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    Chloroplasts
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    Chloroplast Background Information

    Definition

    Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis with the help of the molecule chlorophyll stored inside the chloroplasts.

    Basics

    Chloroplasts are small organelles inside the cells of plants and algae. They absorb light to make sugar in a process called photosynthesis. The sugar can be stored in the form of starch. Chloroplasts contain the molecule chlorophyll, which absorbs sunlight for photosynthesis. In addition to chlorophyll, a chloroplast uses carbon dioxide (CO2) and water (H2O) to form sugar, and gives off oxygen (O2). Chlorophyll is what gives green plants their green colour. Chloroplasts also contain various yellow and orange pigments to assist in photon capture for photosynthesis.

    Structure: Each choloroplast is surrounded by a double walled semi-permeable membrane. In the layered stacks are flat disk-shaped thylakoids. They contain light-absorbing pigments, including chlorophyll and carotenoids, as well as proteins which bind the pigments. Like mitochondria, chloroplasts also contain their own DNA and ribosomes.

    Evolution: Chloroplasts are one of the many different types of organelles in the cell. They are thought to have originated as endosymbiotic cyanobacteria. This was first suggested by Mereschkowsky in 1905 after an observation by Schimper in 1883 that chloroplasts closely resemble cyanobacteria. Almost all chloroplasts are thought to derive directly or indirectly from a single endosymbiotic event.

    Mitochondria also had a similar origin, but chloroplasts are found only in plants and protista. In green plants, chloroplasts are surrounded by two lipid-bilayer membranes. They are thought to correspond to the outer and inner membranes of the ancestral cyanobacterium. Chloroplasts have their own genome, which is much smaller than that of free-living cyanobacteria. The DNA which remains shows clear similarities with the cyanobacterial genome. Plastids may contain 60–100 genes whereas cyanobacteria often contain more than 1500 genes. Many of the missing genes are encoded in the nuclear genome of the host.

    In some algae (such as the heterokonts), 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. In the alga Chlorella, there is only one chloroplast, which is bell-shaped.

    In some groups of mixotrophic protists such as the dinoflagellates, chloroplasts are separated from a captured alga or diatom and used temporarily. These klepto (stolen) chloroplasts may only have a lifetime of a few days and are then replaced.

    Topics of Interest

    Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.

    The word chloroplast is derived from the Greek words chloros, which means green, and plast, which means form or entity. Chloroplasts are members of a class of organelles known as plastids.

    Chloroplasts are observable as flat discs usually 2 to 10 micrometers in diameter and 1 micrometer thick. In land plants, they are, in general, 5 μm in diameter and 2.3 μm 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. A typical parenchyma cell contains about 10 to 100 chloroplasts.

    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; however most of its proteins are encoded by genes contained in the host cell nucleus, with the protein products transported to the chloroplast.

    Recently, chloroplasts have caught attention by developers of genetically modified plants. In most flowering plants, chloroplasts are not inherited from the male parent, although in plants such as pines, chloroplasts are inherited from males. Where chloroplasts are inherited only from the female, 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. While the reliability of this mechanism has not yet been studied for all relevant crop species, recent results in tobacco plants are promising, showing a failed containment rate of transplastomic plants at 3 in 1,000,000.

    Chloroplasts contain several important membranes, vital for their function. Like mitochondria, chloroplasts have a double-membrane envelope, called the chloroplast envelope. Each membrane is a phospholipid bilayer, between 6 and 8 nm thick, and the two are separated by a gap of 10-20 nm, called the intermembrane space. The outer membrane is permeable to most ions and metabolites, but the inner membrane is highly specialised with transport proteins. Carbohydrates are transported across the outer membrane by a triose phosphate translocator.

    The Calvin cycle or Calvin-Benson-Bassham cycle is a series of biochemical reactions that take place in the stroma of chloroplasts in photosynthetic organisms. It was discovered by Melvin Calvin, James Bassham and Andrew Benson at the University of California, Berkeley by using the radioactive element, carbon-14. It is one of the light-independent (dark) reactions, used for carbon fixation.

    The endosymbiotic theory concerns the origins of mitochondria and plastids (e.g. chloroplasts), which are organelles of eukaryotic cells. According to this theory, these organelles originated as separate prokaryotic organisms that were taken inside the cell as endosymbionts. Mitochondria developed from proteobacteria (in particular, Rickettsiales or close relatives) and chloroplasts from cyanobacteria.

    Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)

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