The cell is the fundamental construct of living systems. It is the common building block of all living tissue and the foundation for all living systems, without exception. In many ways it is the best example of a prototypical system that we can conjure up, and it is in many ways the epitome of natural self-organization. Indeed, general systems science really had its birth in the concern with holistic perspectives upon cellular function and development, at a time when the internal organization and happenings of a cell were still pretty much a mysterious "black box." We need only consider how remarkable cellular evolutionary development has been, when we realize that all living organisms have been basically the descendants of a continuous, non-stop process of cellular growth, reproduction and division from the first proto-biotic cellular formations, and it is possible that all life might have originated from a single successful cellular system. I return to the examination of the cell as a system, for beyond ecology, what little formal biological study I've had has been focused upon the cell.

A systems based perspective upon the cell therefore would be based upon not only the examination of the metabolism, behavior and life-cycle of a cell as a prototypical biological system, but upon what can be called "cell ecology" and properly "cellular meta-biotics" or the nature of inter-cellular interactions that influence the outcomes of microbial development and evolution. At some point, microbial populations would begin exerting a significant influence upon their environment, and begin altering their bio-geophysical contexts in the direction that encouraged further evolutionary development taking place. In spite of the self-containing environment within the cell membrane, all cells exhibit specialized environmental adaptations. They exist in rather special and narrow contexts of cell development, that define the limits of their growth and development.

Under normal conditions for a cell, a cell can be expected to respond to its environment by growing in population to the natural limits of its context, and then overstepping these limits in critical ways. Death rates will eventually balance or exceed reproductive rates, and the population will eventually crash or achieve some long term equilibrium.

A phenomenon called "endosymbiosis," the encapsulization of the DNA machinery of one micro-organism by another, and the appropriation of that machinery, seems to me a fitting demonstration of a systems-based meta-biotic framework. Virus's and viroids are similar entities, that, though associated with disease, demonstrate a kind of meta-biotic complication of living systems that do not fit normal hierarchical frameworks. Horizontal transmission of DNA, especially in some forms of soil bacteria, are another case that clearly defies our received models of strict vertical and intergenerational transmission of DNA from parent to offspring.

"Endosymbiosis" becomes the basis for the emergence of Eukarya, more complex cells, and in turn for multi-cellular organization, which features functional subordination and specialization of cell types. A fertilized gamete will carry the blueprints that include the instructions for the development of a large number of different specialized cell types performing a host of integrated functions. The differentiation of so many cell types from a single precursor resembles the controlled internalized embodiment of the entire process of taxon evolution of complex forms from simple parent cells. 

It is evident therefore in the first place that all cells are capable of evolutionary development and speciation through chance genetic variation of structure. Microorganisms have been observed to be capable of quite rapid speciation and evolutionary development, compared to more complex living systems that are slower to replicate and reproduce. The earliest known form of cells, presumably some primitive form of Prokarya, were of minimal possible size and structure, probably less than five micrometers in diameter and a micron in thickness. This was perhaps the optimal size for rapid self-replication and immediate exploitation of whatever growth medium becomes available. It seems that life seized on the basic principles involved in this, and capitalized on it as much as possible.

It is the case therefore that in the war against disease, there is a on-going struggle to develop effective vaccines against new strains of old strains of bacteria that become immune to previous remedies through evolutionary adaptation.

The principle function of the cell can be said to provide a suitable environment for the storage and replication of DNA and the machinery needed for this storage and replication, as well as for replication needed for the components of the cell itself. A cell must therefore be capable of faithfully reproducing its DNA informational database, and itself. It requires transportable and usable energy, in chemical form, for carrying out its many tasks, and it must be capable of somehow capturing and transporting energy into itself across its membrane. Cellular reproduction is a normal part of cell growth and its life-cycle. 

Essential components of cells, besides DNA, are:

1. A cell wall and/or cytoplasmic membrane

2. Cytoplasm

3. A nucleus or nucleoid

4. Cellular Organelles, consisting of ribosomes and/or mitochondrioon, endoplasmic reticula for the manufacture of proteins and for the maintenance of cellular tissue structure, function and equilibrium.

The first distinction we draw is between simple bacteria or prokaryotes, and complex cells referred to as eukarya or eukaryotes. In general, prokaryotes are much smaller and simpler than eukaryotes, averaging between 1 and 5 micrometers (versus about 25 micrometers for eukarya) and about 1 micrometer in depth. They have a cell wall, but otherwise lack many of the structures common to eukarya. 

Prokarya are simple one-celled organisms capable of fairly rapid reproduction. They are presumably the first organisms, or the direct descendants of the first forms of life, to have evolved on earth. Some have even suggested the possibility that they may have been carried to earth in a meteorite that crashed in the best of circumstances (cosmic seeding hypothesis), and from there began reproducing and evolving. Prokarya are grouped into bacteria and archaea, distinguished largely by the complex structures of the cell walls and analytically by their growth characteristics in various mediums and the ability to stain under a cover slip of a microscopic slide. 

Eukarya evolved from Prokarya, with the suggested mechanism of the symbiotic ingestion ("endosymbiosis") of one cell by another to form a more complex structure of organelles like mitochondria and chloroplasts. Eukarya include algae, fungi, protozoa and all multi-cellular life forms we know of. Prokarya in general do not form multi-cellular structures of any form. They lead an independent life contained within the narrow confines of their cell wall. Under the right conditions, they are known to grow rapidly, which growth is defined by the cellular division and propagation by mitosis at a fairly fast doubling rate.

One possible scenario of the development of proto-life was that the first forms to emerge were primitive extremophiles, Archaea, that developed through chemosynthesis. These eventually differentiated and evolved into forms that were less marginal and more tolerable of normal conditions, and that possibly could find a broader range of energy resources. From these developed what we know of as the prototypical generalized bacteria. This bacteria became so successful as an environmental generalist, that the first adaptive radiation of life was its spread by air, water or any other medium to virtually the entire earth, to cover the whole earth in a thin, invisible layer of life. When this prototypical bacteria encountered extreme conditions, it possibly resumed characteristics of an extremophile form.

One of the main functions of the cell-membrane is to provide a medium for transport between the external environment of the cell and the cell's self-maintaining internal environment. We know from the standard general system model that such transport mechanisms and mediums are fundamental to the definition of systems as order creating and order maintaining processes. The cell membrane, therefore, with or without the added structural or buffer support of a cell wall, is a vital component of all cell systems, for they are the principal transport mechanisms of such systems that maintain internal equilibrium and stability of the internal environment across a random and external environment.

In the case of multi-cellular organisms, we must ask what a cell gives up in terms of its independence as a system, and what it gains in turn from the specialization of function and structures. Specialized cells tend to take on definite shapes and configurations of structure, and they tend to be capable of performing fairly specialized functions, either in a chemo-mechanical manner or in terms of the production of special protein structures or cellular metabolism. What is clear is that such cells, in become specialized, become dependent upon the organismic contexts in which they develop--they cannot survive apart from the super-cellular structures in which they develop.

It is clear that from fairly early on cellular evolution became a meta-biotic process, with cells respond to and adapting to the presence and behavior of other cells as a part of their environment. The most basic form of this kind of interaction was probably competition of different strains of bacteria for the same resource substrates. Selection that we can observe at this level is generally geared to those strains that can effectively tolerate, adapt to and exploit a different medium. Symbiotic or complementary relationships between different strains of bacteria may have developed in time. 

The earliest mechanisms of cellular metabolism (catabolism and anabolism) was presumably some form of chemo-synthesis--the derivation of usable chemical energy from some form of enzyme reaction with a chemical substrate. Presumably, one of the first metabiotic forms of relationship that may have developed between different species of bacteria may have been that of predator-prey relations, or the capacity for one strain of bacteria to attack and consume another form of bacteria for its energy and material reserves. Photo-synthesis was known to have evolved from early Eukarya, and presumably one of the first forms of endosymbiosis may have been in terms of photosynthetic cloroplasts capable of synthesizing chemical energy from the energy of the sun. Indirect symbiosis and interdependence also undoubtedly occurred, by which the action and metabolism of one organism created the environment suitable for the growth of a second organism.

Presumably, though there are many kinds of Prokarya we know of today, their basic structure and function is not so very different than that of their original precursors, and the diversity of life forms at this level appears to be far lower overall than the diversity of more complex multi-cellular organisms. Of all varieties of prokarya, the forms possibly most similar to the ancestral "precursor" of life are possibly the archaea though what we know of these prokarya today is that they have fairly specialized cellular membranes and walls.