Viruses can be defined as obligatory intracellular parasites – that is, they absolutely require living host cells in order to multiply. Consequently, they are not typically considered to be organisms because they are not capable of independent reproduction. This is problematic though since some parasites and endosymbionts are incapable of independent life as well. To consider this argument, one must delve further into the genetic differences because viruses and certain “live” parasites. Viruses carry a basic structure of nucleic acid (RNA or DNA) which only contains a few of the genes needed for the synthesis of new viruses; including genes for structural components like capsid proteins. etc. These are the only enzymes, mentioned above, which the viruses carry. However, they are only functional when the virus is inside a host cell and is able to use the cells genetic machinery for the synthsis of these enzymes for multiplication and reproduction. On the contrary, “most endosymbionts and parasites, are acutally multi-cellular eukaryotic animals that generally posses digestive, circulatory, nervous, excertory, and reproductive systems- and are highly specialized to live inside their hosts” (Tortora, 361). In short, even though parasites might only be able to live and also reproduce inside a host – much like a virus, it is evident that they are considered to be living organisms due to the complexity and simality of their structure to other living organisms.
Dispite that, there are essential properties and characteristics which are associated with life. Virsuses, however, can not fully satisy all of them. “All living organisms are alike in these aspects: cellular organization, metabolism, growth and development, reproduction, heredity, and response and adaptation to environment” (Pilet, 128). For something to be described as living, that something must display all of these characteristics. Even though many different people have various opinions about what living means, the following characteristics can be considered general and required characteristics of living organisms.
Primarily, all living things are composed of cells. Although it is a simple concept, it is the foundation for life. “Cells are the basic components of all living things” (Pilet, 129). Some organisms are single celled, like bacteria, or multi-celled, like humans; both consisting of a particular type of genetic material – DNA and RNA. In contrast to prokaryotic and eukaryotic cells, in which DNA is always the primary genetic material (RNA plays an auxiliary role), a virus can have either DNA or RNA, but never both. This creates a problem because cellular replication is dependant on both types of nucleic acids. As mentioned earlier, living things are able to reproduce. In living organisms, this is accomplished by cellular replication either by sexual or asexual means (mitosis and meiosis). Living things also require energy to function, usually in the form of ATP. They use this energy to carry out energy-requiring activities such as metabolism and locomotion. Furthermore, organisms display heredity; again this ties in with the amount of nucleic acids in a given organism. Although viruses have this same type of function, it is carried out in a different manner – this of course also begs the question as to if viruses are living, are they even able to pass on such ‘heredity’. To put it simply, living organisms only inherit traits from the parent organisms that created them.
Something that is specifically unique to living creatures is that they can respond to their environment. “All living things respond to stimuli in their environment such as heat, cold, sun, darkness, and movement. They also have the ability to maintain homeostasis and state of internal balance” (Pilet, 134). Viruses can not respond to their surroundings like living organisms do, in fact their ability to withstand their surroundings is directly dependent on their host’s ability to function. The last characteristic that is generally required of living organisms is that they are not only able to respond but also evolve and adapt to their surroundings. This is something which viruses succeed at. For instance, the influenza virus adapts rapidly all the time; which is why many individuals can receive the “Flu” every year.
Using these characteristics, one can categorize things as living or non-living. Because viruses don’t meet all of these requirements, it can be concluded that they are non-living. It is evident that outside of living cells, viruses cannot grow or reproduce. Ultimately, viruses can be regarded as an exceptionally complex aggregation of nonliving chemicals, unable to perform many of the same tasks that living cells accomplish every second.
Works Cited
Pilet, C. Comparative Immunology, Microbiology and Infectious Diseases. Vol. 27. Elsevier Ltd . 2004.
Tortora, Gerard J. Microbiology: An Introduction. 8th ed. Benjamín & Cummings. 2003.
