Plastids

• Plastids are found in all plant cells and in euglenoides.
• Large sized. Easily observable under the microscope.
• They contain some pigments.
• Based on the type of pigments, plastids are 3 types:
  a. Chloroplasts: Contain chlorophyll and carotenoid pigments. They trap light energy for photosynthesis.
  b. Chromoplasts: Contain fat soluble carotenoid pigments like carotene, xanthophylls etc. This gives a yellow, orange or red colour.
  c. Leucoplasts: These are colourless plastids of varied shapes and sizes with stored nutrients. They include:
  1 Amyloplasts: Store starch. E.g. potato.
  2 Elaioplasts: Store oils and fats.
  3 Aleuroplasts: Store proteins.

Chloroplasts

• These are double membrane bound organelles mainly found in the mesophyll cells of the leaves.
• These are lens-shaped, oval, spherical, discoid or ribbon-like organelles.
• Length: 5-10 mm. Width: 2-4 mm.
• Their number varies from 1 (e.g. Chlamydomonas, a green alga) to 20-40 per cell in the mesophyll.
• Inner membrane of chloroplast is less permeable.
• The space limited by the inner membrane of the chloroplast is called the stroma. It contains a number of organized flattened membranous sacs called thylakoids.
• Membrane of thylakoids encloses a space called lumen.
• Chlorophyll pigments are present in the thylakoids.
• Thylakoids are arranged in stacks called grana or the intergranal thylakoids.
• There are flat membranous tubules called the stroma lamellae connecting the thylakoids of the different grana.
• The stroma contains small, double-stranded circular DNA molecules, ribosomes and enzymes for the synthesis of carbohydrates and proteins.
• The ribosomes of the chloroplasts are smaller (70S) than the cytoplasmic ribosomes (80S).