Life vs Non-Life
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There are three things life must have: a barrier to the environment (i.e. a membrane), metabolism, and a genetic system for replication. Viruses may have a protein coat (and sometimes a lipid membrane) as a barrier, as well as a genetic system for replication. However, a virus cannot metabolize until it infects a living cell. Thus, viruses are not considered to be alive, but merely replicating systems (e.g. selfish DNA, if you will). The smallest unit of life then, is the cell. First of all, what is metabolism? Metabolism is simply the making and degrading of organic molecules by a living system. Catabolism is the term for the degradative processes. Anabolism is the term for biosynthetic processes. Why does a cell need metabolism? It is required to maintain the cell's integrity. Degradation of molecules may occur naturally or driven biologically -- e.g. UV can damage molecules, and enzymes can break down specific molecules. Molecules may be broken down biologically, simply due to the requirement of energy or of materials to make new and useful molecules. Thus, some broken down products are useful, while others are waste products to be discarded. Why is a membrane or barrier necessary? Cells used to be thought of as "bags" of molecules, however, cells are much more than this. A barrier is needed to keep certain molecules in and certain molecules out. A cell would soon use up all of its internal chemicals if it could not replenish them in some way. Thus, the membranes of living cells are selectively permeable. In other words some molecules are allowed in and out, while others prevented from passing through. Why is replication important? Living systems, in general, replicate (with the exception of sterile individuals). They do this with genetic information stored in DNA or RNA, as well as with a structural system (e.g. organelles). A genetic system (e.g. DNA and RNA) is used for more than just replication -- proteins are made from the genetic code. Proteins can be structural or functional. Structural proteins would be involved in maintaining cell shape and membrane integrity. Functional proteins (i.e. enzymes) are involved in metabolic processes. As I mentioned in another section, there are many differences between prokaryotes and eukaryotes. For example, there are many structural and biochemical differences, such as the bacterial flagella versus the eukaryotic flagella. Eukaryotes have organelles and a cytoskeleton, both of which are absent in prokaryotes. Cell division is also different. Eukaryotes undergo processes of nuclear division (involving mitosis and/or meiosis), whereas bacteria divide by binary fission. And, in general, eukaryotes tend to have more complex regulatory mechanisms than prokaryotes, such as in the translation machinery. The question is, how did eukaryotes arise? More specifically -- and assuming that there was only one common ancestor to all life -- how did eukaryotes evolve from prokaryotes? Although the actual details are not really known (after all, no one was around to witness the divergence of eukaryotes from prokaryotes), there are theories as to how it could have happened. In particular, endosymbiosis played a critical role in the development of eukaryotes. This is described below. Symbiosis can collectively refer to several different associations between organisms -- in particular, parasitism and mutualism. Parasitism occurs where one entity (the parasite) lives off another (the host), without benefit to the host (and can even be detrimental or harmful to the host). Mutualism occurs when two or more entities benefit from an association. These symbiotic associations can be permanent or transient. Endosymbiosis is the case where the symbiont is contained within the host. Two examples of permanent endosymbiotic associations are mitochondria (derived from a purple non-sulfur bacterial ancestor) and chloroplasts (derived from a cyanobacteria-like ancestor). Some species are rather good at being mutualists with a wide variety of other species. An example of this is lichens. Lichens are essentially fungi (usually ascomycetes) with endosymbiotic green algae or cyanobacteria. The number of known fungal species involved in such associations belong to several hundred genera, whereas the total number of algae involved are from only about 50 genera (ref. 1). A general principle to derive from this would be that a species, which is "good" at being a symbiont, is able to survive and undergo evolution with different hosts (ultimately having the potential for diversification within that symbiont lineage, as in the case of plastids in the plant kingdom). Alternatively, you could also imagine that an organism which is good at being host to a variety of symbionts, could also be a good survivor -- however, it may also be prone to parasites, which would cause a decrease in the fitness of that host. Serial Endosymbiosis Theory (SET) is the theory that multiple endosymbiotic events have occurred to bacterial ancestors of eukaryotic cells. How can such an event take place? It likely starts off with parasitism. Then at some point in evolution, the parasite takes over a function or process done by the host cell, or does it better than the host cell. This may require some form of genetic transfer. As a result, the host-parasite interaction becomes so tightly linked, that the symbiont and host cells become dependent on each other for survival -- i.e. neither of the symbionts can survive without the other. As such, the union creates a new species. This new species may itself be pararsitized by another organism, and another endosymbiotic event may occur, and so on (i.e. serial or multiple endosymbiotic events). Where is the evidence for SET? In nearly all eukaryotes (animals, plants, and fungi) mitochondria are present, whereas prokaryotes lack mitochondria. The mitochondria is the "power factory" of the cell -- i.e. it produces all the energy a eukaryote needs, by respiration, in the form of ATP (ATP = Adenosine Triphosphate; a high energy molecule -- a nucleic acid -- used to drive metabolism). In plant cells you find chloroplasts or other plastids, which generate energy and release oxygen by photosynthesis. Both mitochondria and chloroplasts contain their own DNA, and they resemble certain species of bacteria. For example, mitochondrial and plastid ribosomes resemble bacterial ribsomes (i.e. 70S ribosomes with 30S and 50S subunits) rather than eukaryotic ribosomes (i.e. 80S, with 40S and 60S subunits). I have written an essay reviewing chloroplast genetics and endosymbiosis, which you may find in the "origins of life" section. In summary, life requires a membrane barrier, metabolism, and a genetic system. The simplest unit of life that fits these criteria is the prokaryotic cell.The eukaryotic cell, is the result of numerous permanent and transient symbiotic events between simpler ancestral cells.
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