There are differing degrees of organisation within all life forms. Single-celled organisms, such as amoeba and bacteria, subsist in two different ways: the cell must either perform all life functions itself or it must live as a parasite, drawing materials from its host. Multicellular organisms consist of multiple cells performing different functions and working together to ensure the survival of the whole. Within multicellular individuals, cells are organised in five different levels; the higher the level, the higher the organizational complexity.
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All organisms, both uni- and multi-cellular, must be able to perform six basic functions in order to be considered alive. According to the Utah State Office of Education, they must be able to grow, reproduce, take in nutrients, eliminate waste, produce energy and react to changes in their environment. Each of these functions requires the concerted effort of the organism's components.
The first level of cell organisation is the cell; it is the highest level of organisation for a single-celled organism. Levels two through five apply to all multi-cellular organism, be they plant or animal. Level two is tissues, or groups of similar cells that perform the same function. Examples of tissues include blood and muscle. The third level is organs, which are made up of groups of tissues. Level four is organ systems, or a group of two or more organs that work together to perform a specific function, such as circulation. The fifth level of organisation is the organism which is able to perform all of the basic life functions.
"Cellular organisation" is the term used to describe the three levels of organisation that exist within each cell. Cells are composed of organised organelles, which are particular structures that perform specific functions within cells. Organelles themselves are made up of organised molecules, and molecules are forms of organised atoms, the building blocks of all matter.
All cells are categorised into two broad groups, prokaryotic and eukaryotic. Prokaryotic cells are generally smaller and more primitive. Their organelles are less specialised, and their genetic material is free-floating within the cell. By contrast, eukaryotic cells have membrane-bound organelles and a nucleus that encapsulates the chromosomes. There's lots of variety in both cell types; however, all cells share some common features: an outer cell membrane, DNA and RNA genetic material, cytoplasm, or the jelly-like fluid that suspends organelles and ribosomes, and the sites of protein synthesis.
Single-celled organisms may be either prokaryotic or eukaryotic, and range from bacteria to algae. Only one known prokaryotic multicellular organism is known to exist, as discovered in 2004 by Dr. C.N. Keim and colleagues of the Federal University in Rio de Janeiro. All other known multicellular organisms in the plant and animal kingdoms are comprised of organised levels of eukaryotic cells.
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