Is a virus a living organism?
Biochemical characteristics include carbon-based macromolecular design of all life forms, including nucleic acids as genetic material. Genetic view draws on the properties of genes, reproduction/replication, mutations, natural selection, adaptation, and evolution. Finally, thermodynamic arguments state that living organisms are open systems that increase their order at the expense of a larger decrease of order of the universe outside.
I think that one could think of two different but related approaches when trying to define the properties of living systems, 1) what are the common properties of life on Earth? 2) what properties should we expect to find in a completely unknown (e.g. extraterrestrial) life form? I believe that the Britannica and related definitions serve the first task quite well - they pinpoint simple empirical properties known from our everyday experiences that cover most (or all) life forms on Earth quite well. But the empirical definition of living systems may be less useful in trying to envision what other life forms we may encounter on other planets. Most scientists now agree that all organisms on Earth probably share a common origin, therefore we know but one example of life. I guess when trying to predict the properties of unknown life forms, the emphasis should be shifted to more general thermodynamic/informational arguments (like replication, metabolism, and complex systems architecture), without necessitating the biochemical particulars of carbon-based life. The emerging science of systems biology may shed some light on general design principles of life. Some people though are convinced that carbon-based living systems are the only form of life that exists in the Universe.
There are of course many difficult cases that challenge our ability to define living objects. Probably the most notorious one is whether viruses are alive or not. The prevailing doctrine has shifted during the years back and forth between considering viruses inanimate objects and fully grouping them with the other living organisms. The arguments both for and against are nicely treated in ref. 1. I agree with the author of the article in Scientific American that whether we consider them alive or not, viruses are an integral part of the existence of biological systems and even a driving force in evolution. These properties certainly put them closer to living organisms.
To sum up, no simple definition would be universal enough to cover all manifestations of life. But if a particular entity possesses most (or all) of the characteristics outlined above, it can with a high degree of certainty be considered alive.
1. L.P. Villarreal. Are viruses alive? Scientific American, December 2004 issue, pp. 101-105.
A thoughtful article that nicely addresses the posed question.
A rather opinionated but interesting and rich collection of attempts at defining properties of life, with multiple related links. Will provide hours of amusing reading.
3. E. Schrodinger. What is Life? (any edition).
A classic. Though some parts read a bit naively now, this is still a great example of a great mind attempting to grasp this huge problem.
A pretty good and succinct analysis of the definition of life on a free encyclopedia site.
5. I recommend checking out a definition of life on the Encyclopedia Britannica website, if you have online access to it.