Cell Biology | Cell Organelles | Plant Cell

Plastids- Types, Structure and function.

A double membraned oval organelle present in all plant cells and euglenoids.

They bear some specific pigments thus giving the specific colors to the plants.

They are generally large in shape and can be easily observed under the microscope.

Based on the amount and the type of the pigments plastids can be classified into 3 parts-

  • Chromoplast
  • Leucoplast
  • Chloroplast

Even though plastids vary from each other in functions and physiology, but they can be interconverted into each other, based upon the need.

Plastids have their own-

  • DNA
  • Ribosome
  • Specific Pigments
  • Storage granule
  • Some times mitochondria

Types of Plastids-

Chromoplast

Chromo means color and as the name indicates it contains the various colored pigment except for the green color i.e. chlorophyll (present only in the chloroplast).

Chromoplast contains the fat-soluble pigments called Cartenoids like Carotene, Xanthophyll, and many others.

These pigments give yellow, orange, red, and many other colors to the cell.

They are mainly found in the pericarp and the petals.

Functions-

Due to the colored nature of the chromoplast, insects and animals are attracted to the flower and fruit and further helps in the pollination and the seed dispersal.

Leucoplast-

Leuco means the white or colorless, so leucoplast is the colorless plastid as the pigments are absent in them.

Function-

Their basic function is the storage of the food.

Based on the type of food they store they can be divided into 3 types-

Amyloplast-

Stores Carbohydrates

Aleuroplast-

Stores Proteins.

Elaioplast-

Store Fats

Leucoplasts are generally present in non-green and underground plant cells, basically in the storage organs of plants (also called ‘Organ of perennation’).

Chloroplast

Chloro means green.

Chloroplast
ChloroplastImg source

Chloroplasts are the most important of all plastid as they make up the food for the entire plant, so also called the Kitchen of the cell.

The majority of the chloroplast is found in the mesophyll cells of the leaves.

Their shapes vary in different organisms they are oval, spherical, discoid, or ribbon-shaped.

Chloroplast contains two major components that are absent in other plastids-

Granum-

It is a stack of the thylakoid interconnected with the help of stroma lamellae. 

Light reaction of photosynthesis occurs in the granum.

Stroma-

It is the liquid part of the chloroplast where the many enzymes for photosynthesis are present.

So, the enzymatic part of the photosynthesis; i.e. the dark reaction occurs here.

Function-

No need to tell that the major function of the chloroplast is the photosynthesis.

Produces Oxygen for the animals to breathe.

Biogenesis of Plastids

Biogenesis gives information on how the structure or organism is formed.

  1. From Pro-plastids.
  2. By the binary fission in the pre-existing plastids.

Interconversion of the Plastids

Plastids can be interconverted into each other based on the demand/necessity.

One may think, how is this possible?

 It is possible as all the plastid contains the same kind of DNA with little moderation and they all have been formed from the same cell Proplastids.

Chromoplast never converts into the chloroplast.

Chloroplast converts into the chromoplast in ripening fruits. 

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