Chloroplast



 

Chloroplasts are organelles found in plant cells and eukaryotic plastids.

Evolutionary origin

Chloroplasts are one of the many unique organelles in the plant cell. They are generally considered to have originated as endosymbiotic DNA and is involved in energy metabolism. Further, it has reticulations, or many infoldings, filling the inner spaces.

In green plants, chloroplasts are surrounded by two lipid-bilayer membranes. The inner membrane is now believed to correspond to the outer membrane of the ancestral cyanobacterium. Chloroplasts have their own genome, which is considerably reduced compared to that of free-living cyanobacteria, but the parts that are still present show clear similarities with the cyanobacterial genome. Plastids may contain 60-100 genes whereas cyanobacteria often contain more than 1500 genes.[3] Many of the missing genes are encoded in the nuclear genome of the host. The transfer of nuclear information has been estimated in tobacco plants at one gene for every 16000 pollen grains.[4]

In some algae (such as the heterokonts and other protists such as Euglenozoa and Cercozoa), chloroplasts seem to have evolved through a secondary event of endosymbiosis, in which a eukaryotic cell engulfed a second eukaryotic cell containing chloroplasts, forming chloroplasts with three or four membrane layers. In some cases, such secondary endosymbionts may have themselves been engulfed by still other eukaryotes, thus forming tertiary endosymbionts.

Structure

  Chloroplasts are observable morphologically as flat discs usually 2 to 10 micrometer in diameter and 1 micrometer thick. The chloroplast is contained by an envelope that consists of an inner and an outer phospholipid membrane. Between these two layers is the intermembrane space.

The material within the chloroplast is called the stroma, corresponding to the cytosol of the original bacterium, and contains one or more molecules of small circular DNA. It also contains ribosomes, although most of its proteins are encoded by genes contained in the host cell nucleus, with the protein products transported to the chloroplast.

Within the stroma are stacks of biosynthesis via the dissipation of a proton electrochemical gradient.

Embedded in the thylakoid membrane is the antenna complex, which consists of proteins, and light-absorbing pigments, including resonance energy transfer. Two chlorophyll molecules are then ionised, producing an excited electron which then passes onto the photochemical reaction centre.

Transplastomic plants

Recently, chloroplasts have caught attention by developers of genetically modified plants. In certain plant species, such as tobacco, chloroplasts are not inherited from the male, and therefore, transgenes in these plastids cannot be disseminated by pollen. This makes plastid transformation a valuable tool for the creation and cultivation of genetically modified plants that are biologically contained, thus posing significantly lower environmental risks. This biological containment strategy is therefore suitable for establishing the coexistence of conventional and organic agriculture. The reliability of this mechanism has not yet been studied for all relevant crop species. However, the research programme Co-Extra recently published results for tobacco plants, demonstrating that the containment of transplastomic plants is highly reliable with a tiny failure rate of 3 in 1,000,000.[5]

See also

  • Chloroplast membrane
  • Inner membrane
  • Outer membrane

References

  • This article contains material from the Science Primer published by the NCBI, which, as a U.S. government publication, is in the public domain.
  1. ^ Mereschkowsky C (1905). "Über Natur und Ursprung der Chromatophoren im Pflanzenreiche". Biol Centralbl 25: 593-604.
  2. ^ Schimper AFW (1883). "Über die Entwicklung der Chlorophyllkörner und Farbkörper". Bot. Zeitung 41: 105-14, 121-31, 137-46, 153-62.
  3. ^ Martin W, Rujan T, Richly E, Hansen A, Cornelson S, Lins T, Leister D, Stoebe B, Hasegawa M, Penny D (2002). "Evolutionary analysis of Arabidopsis, cyanobacterial, and chloroplast genomes reveals plastid phylogeny and thousands of cyanobacterial genes in the nucleus". Proc Natl Acad Sci 99: 12246-12251.
  4. ^ Huang CY, Ayliffe MA, Timmis JN (2003 Mar 6). "Direct measurement of the transfer rate of chloroplast DNA into the nucleus". Nature 422 (6927): 72-6.
  5. ^ Ruf S, Karcher D, Bock R (2007 Apr 24). "Determining the transgene containment level provided by chloroplast transformation". PNAS 104 (17): 6998-7002.
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Chloroplast". A list of authors is available in Wikipedia.