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The cell was first discovered by Robert Hooke in 1665 using a microscope. The first cell theory is credited to the work of Theodor Schwann and Matthias Jakob Schleiden in the 1830s. In this theory the internal contents of cells were called protoplasm and described as a jelly-like
The cell is the unit of structure, physiology, and organization in living things. [32] The cell retains a dual existence as a distinct entity and a building block in the construction of organisms. [32] By the 1860s, these tenets were the accepted basis of cell theory, used to describe the elementary anatomical composition of plants and animals. [3]
Schwann dedicated a chapter of the treatise to explicitly formulate the cell theory, stating that ("the elementary parts of all tissues are formed of cells” and that “there is one universal principle of development for the elementary parts of organisms... and this principle is in the formation of cells" (Henry Smith's translation, 1847).
Bacteria function and reproduce as individual cells, but they can often aggregate in multicellular colonies. [54] Some species such as myxobacteria can aggregate into complex swarming structures, operating as multicellular groups as part of their life cycle , [ 55 ] or form clusters in bacterial colonies such as E.coli .
Illustration of Virchow's cell theory. Virchow is credited with several key discoveries. His most widely known scientific contribution is his cell theory, which built on the work of Theodor Schwann. He was one of the first to accept the work of Robert Remak, who showed that the origin of cells was the division of pre-existing cells. [29]
Many bacteria secrete extracellular polymers outside of their cell walls called glycocalyx. These polymers are usually composed of polysaccharides and sometimes protein. Capsules are relatively impermeable structures that cannot be stained with dyes such as India ink. They are structures that help protect bacteria from phagocytosis and desiccation.
Some species of bacteria kill and then consume other microorganisms; these species are called predatory bacteria. [199] These include organisms such as Myxococcus xanthus , which forms swarms of cells that kill and digest any bacteria they encounter. [ 200 ]
In mammals, major cell types include skin cells, muscle cells, neurons, blood cells, fibroblasts, stem cells, and others. Cell types differ both in appearance and function, yet are genetically identical. Cells are able to be of the same genotype but of different cell type due to the differential expression of the genes they contain.