What does it mean to be alive? It seems like it
would be a pretty obvious answer. If something can move, breathe and
eat, then it must be alive. What about plants and sea anemones? They do
not necessarily move. Some animals, like sponges, filter feed instead of
chewing up food. Then there are plants, which do not eat other animals.
How can we decide what is living and what is not?
Life can be defined by several characteristics that
living organisms display, including cellular organization, reproduction,
metabolism, homeostasis, responsiveness, growth, and adaptation.
All living things are composed of at least one or
more cells. Unicellular organisms are made up of one single cell, the
smallest units capable of life, such as a bacterium. Multi-cellular
organisms consist of many cells, which have specialized jobs within the
organism. For example, skin cells have a different job than muscle
All organisms reproduce in order to continue the
species. In sexual reproduction, the genetic information is combined
from the mother and father. A new organism will have some
characteristics from both parents. In asexual reproduction, a new
organism is an exact copy of the first, because the parent organism
split an existing cell into two offspring cells.
All organisms use energy. Sum of the chemical
energy used is called metabolism. Organisms need energy to grow,
develop, repair damage, and reproduce. Autotrophs, like plants, use
energy from the sun to make their own “food.” Heterotrophs, like
animals, must ingest food to obtain energy. All organisms have internal
conditions that must be maintained in order for the organism to remain
alive. This is called homeostasis. Conditions like body temperature,
blood volume, pH balance, and water balance must be kept stable.
Homeostasis is related to energy use in that a certain level of energy
must also be maintained.
All living things respond to their environment. A
response to stimuli is usually a response of an organism to some
external condition or event. Stimuli include light, temperature, odor,
sound, heat, and water. An example is a plant’s leaves and stems growing
toward the light. All organisms grow and develop. Cells divide to form
new, identical cells. Cells mutate into other forms of cells, making a
more complex organism. Organisms growing, changing and becoming more
complex is called development. Unicellular organisms grow as well, but
they only become slightly larger.
All organisms adapt to their environment through
evolution. Adaptation is the process that enables organisms to become
better suited to their environment. Life requires a complex definition
for it to make sense. The characteristics described above detail all the
current conditions it takes for something to be considered alive.