page_title: Organelle
section_title: Prokaryotic organelles
hierarchical_section_title: Organelle / Prokaryotic organelles
caption_attribution_description: Italiano: Schema della cellula di Brocadia anammoxidans, batterio anammox.
context_page_description: In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name organelle comes from the idea that these structures are parts of cells, as organs are to the body, hence organelle, the suffix -elle being a diminutive. Organelles are either separately enclosed within their own lipid bilayers or are spatially distinct functional units without a surrounding lipid bilayer. Although most organelles are functional units within cells, some functional units that extend outside of cells are often termed organelles, such as cilia, the flagellum and archaellum, and the trichocyst.
  Organelles are identified by microscopy, and can also be purified by cell fractionation. There are many types of organelles, particularly in eukaryotic cells. They include structures that make up the internal endomembrane system, and other structures such as mitochondria and plastids. While prokaryotes do not possess eukaryotic organelles, some do contain protein-shelled bacterial microcompartments, which are thought to act as primitive prokaryotic organelles, and there is also evidence of membrane-bounded structures.
context_section_description: Prokaryotes are not as structurally complex as eukaryotes, and were once thought not to have any internal structures enclosed by lipid membranes. In the past, they were often viewed as having little internal organization, and lack cellular compartments; but slowly, details are emerging about prokaryotic internal structures. An early false turn was the idea developed in the 1970s that bacteria might contain cell membrane folds termed mesosomes, but these were later shown to be artifacts produced by the chemicals used to prepare the cells for electron microscopy.
  However, there is increasing evidence of compartmentalization in at least some prokaryotes. Recent research has revealed 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, reported in 2006. 
  The bacterial phylum Planctomycetes has revealed a number of compartmentalization features. Planctomycetes have an intracytoplasmic membranes that separates the cytoplasm into paryphoplasm (an outer ribosome-free space) and pirellulosome (or riboplasm, an inner ribosome-containing space). Membrane-bound anammoxosomes have been discovered in five Planctomycetes anammox genera. In the Planctomycetes Gemmata obscuriglobus, a nucleus-like structure surrounded by lipid membranes has been reported.
  Compartmentalization is a feature of prokaryotic photosynthetic structures. Purple bacteria have "chromatophores", which are reaction centers found in invaginations of the cell membrane. Green sulfur bacteria have chlorosomes, which are photosynthetic antenna complexes found bonded to cell membranes. Cyanobacteria have internal thylakoid membranes for light-dependent photosynthesis; studies have revealed that the cell membrane and the thylakoid membranes are not continuous with each other.
caption: Structure of Candidatus Brocadia anammoxidans, showing an anammoxosome and intracytoplasmic membrane
exif: {"Image Orientation": "Horizontal (normal)", "Image XResolution": "72", "Image YResolution": "72", "Image ResolutionUnit": "Pixels/Inch", "Image Software": "Adobe Photoshop CS6 (Windows)", "Image DateTime": "2012:11:23 16:36:09", "Image ExifOffset": "164", "Thumbnail Compression": "JPEG (old-style)", "Thumbnail XResolution": "72", "Thumbnail YResolution": "72", "Thumbnail ResolutionUnit": "Pixels/Inch", "Thumbnail JPEGInterchangeFormat": "302", "Thumbnail JPEGInterchangeFormatLength": "5602", "EXIF ColorSpace": "Uncalibrated", "EXIF ExifImageWidth": "703", "EXIF ExifImageLength": "362"}