Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization
© 1975-1979, 2008 Robert A. Freitas Jr. All Rights Reserved.
Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm
7.4 Proteins and Cells
The cell is the fundamental biological unit and a common denominator among all terrestrial lifeforms. Living things on this planet are made up of cells which vary in size from less than one micron to several centimeters in diameter. While the simplest organisms are unicellular, the typical human is an ambulatory assemblage of from fifty to a hundred trillion (1014) individual cells.
The cellular construction of Earth life is remarkably uniform: Similar water content, similar kinds of proteins, similar lipids and so forth. All have at least one membrane, perhaps no more than 100 Angstroms thick, which protects the inner workings from the harsh vagaries of the external environment. The first protobionts undoubtedly had no such complex organizational qualities. But how can structure arise in the first place?
It has been shown by Ilya Prigogine that thermodynamic chemical systems may develop certain states wherein some of the chemical constituents have periodic, oscillating values.2230 A biologist, J. Pringle, has demonstrated that initially homogeneous systems can undergo a progressive change, leading to the appearance of "spatial heterogeneity."2231,2232 That is, structure can arise spontaneously. These two treatments of the problem of organization suggest that mechanisms may exist for collecting material into small, localized concentrations, perhaps leading to ordered structures we would recognize as cells.2368
But to build cells, we must have protein. Protein is the most fundamental construction material, used in building cell walls, enzymes, and so forth. To make proteins, there are two requirements.
First, we need an abundance of amino acids. From our discussion above, we see that this is virtually inevitable on any normal world possessing at least small aqueous oceans and a primitive hydrogenous atmosphere.
Second, there must be some way to hook up a long string of amino acids into a polymer of protein. Polymerization (linking together) of amino acids leads to the production of protein, which can then be used for cell-building.* It is true that even dilute primordial soups can coagulate into gelatinous masses. But such conditions are far from ideal. In all likelihood, most prebiotic syntheses probably took place in local regions of increased concentration. The efficient construction of amino acid polymers undoubtedly occurred elsewhere than in the open seas.
Numerous concentration mechanisms have been proposed which might conceivably lead to the creation of small pockets of more potent broth. The simplest method is evaporation. Primordial soup, caught in a narrow, shallow lagoon, would slowly thicken as the water that held the components in solution evaporated away.85 As suggested by Miller and Orgel, similar effects result from slowly freezing the solution in the lagoon: The solvent freezes out in the pure form first, leaving the solute concentrated in ever-smaller quantities of solvent.521
A combination of air-water and water-solid interfaces provides mechanical consolidation of suspended matter, as evidenced by the accumulation of scums and oil slicks near coastlines.1667 Another possibility is that organic compounds may have been trapped on solid surfaces such as aluminum silicate clays, quartz, and other minerals which allow polymerization reactions to proceed.1430,2381
To date, however, there is really only one proven method which yields polymers of amino acids under plausible prebiotic conditions. Dr. Sidney W. Fox at the University of Miami has obtained long-chain molecules with the following essential properties:
1. They contain all amino acids common in contemporary terrestrial organisms.
2. They have high molecular weights (the chains are relatively long).
3. They are "active" because they interact in the catalytic or rate-enhancing sense. (This anticipates metabolic activities mediated by enzymes -- which are also proteins.
4. They are as heterogeneous as contemporary proteins.
5. They yield "organized units" upon contact with water which have many properties in common with modern cells.1625
Dr. Fox calls his substances "proteinoids," because they greatly resemble living protein polymers. His method for producing them is quite simple. A mixture of amino acids is cooked at 120-170 °C for a few hours, and substantial yields (10-40%) of polymeric material are obtained.
Fox decided to test his method under more realistic field conditions. He secured a large piece of lava from the site of an Hawaiian volcano.1702 The temperature of the rock was raised to 170 °C, and the appropriate amino acids seated in a small depression at the top. Heating continued for several hours, after which the lava was washed off with a small spray of sterilized boiling salty water -- as might have occurred naturally near a volcanic shoreline in ancient times. Proteinoid polymers were formed, but there was more! To Dr. Fox’s surprise, billions of tiny "microspheres" appeared in the wash water: spherical, microscopic particles of uniform diameter bearing a striking resemblance to living cells (Figure 7.3).
Figure 7.3 Possible Model Protobionts: Thermal Proteinoid Microspheres1625
The Miami scientist presented a scenario for the origin of microsphere protocells in prebiotic times: (1) Hot lava meets soup; (2) water boils away, leaving sticky brown goop on lava; (3) contact with water (rain, sea spray, etc.) causes proteinoids to assemble themselves; and (4) microspheres are washed back into the soup.
These initial experiments were completed nearly two decades ago,1702 and since that time Fox and his colleagues have refined their methods and perfected their theories on the origin of cells and life. Protein-like materials are now produced with molecular weights ranging from 3000 to 10,000 under plausible primitive Earth conditions.2371 And it has been shown that a primitive “cell” with most of the attributes of life can arise spontaneously in a very brief period of time.
Detailed studies of microspheres have confirmed the researchers’ initial optimism. What makes these spherules so unique is their “active” nature. Dr. Fox has observed and recorded the following characteristic behavior of his proteinoid microspheres under various chemical and physical conditions:
1. Spherical shape -- 0.5 to 7.0 microns, uniformly.
2. Single-walled membranes (like plants) and double-walled membranes (like animals).
3. Simulation of osmosis -- microspheres swell and shrink in response to changes in the chemical environment.
4. Selectivity of diffusion -- microspheres possess semipermeable membranes analogous to those of living cells. For instance, in one case Fox discovered that polysaccharides were selectively retained under conditions in which monosaccharides diffused freely through the microsphere walls. (Polynucleotides and other organics are also absorbed from aqueous solution.1624)
5. Cleavage -- a kind of binary fission of a single “cell” has been observed in acidic proteinoid microspheres.
6. Motility -- the microspheres, when viewed under a microscope, move non-randomly in preferred directions under certain special conditions. The addition of ATP appears to enhance the movement.
7. Budding -- buds appear spontaneously on proteinoid microspheres allowed to stand undisturbed in their mother liquor.
8. Growth by accretion -- buds which have been liberated by mild heating or electric shock will swell by diffusion to the same approximate size as the “parent” cell.
9. Proliferation through budding -- second generation budding has frequently been observed on buds that grew to the size of normal microspheres. The buds are apparently engaging in a kind of “reproduction.”
10. Formation of junctions -- microspheres are capable of approaching one another and physically attaching together in a more or less permanent fashion.
11. Transmission of information -- when two spheres have joined, small proteinoid microparticles within the larger sphere are observed to pass through the junctions. The whole process is highly suggestive of microbial conjugation.
12. Stability -- the activity of the proteinoids does not diminish with storage over a period of 5-10 years.2370
The best-known of all physical cell models prior to the discovery of proteinoids was the coacervates, thoroughly researched by the Soviet biochemist A. I. Oparin, the Dutch biochemist H. G. B. de Jong, and others. Coacervates are produced by combining solutions of oppositely charged colloids such as gelatin or histone with gum arabic. When solutions of the two substances are commingled, they interact to yield clusters of microscopic structures having the appearance of tiny liquid droplets. Coacervates have many interesting properties from the point of view of the origin of life.
For instance, after these uniform spherules have aggregated, they are able to absorb various simple organic molecules from the external medium (sugars, dyes, etc.). However, Oparin has admitted that this process quickly leads to static equilibrium, and the coacervate "protocell" then becomes a passive system, unstable and prone to break-up upon standing. Another property of coacervates is their ability to convert certain chemical monomers to polymers after diffusion through the "cell" wall, although it is generally recognized that the dynamic behavior of these droplets is fairly limited.
There is another reason why coacervates,1432 sulphobes,1625 "biphasic vesicles,"1630 and many other prospective pseudocells1211,1415 do not compare favorably with Dr. Sidney Fox’s microspheres as model protocells. Coacervate droplets are formed from polymers which themselves were synthesized by living organisms. The gum arabic used to manufacture Oparin’s droplets was not produced abiogenetically, nor is it at all clear how this might be done. The great advantage of the microspheres is that they are the direct product of single, simple amino acids -- amino acids that must have been common on the shores and seas of the primitive Earth eons ago.
Of course, no biologist today would claim that proteinoid microspheres are alive in the sense of representing the first protocell. And yet, to the extent that they self-organize, accumulate information from their surroundings, and exhibit both structure and behavior, they are certainly near the borderline of life.
* We will not discuss here the significance of molecular optical activity. The curious reader is referred to Sagan,20 Jackson and Moore,47 Glasstone,72 Gabel and Ponnamperuma,315 Miller and Orgel,521 Ulbricht,1445 Hochstim,1446 Bonner et al,1447 Wald,1665 and Walker.2382
Last updated on 6 December 2008