Class 11 Biology Cell : The Unit of Life Plastids


  • Plastids are the organelle present in the plant cells which bear some specific pigments, thus imparting specific colours to the plants.
  • Based on the type of pigments plastids can be classified into chloroplasts, chromoplasts and leucoplasts.
  • The chloroplasts contain chlorophyll and carotenoid pigments which are responsible for trapping light energy essential for photosynthesis.
  • In the chromoplasts, fat soluble carotenoid pigments like carotene, xanthophylls and others are present.
  • The leucoplasts are the colourless plastids of varied shapes and sizes with stored nutrients like amyloplasts which store carbohydrates, elaioplasts store oils and fats, whereas the aleuroplasts store proteins.
  • Chloroplasts of the green plants are found in the mesophyll cells of the leaves.
  • The chloroplasts are double membrane bound.
  • The space limited by the inner membrane of the chloroplast is called the stroma.
  • A number of organised flattened membranous sacs called the thylakoids.
  • Thylakoids are arranged in stacks like the piles of coins called grana.
  • There are flat membranous tubules called the stroma lamellae connecting the thylakoids of the different grana.
  • The membrane of the thylakoids enclose a space called a lumen.
  • The stroma contains enzymes, double stranded DNA molecules, ribosomes.


Fig. Chloroplast

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