was shown by experiments using the chemicals presumed to compose the original earth atmosphere and bombarding the chemicals with energy such as ultra-violet radiation. Such a scenario was indeed possible, for the early atmosphere was methane (CH₄), ammonia (NH₃) and hydrogen sulphide rich (H₂S) rich. This together with water and later CO₂ could readily be transformed into monomers by ultraviolet radiation. However, recently we have discovered that this complexity is not necessary. We now know that in second generation stellar systems the interstellar space is rich in carbon and that numerous kinds of carbon compounds are produced in space. Formaldehyde (H₂C0), formic acid (HCOOH), methanimine (H₂CHN), cellulose, and many others occur. The significant point is two-fold. The pre-cursors to life-forming monomers exist in space and were probably here on earth at the very instant that the earth was formed. Secondly, many of these pre-cursor biochemicals undergo simple reactions to form monomers. Formic acid and methanimine for example, react to form the simplest amino acid GLYCINE (NH₂CH₂COOH); and, formaldehyde is the common component of sugars and many other monomers. The HALDANE-WICKRAMASINGHE THEORY propounded in the book LIFECLOUD goes so far as to suggest that macromolecules could build-up in the interstellar clouds and even life-forms exist in such places. This is the regeneration of an old theory called PANSPERMIA: that life originates everywhere in the universe. It also is somewhat reminiscent of an old Startrek episode! Whether the macromolecules built-up in interstellar clouds or at the earth's surface is not relevant to the overall theory of a biochemical origin to life. The critical stage is still the development of the containing cell membrane. However, between the pre-life stage of biological carbon complexity and cellular life itself, a proto-biological stage of a naked cell might have occurred. In such a naked cell the nucleic acids, proteins, and other organic molecules interacted without being enclosed in a membrane. The modern viruses may represent something akin to this stage. A virus is mainly DNA or RNA surrounded by protein. Today they need a synthetic host to exist but this may be an adaptation that occurred after cellular life formed.

Cell membranes can be considered as hollow organic molecules. They are composed of protein molecules and phospho-lipid molecules linked together. Their important characteristics are that they are hollow and that they can pass certain chemicals through their structure.

The basic idea for the development of the procaryotic cell then is that the fundamental units developed either on earth or in space, at some stage the spherical molecules developed. Alternatively, a kind of skin developed around a virus so that there never was a spherical molecule. Such a skin might have been a waste product from viral activity. The first scenario maintains that the procaryotics and eucaryotics developed simultaneously as the enclosing spherical molecules randomly enclosed any reaction that was taking place. Thus some cells could become enclosed in other cells! The idea is that many varied kinds of cells developed but only the procaryotic cell of today and the eucaryotic cell of today were competitively successful. It is possible that all of the major cell types that could originate actually did originate and occur today forming the variety of eucaryotics that coexist. The second scenario presents an orderly development of procaryotic life forms through the virus-like organisms. This scenario is useful if we do not accept that prokaryotes and Eucaryotes developed together because in that case a mechanism must be found whereby the prokaryotes can evolve into the Eucaryotes. Such a mechanism is not difficult to visualize for some procaryotic cells have specialized chemical reactions going on the outside of their membrane. Slight invaginations of the membrane form a protective enclosure for such reactions (compare the gut of man) and it is possible that these invaginations simply pinched-off internally to form organelles of a eucaryotic cell. Currently the following model is favored.

Monomer -> Macromolecules -> Protolife (viral stage) -> Procaryotes -> Eucaryotes

The early life forms were essentially uni-cellular organisms. If colonial they were very simple groupings of cells. The organization of some cells into highly specialized bundles [tissue] was a much later stage in the development of multi-cellular life. Moreover reproduction in these simple early life forms was by mitosis and involved simple splitting. Reproduction of life was entirely asexual.