Biological cellThe cell is the basic unit of life. All known cells (except a few specialized cell types) have certain basic components in common: * cytoplasm (the living substance which makes up most of the cell volume) * DNA, the genetic information that acts as a blueprint for the other components. * Proteins, the machinery of the cell. * Membranes, which separate the cell form its environment, work as a filter, and act as a communication relay with the outside. They also share several abilities: * Reproduction by cell division. * Metabolism, the taking in of raw material, using it to build cell components or breaking it down for energy, and releasing byproducts. * Protein biosynthesis by transcription of DNA to RNA and then translation to protein. Organisms vary from single cells which survive individually, through colonial forms with multiple similar cells living together, to multicellular forms in which cells are specialized and do not generally survive if separated. There are 220 types of cells and tissues that make up the human body. There are two basic types of cells: prokaryotic and eukaryotic. Prokaryotic cells are structurally simple. They are found only in single-celled and colonial organisms. In the three-domain system of taxonomy, prokaryotic cells are placed in the domains Archaea and Eubacteria. Eukaryotic cells have organelles with their own cell membranes. Single-celled eukaryotic organisms are very diverse, but many colonial and multicellular forms also exist. (The multicellular kingdoms, Animalia, Plantae and Fungi, are all eukaryotic.) Features of prokaryotic and eukaryotic cells Ê Prokaryotes Eukaryotes typical organisms bacteria protists, fungi, plants, animals typical size ~ 1-10 um ~ 10-100 um type of nucleus nucleoid region; no real nucleus with double real nucleus membrane DNA circular linear molecules (chromosomes) with histone proteins RNA-/protein-synthesis coupled in RNA-synthesis inside the core cytoplasm protein synthesis in cytoplasm ribosomes 50S+30S 60S+40S highly structured by cytoplasmatic structure very few structures intercellular membranes and a cytoskeleton cell movement flagella made of flagella and cilia made of flagellin tubulin mitochondria none one to several dozen chloroplasts none in algae and plants single cells, colonies, higher organization usually single organisms with specialized cells cells Mitosis (core division) cell division simple division Cytokinesis (cytoplasmatic division) Prokaryotic cells * The cytoplasm of prokaryotes (the liquid which makes up most of the cell volume) is diffuse and granular due to ribosomes (protein factories) floating in the cell. * The plasma membrane (a phospholipid bilayer) separates the interior of the cell from its environment and serves as a filter and communications beacon. * Most prokaryotes (except for the bacterial Mycoplasma and the Ureaplasma of the archaea) have a cell wall. It consists of peptidoglycan and acts as an additional barrier against exterior forces. It also prevents the cell from "exploding" of osmotic pressure against a hypotonic environment. * The prokaryotic DNA is a circular molecule. Even without a real nucleus, the DNA is somehow condensed in a nucleoid. Prokaryotes can have extrachromosomal DNA organized in circular molecules called plasmids. Plasmids can carry additional functions, such as antibiotics resistance. * Some prokaryotes have flagella which enable them to move actively instead of passively drifting. Eukaryotic cells * The cytoplasm of eukaryotes does not appear as granular as that of prokaryotes, since an important part of the ribosomes are bound to the endoplasmatic reticulum. * The plasma membrane resembles that of prokaryotes in function, with minor differences in the setup. Cell walls may or may not be present. * The eukaryotic DNA is organized in one or more linear molecules, called chromosomes, which are highly condensed (e.g. folded around histones). All chromosomal DNA is stored in the cell nucleus, separated from the cytoplasm by a membrane. Some eukaryotic organelles can contain some DNA. * Eukaryotes can become mobile using cilia or flagella. The flagella are more complex than those of prokaryotes. History * 1665 : Robert Hooke discovers cells in cork, then in living plants using the first microscopes. * 1839 : Theodor Schwann discovers that plants and animals are made of cells, concluding that cells are a common unit of structure and development, thus founding cell theory. Schwann cells are named after him. * The belief that life forms are able to occur spontaneously ("generatio spontanea") is contradicted by Louis Pasteur (1822-1895). * Rudolph Virchow states that cells always emerge from cell divisions ("omnis cellula ex cellula").