Organelle facts for kids

A typical animal cell. Within the cytoplasm, the major organelles and cellular structures include: (1) nucleolus (2) nucleus (3) ribosome (4) vesicle (5) rough endoplasmic reticulum (6) Golgi apparatus (7) cytoskeleton (8) smooth endoplasmic reticulum (9) mitochondria (10) vacuole (11) cytosol (12) lysosome (13) centriole.

In cell biology, an organelle is a part of a cell that does a specific job.

Organelles typically have their own plasma membrane round them. Most of the cell's organelles are in the cytoplasm.

The name organelle comes from the idea that these structures are to cells what an organ is to the body.

Organelles are identified by microscopy, and can also be purified by cell fractionation.

There are many types of organelles in eukaryotic cells. Prokaryotes were once thought not to have organelles, but some examples have now been found. They are not organised like eukaryote organelles, and are not bounded by plasma membranes. They are called bacterial microcompartments.

Scope of the term

The term is now widely used to refer to cellular structures surrounded by single or double plasma membranes. However, the older definition of a 'subcellular functional unit' still coexists. That is, the term is sometimes used for structures which are not membrane-bound.

The plasma membrane is a lipid bilayer with some proteins embedded in it. It keeps the ions and molecules of the organelle from merging with the surroundings.

Cell structure

The non-membrane bounded organelles, also called large biomolecular complexes, are large assemblies of macromolecules that carry out particular and specialized functions, but they lack membrane boundaries. Many of these are referred to as "proteinaceous organelles" as their main structure is made of proteins. Such cell structures include:

• large RNA and protein complexes: ribosome, spliceosome, vault
• large protein complexes: proteasome, DNA polymerase III holoenzyme, RNA polymerase II holoenzyme, symmetric viral capsids, complex of GroEL and GroES; membrane protein complexes: porosome, photosystem I, ATP synthase
• large DNA and protein complexes: nucleosome
• centriole and microtubule-organizing center (MTOC)
• cytoskeleton
• flagellum
• nucleolus
• stress granule
• germ cell granule
• neuronal transport granule

The mechanisms by which such non-membrane bounded organelles form and retain their spatial integrity have been likened to liquid-liquid phase separation.

Origin of organelles

Mitochondria and chloroplasts, which have double-membranes and their own DNA, are believed to have originated from incompletely consumed or invading prokaryotic organisms, which were adopted as a part of the invaded cell. This idea is supported in the endosymbiotic theory.

Eukaryotic organelles

Major organelles

Minor organelles

Prokaryotic organelles

Prokaryotes are not so complex as eukaryotes. They were thought to have no internal structures enclosed by lipid membranes.

However, recent research has shown that at least some prokaryotes have microcompartments such as carboxysomes. These subcellular compartments are 100–200 nm in diameter and are enclosed by a shell of proteins. Even more striking is the description of membrane-bound magnetosomes in bacteria. as well as the nucleus-like structures of the Planctomycetes that are surrounded by lipid membranes.

Images for kids

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  (A) Electron micrograph of Halothiobacillus neapolitanus cells, arrows highlight carboxysomes. (B) Image of intact carboxysomes isolated from H. neapolitanus. Scale bars are 100 nm.

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  Structure of Candidatus Brocadia anammoxidans, showing an anammoxosome and intracytoplasmic membrane

See also

In Spanish: Orgánulo para niños