Life can be studied as a hierarchical
structure
The structure of living organisms
including those of entire populations and ecosystems is organized
in a hierarchical fashion that allows a systematic exploration of
the question ‘What is life?’. In the biological sciences, the following
structural levels are recognized:
|
Level |
Examples |
Special consideration |
|
Ecosystem
|
Rain
forest, desert, fresh water lake, digestive tract of animal
for bacteria |
Includes
all living organisms and non living matter such as air, water
and minerals |
|
Community
|
All
species in an ecosystem |
Only
includes living things from bacteria, to fungi, to plant to
animal |
|
Population
|
All
individuals of a single species |
Includes
only individuals from a specific species such as a plant,
an animal, a bacterial colony |
|
Organism
|
One
single individual |
Serves
as a representative of the species and describes overall form
and function of an organism |
|
Organ
system |
A
specialized functional system of an organism |
The
nervous system or immune system of an animal |
|
Organ
|
A
specialized structural system of an organism |
The
brain or the thymus of an animal |
|
Tissue |
A
specialized substructure of an organ |
The
nervous tissue and epithelial tissue are both part of the
brain |
|
Cell |
A
single cell |
A
neuron, a skin cell, a root cell, bacteria, yeast, paramecium |
|
Molecule |
A
single large or small molecule such as a protein, DNA, sugar
or fatty acid |
Molecules
are the smallest part of biological systems; they can be studied
for their chemical, physical properties, but are of particular
interest for their usefulness in biological systems. |
In this hierarchical organization,
each higher level exists only with all lower levels intact (single
celled organism don’t include tissue and organ levels). Importantly,
each higher level provides novel (emerging) properties not found
at any lower level, an important feature of hierarchical systems.
This phenomenon is known as ‘the whole is more than the sum of its
parts’.
Lower level disturbances or changes
affect higher level properties. This hierarchical model is well
suited to explain the cause of diseases and the mechanism of evolution.
The latter depends on random mutations occurring at the DNA level
affecting higher order properties at the cellular and organism level.
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Copyright © 1999-2009
Lukas K. Buehler |
|
