5. Cell Structure And Organization - part 11 - Plastids

 

5. Cell Structure And Organization - part 11 - Plastids


Plastids :

  • Like mitochondria, plastids too are double walled organelles containing - 
  1. DNA
  2. RNA and 
  3. 70S ribosomes. 
  • But they are larger in size and can be observed under light microscope. 
  • Plastids are classified according to the pigments present in it as - 
  1. leucoplasts,
  2. chromoplasts and 
  3. chloroplasts.

1. Leucoplasts:

  • Do not contain any pigments, they are of various shapes and sizes.
  • These are meant for storage of nutrients. 
         e.g. 1. Amyloplasts that store starch
                2. Elaioplasts that store oils and
                3. Aleuroplasts that store proteins.


Something interesting - Endosymbiont theory : 

  • Both mitochondria and chloroplast are double walled organelles, they have DNA and ribosomes and can duplicate within the cell on their own! 
  • It is considered that primitive eukaryotic cell engulfed an aerobic nonphotosynthetic prokaryotic cell. 
  • This guest cell developd symbiotic relationship with the host cell. 
  • In course of evolution, both merged as a single cell with a mitochondrion.
  • One of these cells might have engulfed photosynthetic prokaryote and evolved into photosynthetic eukaryotic cells. This is called ‘Endosymbiont theory’ i.e. coexistence of cell within cell!

2. Chromoplasts 

  • Contain pigments like carotene and xanthophyll, etc. 
  • They impart red, yellow or orange colour to flowers and fruits. 

3. chloroplasts :


  • Plant cells, cells of algae and few protists like Euglena contain chloroplasts.
  • e.g. ribbon shaped chloroplast in Spirogyra. 
  • It differs in size, number and shape in various cells in which it is found. 
  • In plants, it is found in green regions; mainly in mesophyll of leaf. This chloroplast is lens shaped. 
  • But it can also be oval, spherical, discoid or ribbon like. 
  • A cell may contain single large chloroplast as in Chlamydomonas or there are 20 to 40 chloroplasts per cell seen in mesophyll cells. 
  • Chloroplasts contain green pigment - chlorophyll along with other enzymes that help in production of sugar by photosynthesis.
  • Inner membrane of double walled chlorophyll is comparatively less permeable.
  • Inside the cavity of inner membrane, there is another set of membranous sacs called thylakoids. 
  • Thylakoids are arranged in the form of stacks called grana (singular: granum).
  • The grana are connected to each other by means of membranous tubules called stroma lamellae. 
  • Space outside thylakoids is is filled with stroma. 
  • The stroma, and the space inside thylakoids contain various enzymes essential for photosynthesis. 
  • Like other plastids, stroma of chloroplast also contains DNA and ribosomes.

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