Two forms of life

The greatest divide of the living world is not between plants and animals, as was thought for thousands of years, but between cells without a nucleus (prokaryotes) and nucleated cells (eukaryotes). Prokaryotes, or bacteria, have only one DNA molecule, arranged in a circle, and a single cytoplasmic compartment where all biochemical reactions take place in solution, and normally the form of the cell is due to an external wall (an exoskeleton) which surrounds the cell s plasma membrane. 

Eukaryotes have various DNA molecules, arranged in linear fibers which are repeatedly coiled and folded to produce highly organised chromosomes, and a composite cytoplasm which is divided into distinct compartments and houses a variety of cell organelles (mitochondria, chloroplasts, lysosomes, the endoplasmic reticulum, etc.) the form of the cell is due to an internal cytoskeleton which is made of three different types of filaments (microtubules, microfilaments and intermediate filaments). 

In 1866, Ernst Haeckel proposed a phylogenetic tree where the first forms of life were cells without a nucleus (which he called Monera), which later generated nucleated cells (Protista), which in turn gave rise to all multicellular organisms. Already in 1883, Schimper proposed that chloroplasts had once been free-living bacteria that happened to be incorporated, by a kind of symbiosis, into some eukaryotes, and from 1905 to 1930 this hypothesis was not only reproposed but also extended to mitochondria by Mereschowsky, by Portier and by Wallin. 

In the 1970s, the symbiosis hypothesis was forcefully reproposed by Lynn Margulis, and within a few years it received the support of an astonishing number of experimental discoveries. Mitochondria and chloroplasts are still carrying fragments of their ancient circular DNA, 

All this, however, tells us nothing about the cells that acquired orgenelles by symbiosis, and on this point biologists are divided into two opposing camps. Some maintain that the cells which engulfed bacteria were themselves bacteria, a hypothesis which leads to two precise conclusions (1) the first living cells were bacteria, and (2) eukaryotes are chimeras of bacteria (the bacterial theory of life). 

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The MAOIs are as effective as the heterocyclic antidepressants and the newer agents, such as the SSRIs. However, at least two forms of life-threatening toxicity (hepatotoxicity and dietary tyramine-induced hypertensive crisis ) have been associated with their chronic use. For this reason, the MAOIs are not considered first-line agents in the treatment of depression. They are generally reserved for treatment of depressions that resist therapeutic trials of the newer, safer antidepressants. However, a new transdermal formulation of selegiline undergoing clinical trials demonstrates antidepressant efficacy without concerns of liver toxicity or dietary tyramine-induced hypertension. 

All these uncertainties would be dispelled if clear evidence of extraterrestrial life could be obtained. Even then, kinship with earth life would have to be ruled out. Discovering life on Mars, for example, might not be decisive in itself. It is not considered impossible that Martian life could originate from earth, or terrestrial life from Mars, or both from some third site in the solar system. The two forms of life would have to differ in a significant way, for example in the chirality of some key constituent, for their independent origin to be incontrovertibly established. 

Two forms of gold provide medical treatments. The radioactive isotope Au-198, with a short half-life of 2.7 days, is used to treat cancer and is produced by subjecting pure gold to neutrons within a nuclear reactor. A gold salt, a solution called sodium thiosulfate (AuNa O Cl ), is injected as an internal treatment for rheumatoid arthritis. However, since gold and some of its compounds are toxic when ingested, this treatment may cause complications such as skin rashes and kidney failure. It is a less popular treatment, particularly with the development of newer and more effective medications. 

Firstly, I believe that any of the above descriptions of minimal life should permit one to discriminate between the living and the non-living. All forms of life we empirically know about should be covered by such a dehnition - and conversely one should not be able to hnd forms of life that are contradictory to such a definition. Secondly, there is the intellectual challenge to capture in an explicit formulation the quality of life how can one express the common denominator of micro-organisms, plants, animals, mushrooms, and mammals which set them apart from the inanimate world of rocks and machines Clearly, even if we do not arrive at an unique definition of life, the two above conditions are capable of fostering useful discussion and progress in the field. 

Life with carbenes is substantially complicated by the fact that there are two different forms (singlet and triplet) of CH3 and presumably of all other carbenes. The two forms of CH2 differ considerably in their reactivity. One is the singlet, which has its unshared electrons paired, while the other is the triplet with the same electrons unpaired. For CH2, the singlet form is the less stable and more reactive, whereas with CC12, the triplet is the less stable and more reactive. 

It is probably correct that a random genetic difference would have been sufficient to enable one of two chiral forms of living matter to become dominant. It might also have been that both chiral life forms developed in the primordeal ocean when it was rich in nutrient molecules. They need not initially have competed with each other. However, the metabolic waste products like carbon dioxide and methane are achiral. The form of life that developed photosynthesis could recycle those waste products into chiral material of use only to it and would then have had an overwhelming selective advantage. 

Archaea comprise one of the three-domains that separate all forms of life the other two are bacteria and eukarytoa. 

Capitalizing on this metabolic difference between higher forms of life and micro-organisms is the basis for this research approach to wood protection. Compounds are available which inhibit the cellulase enzyme systems however, their specificity has not been determined. Mandels and Reese (21,22) found that the extracts from the immature fruit of persimmon or the extract from leaves of bayberry were very effective inhibitors of the cellulase system. At concentration levels of. 00005 and. 00018%, respectively, these two extracts inhibited the cellulase enzymes isolated from Trichoderma viride. It is not known what the active component(s) are in these two extracts. 

Embley TM, van der GM, Horner DS, Dyal PL, Bell S, Foster PG (2003a) Hydrogenosomes, mitochondria and early eukaryotic evolution. IUBMB Life 55 387-395 Embley TM, van der GM, Horner DS, Dyal PL, Foster P (2003b) Mitochondria and hydrogenosomes are two forms of the same fundamental organelle. Philos Trans R Soc Lond B Biol Sci 358 191-201... 

Broad empirical experience shows that organic reactivity in hydrocarbon solvents is no less versatile than in water. Indeed, many terran enzymes are believed to catalyze reactions by having an active site that is not waterlike. Further, with ethane as a solvent, a hypothetical form of life would be able to use hydrogen bonding more effectively these bonds would have the strength appropriate for the low temperature. Further, hydrocarbons with polar groups can be hydrocarbon-phobic acetonitrile and hexane, for example, form two phases. It is possible to conceive of liquid/liquid phase separation in bulk hydrocarbons that could achieve the isolation necessary for Darwinian evolution. 

All this tells us that the evolution of primitive ribosoids into protoribomes and ribogenomes could have produced - at equal thermodynamic conditions - a countless number of other protein worlds, and therefore countless other forms of life. In the course of precellular evolution, therefore, two distinct processes went on in parallel the development of metabolic structures, and the development of a particular genetic code that gave life the familiar forms of our world, and not those of countless other possible worlds. 

Almost every aspect of human life involves carbohydrates in one form or another. Like other animals, we use the energy content of carbohydrates in our food to produce and store energy in our cells. Clothing is made from cotton and linen, two forms of cellulose. Other fabrics are made by manipulating cellulose to convert it to the semisynthetic fibers rayon and cellulose acetate. In the form of wood, we use cellulose to construct our houses and as a fuel to heat them. Even this page is made from cellulose fibers. 

Copper is the third most abundant metallic element in the human body, following iron and zinc. It also occurs in all other forms of life and it plays a role in the action of a multitude of enzymes that catalyze a great variety of reactions. There are two cross-cutting ways to classify the copper-containing enzymes (1) According to the structural and spectroscopic characteristics of the copper complex at the active site. (2) According to the function of the enzyme. We shall base discussion on the first of these, with allusions to function as we go along. 

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