Plastids and three different Types with Structure and Structural Components
The membranous bound and pigment-containing bodies present in the cell are called plastids. Plastids are present only in plant cells. There are three types of plastids:
The membrane-bound structure, containing green pigment is called a chloroplast. The chloroplasts are present only in photosynthetic plants. 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 that transports oxygen in animals. There is the main difference between two molecules. The central atom of chlorophyll is Mg++, while the central atom of haem is Fe++.
Structure of chloroplast
The chloroplast has different shapes and structures. Its diameter is about 4 – 6 pm. 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 the envelope, stroma, thylakoid, and granum.
It is the outer covering of the chloroplast. It formed by a double membrane.
It covers most of the volume of the chloroplast. It is a fluid that surrounds the thylakoids. It contains proteins, some ribosomes, and a small circular DNA. The CO2 is fixed and sugars are synthesized in the stroma. Some proteins are also synthesized in this part.
These are flattened vesicles. They arrange themselves to form grana and intergrade.
A granum is a pile of thylakoids, stacked on each other like coins. On the 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.
The grana are interconnected with each other by the non-green part, called intergranum.
Chromoplats give color to plants other than green. They are present in the petals of flowers, and in the ripened fruits. They help in pollination and dispersal of seeds.
They are colorless. Their shapes may be triangular tubular or some other. They are found in the underground parts of the plant and store food.