[Home]Biological cell

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All known life forms are made of elementary units, the cells. A cell is the smallest self-preserving and self-reproducing unit.

All known cells (except a few specialized cell types) have certain basic components in common :

They also share several abilities :

There are 220 types of cells and tissues that make up the human body.

Kingdoms There are three basic cell types, called kingdoms, namely Prokaryotes, Eukaryotes and Archaea. Archaea are a very old and basic form of life. Prokaryotes have a simple cell structure. Eukaryotes emerged from Prokaryotes and Archaea, but have a much more sophisticated structure.

  Prokaryotes Eukaryotes
typical organisms bacteria protists, fungi, plants, animals
typical size ~ 1-10 [[Ám]] ~ 10-100 [[Ám]]
type of nucleus nucleoid?; no real nucleus real nucleus with double membrane
DNA circular linear molecules (chromosomes) with histone proteins
RNA-/protein-synthesis coupled in cytoplasm RNA-synthesis inside the core
protein synthesis in cytoplasm
ribosomes 50S+30S 60S+40S
cytoplasmatic structure very few structures highly structured by intercellular membranes and a cytoskeleton?
[cell movement]? flagella? made of flagellin? flagella and cilia? made of tubulin?
metabolism anaerob? or aerob? aerob?
mitochondria none one to several dozen
chloroplasts none in algae and plants
organization usually single cells single cells, colonies, higher organisms with specialized cells
cell division simple division Mitosis (core division)
Cytokinesis? (cytoplasmatic division)


Eukaryotes can have extrachromosomal DNA organized in circular molecules called plasmids. Then can carry additional functions, such as antibiotics resistance.


Eukaryotes contain several organelles with specific functions. Organelles are separated from the cytoplasm and from each other by membranes.

Eukaryotes can build multicellular colonies. These can consist of several equal cells that are able to stay alive when separated from the colony, or of specialized cells that depend on each other. The latter are "higher" organisms, including plants, animals and the reader of this article.


Archaea are the oldest organisms existing. According to common theory, they are the ancestors of eukaryotes. By absorbing certain prokaryotes, they added new functions to themselves, becoming more complex in the process ([endosymbiotic theory]?). Mitochondria and chloroplasts in eukaryotes, for example, are basically adapted bacteria.

Today, several Archaea can only survive by adopting to extreme environments, e.g. geyser?s or [black smoker]?s. They can survive extreme pressure and heat, and can feed on methane or sulfur.


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Last edited December 9, 2001 7:36 pm by Magnus Manske (diff)