Principles of Biology |
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Cell structures There are two basic forms of cells found in nature; the prokaryotic and eukaryotic cell type. The prokaryotic cell type is a single cell with just one compartment that contains all the DNA and proteins necessary for reproduction and growth. It is used by bacteria and archaea organisms. The eukaryotic cell type is a much larger cell and contains multiple internal organelles that divide these larger cells into specialized compartments that are the nucleus (containing genes), the mitochondria and chloroplasts (producing energy), and endomembrane systems (secreting, importing and redistribution cellular components). In addition, eukaryotic cells contain a cytoskeleton structure important for internal movement and overall structure. Multicellular organisms contain only eukaryotic cells. These cells also need surface junctions that allow them to connect and communicate with each other to provide proper stability and physiological integration into the often very large body structures (animals, plants). For biological macromolecules and cells, and for any hierarchical level for that matter, the basic principle is ‘function follows form’ (*). This means that over evolutionary time, the structure of proteins, DNA, cells and shape of organisms have evolved altering specific functions which may or may not improve the chance for survival of the organism ('survival of the fittest'). * Note: most biology textbooks introduce the notion of 'form follows function', a paradigm of design first articulated by the American architect Louis Sullivan (1856 - 1924). Evolution is not design of a structure for a purpose (e.g. use windows to have natural light inside a house), the rational behind Sullivan's dictum. Rather, evolution is design by trial and error. Evolution does not ‘plan ahead’, but tinkers with changes in structures (mutations) subject to selection ('trial') allowing novel functions (an 'error') to propagate to the next generation (see our discussion of intelligent design theory and why it is not a scientific theory, but theistic belief). H
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