Biological organisation

"Hierarchy of life" and "Levels of organization" redirect here. For the hierarchical ordering and organization of all organisms, see Biological classification. For the evolutionary hierarchy of organisms and interspecial relationships, see Phylogenetic tree. For the concept of hierarchical organization outside of biology, see Integrative level.
A population of bees shimmers in response to a predator.

Biological organization is the hierarchy of complex biological structures and systems that define life using a reductionistic approach.[1] The traditional hierarchy, as detailed below, extends from atoms to biospheres. The higher levels of this scheme are often referred to as a ecological organization concept, or as the field, hierarchical ecology.

Each level in the hierarchy represents an increase in organizational complexity, with each "object" being primarily composed of the previous level's basic unit.[2] 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 is the high degree of order of an organism (in comparison to general objects).[3] 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 physiology (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 inseciticidal 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.

Levels

The simplest unit of life is the atom, like oxygen. Two or more atoms is a molecule, like dioxide. Many molecules is a macromolecule, such as a phospholipid. Multiple macromolecules form a cell, like a club cell. A group of cells functioning together is a tissue, for example, Epithelial tissue. Different tissues make up an organ, like a lung. Organs work together to form an organ system, such as the Respiratory System. All of the organ systems make a living organism, like a lion. A group of the same organism living together in an area is a population, such as a pride of lions. Two or more populations interacting with each other form a community, for example, lion and zebra populations interacting with each other. Communities interacting not only with each other but also with the physical environment encompass an ecosystem, such as the Savanna ecosystem. All of the ecosystems make up the biosphere, the area of life on Earth.

The simple standard biological organization scheme, from the lowest level to the highest level, is as follows:[1]

For particles smaller than atoms see subatomic particles
A-cellular level
and
Pre-cellular level 		Atoms
Molecule Groups of atoms
Biomolecular complex Groups of (bio)molecules
Sub-cellular level Organelle Functional groups of biomolecules, biochemical reactions and interactions
Cellular level Cell Basic unit of all life and the grouping of organelles
Super-cellular level
(Multicellular level) 		Tissue Functional groups of cells
Organ Functional groups of tissues
Organ system Functional groups of organs
Ecological levels Organism The basic living system, a functional grouping of the lower-level components, including at least one cell
Population Groups of organisms of the same species
Community
(or biocoenosis) 		Interspecific groups of interacting populations
Ecosystem Groups of organisms from all biological domains in conjunction with the physical (abiotic) environment
Biome Continental scale (climatically and geographically contiguous areas with similar climatic conditions) grouping of ecosystems.
Biosphere or
Ecosphere 		All life on Earth or all life plus the physical (abiotic) environment[4]
For levels larger than the planet, see Earth's location in the Universe

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.[5]

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.

Every organism is organized, though not necessarily to the same degree.[6] An organism can not be organized at the histological (tissue) level if it is not composed of tissues in the first place.

Fundamentals

Empirically, a large proportion of the (complex) biological systems we observe in nature exhibit hierarchic structure. On theoretical grounds we could expect complex systems to be hierarchies in a world in which complexity had to evolve from simplicity. System hierarchies analysis performed in the 1950s,[7][8] laid the empirical foundations for a field that would be, from 1980's, hierarchical ecology.[9][10][11][12][13]

The theoretical foundations are summarized by Thermodynamics. When biological systems are modeled as physical systems, in its most general abstraction, they are thermodynamic open systems that exhibit self-organized behavior, and the set/subset relations between dissipative structures can be characterized in an hierarchy.

Another way, more simple and direct to explain the fundamentals of the "hierarchical organization of life", was introduced in Ecology by Odum and others as the "Simon's hierarchical principle";[14] Simon[15] emphasized that hierarchy "emerges almost inevitably through a wide variety of evolutionary processes, for the simple reason that hierarchical structures are stable".

To motivate this deep idea, he offered his "parable" about imaginary watchmakers.

Parable of the Watchmakers

There once were two watchmakers, named Hora and Tempus, who made very fine watches. The phones in their workshops rang frequently; new customers were constantly calling them. However, Hora prospered while Tempus became poorer and poorer. In the end, Tempus lost his shop. What was the reason behind this?

The watches consisted of about 1000 parts each. The watches that Tempus made were designed such that, when he had to put down a partly assembled watch (for instance, to answer the phone), it immediately fell into pieces and had to be reassembled from the basic elements.

Hora had designed his watches so that he could put together subassemblies of about ten components each. Ten of these subassemblies could be put together to make a larger sub-assembly. Finally, ten of the larger subassemblies constituted the whole watch. Each subassembly could be put down without falling apart.

See also

Notes

References

External links

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