Types of Plastid

Proplastids (undifferentiated plastids) may undergo differentiation into many forms, depending upon their specialized functions in the cell. Plastid may develop as  following variants:

• Chloroplasts 
• Chromoplasts 
• Leucoplasts 
• Gerontoplasts

Chloroplasts 

Chloroplasts (green plastids) are 5 to 10 μm long organelles. They are surrounded by a double membrane known as the chloroplast envelope. In addition to the double membrane, chloroplasts have a third internal membrane system, known as the thylakoid membrane. They are located in the mesophyll of plant cells. They perform the process of photosynthesis and synthesize their food.

Other functions carried out by plant cells include;

• Synthesis of  fatty acid
• Synthesis of amino acids, and 
• Plays an important role in regulating the immune response in plants.

Structure of Chloroplast

The majority of chloroplasts (green plastid) of green plants exist in the leaf mesophyll cells. Their shape resembles a lens and is discoid, oval, spherical, or ribbon-like organelles. Their length varies from 5-10 µm and their width varies from 2-4 µm.

These organelles are encircled by double membrane-like mitochondria. The inner membrane of chloroplast is comparatively less permeable. Stroma is the space surrounded by the internal membrane of the chloroplast. This internal space is filled with fluid that surrounds the grana, thylakoids.  It also contains ribosomes, starch, and DNA

Chromoplasts

These are colored plastids, which are responsible for pigment synthesis and storage. They can be found in a variety of plants, including flowering plants, fruit roots like carrots and sweet potatoes, and aging leaves undergoing senescence. The unique coloration of these plastids is due to carotenoids. Chromoplasts are a type of plastid that synthesize and store pigments like xanthophylls (yellow), carotene (orange), and various red pigments. These vibrant colors help attract pollinators to the plants.

Leucoplasts

These are unpigmented i.e. colorless plastids. They are found in non-photosynthetic roots, bulbs, tubers, and seeds.

Leucoplast exists in four types

• Amyloplasts: These are double enveloped organelles. For the plant,   they store and synthesize starch through glucose polymerization
• Proteinoplasts: They are known as proteoplasts aleuroplasts, or aleuronaplasts. They store proteins and exist in seeds (pulses).
• Elaioplasts: Elaioplasts are also known as nucleosomes. They store fats and oils that are needed for the plant. They exist in seeds.
• Tannosomes: They are found in vascular plants. They synthesize polyphenols and tannins.

Gerontoplasts 

They arise from a chloroplast during the aging of plant foliage. Structural changes in the thylakoid membrane cause the formation of gerontoplasts which are originally chloroplasts that are no longer viable for photosynthesis.

Plastids are cell organelles that are found in green plants and algae. Plastids are responsible for containing photosynthetic pigments, other coloring pigments, starch, proteins, fats, etc. substances. The credit for the discovery of plastid goes to Ernst Haeckel, however, the proper definition of plastid was given by Schimper.

Table of Content

• What are Plastids?
• Types of Plastid
  • Chloroplasts
  • Structure of Chloroplast
  • Chromoplasts
  • Leucoplasts
  • Gerontoplasts
• Inheritance of Plastids
• Functions of Plastids
• FAQs on Plastid