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Plastids – Definition, Classification, Structure, Functions

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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

What are Plastids?

Plastids are a double membrane diverse group of physiologically, phylogenetically, and genetically related eukaryotic organelles that play important roles in plant metabolism through different processes like;

  • Photosynthesis
  • Imparting colors of fruits and other parts
  • Nitrogen, and sulfur assimilation 
  • Amino acid synthesis

They are used as an important factor to differentiate plants and animal cells. Plastid contains a double-stranded DNA molecule that is circular and resembles the circular chromosome of cells of prokaryotes and ribosomes for protein synthesis. The outer and inner membranes have an intermembrane space between them.

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

Plastid

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.

Inheritance of Plastids 

In numerous green algae and land plants, plastid inheritance comes from only one parent. In species with oogamy, plastids are typically inherited from the maternal parent. If both parents contribute to plastid inheritance, the maternal plastids usually dominate. However, conifers have paternal plastid inheritance, despite the maternal influence being strong. Some species even shift from complete maternal inheritance to complete paternal inheritance.

Functions of Plastids

Plastids are responsible for many core functions of the plants, some of which are listed below;

  1. Due to the presence of DNA and ribosomes the plastids can be used for phylogenetic studies of plants.
  2. The pigments contained in plastids are responsible for the different coloration of plant’s parts.
  3. They help of storage of many important organic chemical compounds.
  4. Helps in photosynthesis.

FAQs on Plastid

1. Lack of which plastids may affect the process of pollination?

Answer: 

Chromoplasts are responsible for synthesizing and storing pigments, giving plants their distinct color. They can be found in flowering plants, fruit roots, and aging leaves during senescence. These pigments, like carotenoids, attract pollinators to the plants.

2. List the structures that makes up a plastid.

Answer:  

Plastids are made of double membrane with its own DNA and ribosomes along with the pigments and the organic substances stored in it.

3. What are plastid used for?

Answer: 

Plastid is a plant cell organelle which perform various function according to the type of plastid. They are the storehouse of important chemical compound, site of photosynthesis, and also determine the color of cell.

4. Colored plastids present in carrots and sweet potatoes are?

Answer:

Chromoplasts– These are colored plastids. Their main function includes pigment synthesis and storage. They exist in flowering plants, fruit roots (carrots and sweet potato), aging leaves i.e. senescence, and are responsible for their color. Their distinctive color formation is due to carotenoids.

5. The places where plastids can be found in the living system are?

Answer:  

Plastids can be found only in plant cells and not in any other cells.



Last Updated : 28 Aug, 2023
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