Characteristics of Life

Conception What is life?
If this strikes you as an odd question, consider it for a moment. (It's almost as hard as describing the origin of life). Of course we all have an intuitive sense of what life is, but how would you actually define it? Given the incredible diversity of living things, from the mold on your old tuna sandwich to monkeys in the rainforest, how could one word encompass all that "life" represents?

Even biologists have a difficult time defining the very concept they study. However, given our knowledge of the life forms present on Earth, it is recognized that all living things do share a set of characteristics in common:

1) Living things take in and use energy

2) Living things eliminate waste products

3) Living things grow and develop

4) Living things respond to their environment

5) Living things reproduce

6) Living things show variation based on heredity

7) Over time, living things evolve (change slowly) in response to their environment.

Therefore, in order for something to be considered to be "alive" or to "have life" as we know it, it must possess these characteristics.



Living Things Get Energy from Different Sources

We all know that living organisms need food to survive. Not all life forms consume the same things.

autotroph Autotrophs, known as *self-feeders* are organisms which create "food" using energy from the sun, thermal energy from the Earth, or other such means to feed themselves. All members of the kingdom plantae as well as some protista and monera are autotrophs.

Heterotrophs, known as *other-feeders* are organisms which feed themselves by eating other creatures, plant life, or organic molecules which exist outside of themselves. All members of the kingdoms animalia and fungi as well as some protista and monera are heterotrophs. On Earth, nature provides the following three mechanisms for living creatures to be supplied with energy.

Photosynthesis

Respiration

Chemosynthesis

Photosynthesis and Chemosynthesis are used only by autotrophs.
Both autotrophs and heterotrophs use respiration.