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 :
- 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, act as a communication relay with the outside and divide more complex cells in compartments (organelles).
They also share several abilities :
- Reproduction by cell division.
- Metabolism, taking in raw material, converting it to cell components and energy, and releasing byproducts.
- Protein synthesis by converting DNA to RNA to protein.
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.
|typical organisms||bacteria||protists, fungi, plants, animals|
|typical size||~ 1-10 µm||~ 10-100 µm|
|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
|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)
- 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.
- All prokaryotes (except the mycoplasma) 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 consists of a circular molecule. Even without a real nucleus, the DNA is somehow condensed in a nucleoid.
Eukaryotes can have extrachromosomal DNA organized in circular molecules called plasmids. Then can carry additional functions, such as antibiotics resistance.
- 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 the ribosomes are part of the endoplasmatic reticulum.
- The plasma membrane resembles that of prokaryotes in function, with minor differences in the setup.
- Within the eukaryotes, only plants have a cell wall.
- The eukaryotic DNA is organized in one or more linear molecules, which are highly condensed (e.g. folded around histones). All chromosomal DNA is stored in the nucleus, separated from the cytoplasm by a membrane. Eukaryotes have no plasmids, only some organelles can contain some DNA.
- Eukaryotes can become mobile using cilia or flagella. The flagella are more advanced than these of prokaryotes.
Eukaryotes contain several organelles with specific functions. Organelles are separated from the cytoplasm and from each other by membranes.
- The Endoplasmatic Reticulum (ER) is an extension of the nucleus membrane. The ER can be divided into the smooth ER and the rough ER, by its appearance on a microscopic image. The surface of the rough ER is covered with ribosomes that insert freshly synthesized proteins into the ER. Through the ER, the proteins are transported towards their destination.
- The Golgi Complex is an extension of the ER, transforming and transporting proteins towards the plasma membrane.
- The mitochondria play an important role in cell metabolism. Derived from bacteria, mitochondria contain their own, small amount of DNA.
- The cytoskeleton keeps the cell in shape. It is also important during cell division and in the intracellular transportation system.
- Chloroplasts occur in plants and algae. They collect the energy of light and convert it so it can be used by the cell. Since chloroplasts are derived from cyanobacteria, they contain some DNA.
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.
- 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").