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The 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
* 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
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,
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
mitochondria none one to several dozen
chloroplasts none in algae and plants
single cells, colonies, higher
organization usually single organisms with specialized
Mitosis (core division)
cell division simple division Cytokinesis (cytoplasmatic division)
* 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
* 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
* Some prokaryotes have flagella which enable them to move actively
instead of passively drifting.
* The cytoplasm of eukaryotes does not appear as granular as that of
prokaryotes, since an important part of the ribosomes are bound to the
* 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
* Eukaryotes can become mobile using cilia or flagella. The flagella are
more complex than those of prokaryotes.
* 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
* 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").