Biological organisation

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"Hierarchy of life" redirects here. For the hierarchical ordering and organization of all organisms, see Biological classification. For the evolutionary hierarchy or organisms and interspecial relationships, see Phylogenetic tree.
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 (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.[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 refers to 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 body function. For example, fields such as cognitive and behavioural 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.

Levels[edit]

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 Clara 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]

A-cellular level
and
Pre-cellular level
* The atom
* The molecule, a grouping of atoms
* The biomolecular complex, a grouping of (bio)molecules
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
Super-cellular level
(Multicellular level)
* 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 community (or biocoenosis), a interspecific grouping of interacting populations
* The ecosystem, a grouping of organisms from all biological domains in conjunction with the physical (abiotic) environment
* The Biome, a continental scale (climatically and geographically contiguous areas with similar climatic conditions) grouping of ecosystems.
* The biosphere or ecosphere, all life on Earth or all life plus the physical (abiotic) environment[4]

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.

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