Biological organization, or the hierarchy of life, is the hierarchy of complex biological structures and systems that define life using a reductionistic approach. The traditional hierarchy, as detailed below, extends from atoms (or lower) to biospheres. The higher levels of this scheme are often referred to as ecological organization.
Each level in the hierarchy represents an increase in organizational complexity, with each "object" being primarily composed of the previous level's basic unit. The basic principle behind the organization is the concept of emergence—the properties and functions found at a hierarchical level are not present and irrelevant at the lower levels.
Organization furthermore refers to the high degree of order of an organism (in comparison to general objects). Ideally, individual organisms of the same species have the same arrangement of the same structures. For example, the typical human has a torso with two legs at the bottom and two arms on the sides and a head on top. It is extremely rare (and usually impossible, due to physiological and biomechanical factors) to find a human that has all of these structures but in a different arrangement.
The biological organization of life is a fundamental premise for numerous areas of scientific research, particularly in the medical sciences. Without this necessary degree of organization, it would be much more difficult—and likely impossible—to apply the study of the effects of various physical and chemical phenomena to diseases and body function. For example, fields such as cognitive and behavioral neuroscience could not exist if the brain was not composed of specific types of cells, and the basic concepts of pharmacology could not exist if it was not known that a change at the cellular level can affect an entire organism. These applications extend into the ecological levels as well. For example, DDT's direct effect occurs at the subcellular level, but affects higher levels up to and including multiple ecosystems. Theoretically, a change in one atom could change the entire biosphere.
The simple standard biological organization scheme, from the lowest level to the highest level, is as follows:
|* The atom|
|* The molecule, a grouping of atoms|
|Sub-cellular level||* The organelle, a functional grouping of biomolecules; biochemical reactions and interactions|
|Cellular level||* The cell, the basic unit of all life and the grouping of organelles|
|* The tissue, a functional grouping of cells|
|* The organ, a functional grouping of tissues|
|* The organ system, a functional grouping of organs|
|* The organism, the basic living system, a functional grouping of the lower-level components, including at least one cell|
|* The population, a grouping of organisms of the same species|
|* The biocoenosis or community, an interspecific grouping of interacting populations|
|* The ecosystem, a grouping of organisms from all biological domains in conjunction with the physical (abiotic) environment|
|* The biosphere, the complete set of communities|
|* The ecosphere, the complete set of ecosystems|
More complex schemes incorporate many more levels. For example, a molecule can be viewed as a grouping of elements, and an atom can be further divided into subatomic particles (these levels are outside the scope of biological organization). Each level can also be broken down into its own hierarchy, and specific types of these biological objects can have their own hierarchical scheme. For example, genomes can be further subdivided into a hierarchy of genes.
Each level in the hierarchy can be described by its lower levels. For example, the organism may be described at any of its component levels, including the atomic, molecular, cellular, histological (tissue), organ and organ system levels. Furthermore, at every level of the hierarchy, new functions necessary for the control of life appear. These new roles are not functions that the lower level components are capable of and are thus referred to as emergent properties.
See also 
- Cell theory
- Cellular differentiation
- Composition of the human body
- Evolutionary biology
- Hierachy of matter
- Human ecology
- Living systems
- Spontaneous order
- Griswold, Joseph G.; McDaniel, Nichole (Spring 2006), "Module 1:Overview and Hierarchy of Life", Progressions (New York, New York) 7 (3), ISSN 1539-1752
- Pavé, Alain (2006), "Biological and Ecological Systems Hierarchical Organization", in Pumain, Denise, Hierarchy in Natural and Social Sciences (New York, New York: Springer-Verlag), ISBN 978-1-4020-4126-6
- Postlethwait, John H.; Hopson, Janet L. (2006), Modern Biology, Holt, Rinehart and Winston, ISBN 0-03-065178-6
- Solomon, Eldra P.; Berg, Linda R.; Martin, Diana W. (2002), Biology (6th ed.), Brooks/Cole, ISBN 0-534-39175-3, LCCN 2001095366
- Cell physiology (in Human Physiology) at Wikibooks
- Characteristics of life and the nature of molecules (in General Biology) at Wikibooks
- Organization within the biosphere (in Ecology) at Wikibooks