|MadSci Network: Cell Biology|
Hi, Plastids are plant organelles which have diverse functions include photosynthesis, gravity perception, and biogenesis of micro- and macromolecules. All kinds of plastids develop from proplastids, which are small bodies found in plants growing in dark as well as in the light. Leucoplast is the general name for colorless plastids, which are usually found in root and dark-adapted plants. Some leucoplasts contain proteins or starch, performing storage function. Leucoplasts may also take part in gravity perception, but I didn't find reference on mechanism of this function. Chloroplast is the location of photosynthesis, as I think you’ve already known. Chromoplast are considered derived from chloroplast, but may arise from less differentiated plastids. Chromoplast is the location of forming carotenoid pigments such as carotene or lycopene. Chromoplast can be found in flower, fruit, and leaves in stress or senescence. In flower, carotenoid pigment enables the forming of different color, which can attract insects. Also, chromoplast carotenoids are known to accumulate in green tissues experiencing stress conditions, and studies indicate that they provide efficient protection against oxidative stress. For example, it is shown that oxidative stress is a potent driving force for the expression of carotenoid biosynthetic genes during the chloroplast to chromoplast transition. This is very important for plants. Being immobile and because of oxygenic photosynthesis, plants have the highest internal oxygen concentrations of any other organism. The concentration of molecular oxygen in plant leaf cells is 250 µM, which is higher than the concentration of 0.1 µM found in mammalian mitochondria. It has been estimated that 1% of the oxygen consumed by plants is diverted into active oxygen. As a consequence, plant cells have developed some nonenzymatic and enzymatic mechanisms for scavenging this toxic component. Neutralization by carotenoids is example of nonenzymatic mechanisms. Reference: Florence Bouvier, Ralph A. Backhaus, and Bilal Camara: Induction and Control of Chromoplast-specific Carotenoid Genes by Oxidative Stress J. Biol. Chem. 1998 273: 30651-30659 Anatomy of Seed Plants, 2nd Edition, Katherine Esau, 1976, John Wiley and sons publishers. Plant Physiology, Fourth Editio, Salisbury & Ross, 1991, Wadsworth Publishing
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