12. The Semi-Autonomic Organelle: Plastid

Ernst Haeckel coined the term plastid in the year \(1866\). Plastids are only found in plant cells. They are absent in an animal cell. It has various shapes. They may be plate-like (Ulothrix), cup-shaped (Chlamydomonas), ribbon-like (Spirogyra).
 
The internal organization of plastids contains several membrane layers embedded in a material called the stroma or matrix. It is a semi-fluid colloidal complex containing DNA, ribosomes and enzymes.

 

Grana are stacks of membrane-bound, flattened sacs containing the molecules of chlorophyll. One thylakoid stack is known as granum. Each thylakoid has chlorophyll on its surface that trap sunlight and take part in the process of photosynthesis. The external structure of plastids is similar to mitochondria. They are double-layered; being semi-autonomous organelle (like mitochondria), they have DNA and ribosomes.

 

Plastid is found in the cell of plants, algae and some other eukaryotic organisms. It is the site of synthesis and storage of essential chemical compounds which are used by the cells of autotrophic organisms (an autotroph is an organism that can prepare its food by using sunlight, water, carbon dioxide, or other chemicals. They are also called producers.
 
Plastids contain a pigment called chlorophyll, which are used in the process of photosynthesis, and the types of pigments determine the cell's colour.
 
The types of plastids are,
1) Chloroplasts: These are the green plastids that contain chlorophyll pigment for photosynthesis.
2) Chromoplasts: These are the coloured plastids for pigment synthesis and storage. The pigment present in them is carotenoids which gives a various colour to fruits and flowers.
3) Leucoplasts: These are colourless plastids. They are found in storage cells of roots, seeds and underground stems. Further, leucoplasts are classified into three types based on their storage products.
 
Aleuroplasts or Protoplasts: A leucoplast type of plastid that is involved in the storage of proteins.

 

Amyloplasts: It is a colourless plant plastid that forms and stores starch.

 

Elaioplasts: These are colourless plastids found in plant cells used for the synthesis and storage of fatty acids, terpenes, and other lipids.