Characteristics of living things
Some automated robots nowadays are very lifelike in their actions
and abilities: they can move, work and respond to light and sound. Would
a scientist say they are "living"?
The major difference between living things and non-living things is
that all living things (organisms) are made up of one or more cells.
These basic units can perform all processes associated with life. Living
things all show the following seven main life functions at some time:
Movement
This allows them 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.
Plants are slower and more limited in their movement, though a fly
caught by an insectivorous (insect eating) plant might disagree!
Reproduction
Unlike a stone, living things cannot continue indefinitely, so
they need to be able to create similar organisms to themselves to survive
through time. Extinction may result if reproduction is not successful.
Sensitivity
It
is important that living things can sense, and respond to, changing
factors in their environment. Information taken in through the senses
is processed to allow a response which helps it to survive. For example,
if you are too close to a fire, you usually back away to prevent yourself
being burnt. You know whether you are too close by sensing with your
eyes (you see flames), heat detectors in the skin pick up warmth, your
nose picks up the smell of burning and your ears hear the crackling
fire.
Similarly, plants can sense and respond to light, water and gravity.
Growth
This involves both an increase in size, and repair of damaged parts.
Unlike crystals which grow only on their outer edges, living things
take in nutrients, change them and then use them as building blocks
wherever required throughout the organism.
Respiration
All life processes require energy. The function of respiration is to
release energy from food for use by the organism. This process usually
requires oxygen, but not always. Plants differ from animals by being
able to produce their own food, using it later as an energy source.
Elimination
All life functions create wastes which must be removed from the organism
or these would build up and become poisonous. In humans, these include
carbon dioxide (removed via the lungs), excess salts and water (removed
through the skin and the kidneys), and unused waste food (removed as
faeces). Excess heat is also continuously lost from the body through
the skin.
Nutrition
In all living things, there is a continual need to take in food for
the nutrients which are required for growth, as well as energy.
The acronym MRS GREN makes these seven characteristics easier to remember.
Two other terms often confused in discussions of "living"
are "dead" and "non-living".
"DEAD" implies that the object was once living.
"NON-LIVING" suggests that the object has never had the ability
to carry out the life functions outlined above e.g. metals, water, air,
soil.
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