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How to Characterize Living Things Based on Four Broad Life Functions

Living things seem different from nonliving things, but sometimes it is not clear exactly what the difference is. Biologist differ on the exact criteria required to establish life status, but they generally fall into the four basic life functions: growth and development, metabolizing part of the environment, response to stimulus from the environment and the ability to reproduce. Several mechanisms, such as robots, have one or more of these criteria, but only living things have all four.

Instructions

- 1
  Look for growth and development, which is a fundamental function of all living things. Organisms are born small and go through a period of growth and development before they are mature -- and can start making offspring. At the smallest scale -- the single cell -- the organism merely splits and the daughters look just like the mother, only smaller. Some small organisms bud offspring that are miniature copies of the adults, but most larger organisms have offspring that start off very different from their parents. This is significantly different from robots, which are assembled in their finished form and never go through a growth and development stage.
- 2
  See if the organism under study metabolizes -- takes in energy from the environment and use it to accomplish life functions like growth, development, reproduction and -- for some organisms -- locomotion. This is the criteria that separates viruses from living things. Viruses do not metabolize -- they always use the metabolizing and reproduction processes of the host they infect. This is also a place where robots are not completely different. Many robots carry all the power they need, but more and more they use energy sources like the solar panels on the NASA robots.
- 3
  Test the response of the organism to the environment. This is the one life function that has been implemented by robots. The responses of living things are more complex and subtle than the responses of automatic systems, but this is the basic life functions where robots are closest to living things.
- 4
  Check for a reproductive process. This is the absolutely necessary process for living things. Robots can not do this step at all. Viruses appear to, but they actually only reproduce when they find a host to hijack. There might have once evolved a species that could not reproduce, but they are no longer around.

Nature