What are Plastids? -Definition and Types | Structure of Chloroplast

The membranous bound and pigment-containing bodies present in the cell are called plastids.

Plastids are present only in the plant cell. They are present in the cytoplasm. Chloroplasts and other plastids of plant cells contain their genome’s double-stranded DNA. 

That is involved in the genetic anchoring of at least some of these processes, such as photo-autotrophy. DNA present in chloroplast and mitochondria differ from the DNA (heredity material) found in the nucleus.

Discovery of Plastids

The answer to this question is explained by a scientific theory called endosymbiosis, wherein a unicellular heterotrophic protist engulfed a free-living photosynthetic cyanobacterium and retained it, instead of digesting it in the food vacuole.

The first definitive description of a chloroplast (“grain of chlorophyll“) was given by Hugo von Mohl in 1837. He stated them as discrete bodies within the green plant cell. In 1884, Eduard Strasburger adopted the term “chloroplasts”.

Types of plastids

There are three types of plastids.

• Chloroplastid

• Chromoplast

• Leukoplast

Chloroplast

In photosynthetic plant cells, membrane-bound structures containing green pigment are called chloroplast. Chloroplasts are self-replicating organelles.

The green pigment is an organic compound called chlorophyll.

It helps the plant to absorb light energy and manufacture food: chlorophyll molecule resembles the haem group of hemoglobin. Hemoglobin is a protein used in the transport of oxygen in animals.

There is a main difference between the two molecules. The central atom of chlorophyll is Mg”. But the central atom of haem is Fe”.

Structure of chloroplast

The chloroplasts vary in shape and size. Its diameter is about 4 – 6nm. It appears heterogeneous structure under a light microscope. It has small granule-like structures called grana. These grana are embedded in the matrix.

The chloroplast shows three main components under an electron microscope. These are

• Envelope

• Stroma

• Thylakoid

• Granum

• Intergranum

The envelope:

It is the outer covering of the chloroplast. It formed by a double membrane.

Stroma:

Stroma is a fluid that surrounds the Thylakoids. It covers most of the volume of the chloroplast. It contains proteins, some ribosomes, and a small circular DNA. Here CO₂ is fixed (form glucose) to manufacture sugars. Protein synthesis occurs in this part.

Thylakoids are flattened vesicles. Thylakoids arrange themselves to form granum and intergranular.

Granum:

A granum is a pile of thylakoids, stacked on each other like coins. On average, 50 or more thylakoids are piled to form one granum. The chlorophyll molecules are arranged on the layers of thylakoids. So granum appears green. The light energy is trapped by the membranes of grana and ATP is synthesized.

 Intergranum:

 Each granum is interconnected with others by a non-green part called intergranum.

Chromoplasts

Chromoplasts give color to plants other than green. They are present in the petals of plant flowers and ripened fruits. They help in the pollination and dispersal of seeds.

Leucoplasts

They are colorless. Their shapes may be triangular, tubular, or some other. They are found in underground parts like the root of plants and store food.