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What is Living?  What is "Life?"  These are difficult questions to answer...

The acronym MRS GREN makes the seven characteristics of living things easier to remember:

Movement –allows living things to change their position in their environment to obtain essential requirements

     such as water, air and food as well as to protect themselves, or locate a mate

Respiration – all life processes require energy. Respiration releases energy from food for use by the organism.
     Plants differ from animals by being able to produce their own food, using it later as an energy source.

Sensitivity – all living things react to changes in the environment around them

Growth – involves both an increase in size, and repair of damaged parts

Reproduction – all living things need to be able to create similar organisms to themselves to survive through time.

Excretion - all life functions create wastes which must be removed from the organism or these would build
     up and become poisonous. (Examples: CO2, salts, water, unused food, heat.)

Nutrition - there is a continual need to take in food for the nutrients which are required for growth, as well as energy.

Other terms often confused in discussions of "living" are "dead," "non-living" or "never-living," "biotic" and "abiotic."

"DEAD" (or "Was Alive") implies that the object was once living.

"NON-LIVING" (or "Never Living") suggests that the object has never had the ability to carry out the life functions outlined above e.g. metals, water, air, soil.

"BIOTIC" means "pertaining to life" and refers to anything that is alive, was alive, or is part of something that was alive. A leaf can be said to be BIOTIC, as it is part of a living organism, but the same leaf would not normally be classified as LIVING as it is just part of a living system.