As one takes an evening stroll, one can probably distinguish between all the living and non-living objects that one encounters. In spite of a classic book by Erwin Schrodinger, with a title that seems to ask one for a definition of life, published in 1943 , the scientific community has still not been able to come up with a single answer to this fundamental question that satisfies every scientist. Part of the reason for this ambiguity is because, to date, there remains a controversy over which objects should be considered as living beings . For example, can a virus be considered as a living being?
But, first let's try to discuss some of the traits of living things:
1. Metabolism: A living being consumes energy from the surroundings by converting one form of energy to other forms of energy by a process called metabolism. Metabolism, the Greek word for change, designates all the chemical reactions carried out within a living organism.
2. Organization: The energy gained from metabolism helps organisms to remain far more organized than non-living things. Organization here refers to the fact that one can not reduce an organism into smaller independent parts. All living organisms are formed of the basic biological unit called the cell. Within each cell, there are membranes that divide the living world from the non-living world and within the membranes, the cellular constituents are organized hierarchically to form a live entity. All the molecular constituents within the cell serve a function. These molecules are organized into an integrative system and serve the activities of the cell as a whole. Some people even argue that keeping this organization going is the basic entity of life, and the minute an object is dead, this organization is lost. One can study independent parts (as molecular biology) or cells for that matter, but in reality, life as we know it, can not exist without being organized at various levels hierarchically.
3. Reproduction: Living things can reproduce on their own to produce new organisms of the same kind. The instructions to reproduce are also inherent within an organism and are inherited by each generation from their parents.
4. Evolution: Living things are able to evolve over time on their own according to their environment. They evolve due to the occasional errors that crop up while copying the instruction from one generation to another. These errors track changes in the environment and an organism that is better adapted to the environment survives. Darwin's central contention was that this adaptation stems from the interplay of random variation and natural selection. So, the history is as important as organization to understand the workings of the present day organism.
An object is traditionally considered to be living if it has all the above characteristics . In addition, the definition is applied at a global level to a whole species and not to individual beings . In other words, sterile organisms are also considered to be alive even though they may have lost the ability to reproduce.
Non-living things may have one or more of the above mentioned traits, but do not possess all the above mentioned characteristics. For example, a flame can use up energy and convert chemical energy to light and heat energy, using up energy in this process. However, it can not reproduce on it's own and neither does a flame evolve according to it's environment.
Viruses on the other hand are a little more difficult to distinguish. They can evolve and they can reproduce (albeit, inside another organism and not on their own), but they do not possess any metabolic capabilities, and hence, it may be argued, should be considered as not living. A small minority of the biologists have postulated that the abilities to reproduce and evolve are the only criteria for life, and that viruses should hence be considered alive.
Seeds also form an interesting example. Do we consider seeds as living or non living? Well, I did a google search and they are considered to be alive. They certainly have the ability to reproduce and, hence, evolve under the "right conditions". In addition, they are as organized as a living organism, but the real question was whether metabolism takes place in a seed under dry storage conditions. I was pleasantly surprised to find many papers reporting that seeds do undergo metabolism even during storage (an example is ), and hence, they do have all the criteria to be considered alive.
Physicists and chemists tend to argue over whether all the four properties are really required for life. While some chemists argue that metabolism is the real criterion for life, physicists argue that the level of organization in a cell is what really demarcates the difference between a living cell and a non-living cell. In fact, an algebraic information theoretic framework was developed to define the amount of information required to define an organism and the amount of organization in an organism .
What should be considered as living is not only an academic issue, but is equally important for space probes that look for signs of extraterrestrial life. In addition, it is equally important when one studies the origin of life from non-living entities. When does one consider that there is enough complexity in a system to call it a living cell? I will continue this post with a post on the quest for the origin of life and also, on a separate series of posts, on molecular evolution of living organisms.
 What is Life? by Erwin Schrodinger.
 Chapters 1 and 2 of The Way of the Cell by Franklin Harold.
 Wikipedia entry on Life.
 Brittanica Encyclopedia.
 Metabolic activities of dormant seeds during dry storage. Naturwissenschaften, 59:3, 1972, 73-74.
 Toward a Mathematical Definition of "Life" by Gregory C Chaitin.