Life can be studied as a hierarchical
systems
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
hierarchical 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 as well as heat (temperature) and pressure. |
|
Community
|
All
species in an ecosystem |
Only
includes living things from bacteria, to fungi, to plant to
animal and accompanying viruses. |
|
Population
|
All
individuals of a single species in a given area |
Includes
only individuals from a specific species such as a plant,
an animal, or microbes |
|
Organism
|
One
single individual |
Serves
as a representative of the species and describes overall form
and function of an organism (can be multicellular or unicellular). |
|
Organ
system |
A
specialized functional system of a multicellular 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 |
| Macromolecule |
A
polymer such as a protein, DNA, polysaccharide
or fat |
Macromolecules
are the functional and structural building blocks of cells and their organelles. |
|
Molecule |
A
single small molecule (often building block of a macromolecule) such as an amino acid, nucleotide, monosaccharide
or fatty acid. |
Molecules
are the smallest unit of biological systems; they can be studied
for their chemical and physical properties and are of particular
interest for their role they play in metabolism and signaling. |
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 or emerging properties not found
at any lower level, an important feature of complex 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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o m e
Copyright © 1999-2016
Lukas K. Buehler |
|
