Plastids are double-membrane bounded cell organelle which is mainly found in the plant cell.
On the basis of color, there are three types of plastids they are;
Leucoplasts are colorless or white plastid without any pigment. They occur in the underground parts of the plant like root, rhizome. They help in the storage of food like starch, protein, fat etc.
They have colored plastid other than green. They usually occur in red, yellow and orange color. They contain carotenoids pigment. They occur in flowers and fruits and helps in pollination and dispersal of fruits.
These are green colored plastid which helps in photosynthesis. They contain pigments like chlorophyll and carotenoid.
Shape, size, and number of Chloroplasts
The shape of a chloroplast is variable. In higher plants, it occurs in the disc-shaped or lens-shaped form. But in lower plants, there is great variation in shape. It occurs in cup-shaped, ribbon-shaped, star-shaped or in reticulate shape.
The number is also variable. In the leaf cells of higher plants about 20-40 chloroplasts occur in one cell and in lower plants like Spirogyra its number range from 1-16.
Its size may range from 4-10µm.
Structure of chloroplast
fig:Structure of chloroplast
The structure of chloroplast consists of following important components;
The chloroplast is bounded by an envelope consisting of two membranes outer and inner. It protects the chloroplast inside.
Stroma or Matrix is a semi-fluid ground substance in which thylakoids or granum are subtended. It contains DNA, ribosomes 70s type, RNA, stored starch, lipid and enzymes of dark reaction of photosynthesis(Rubisco).
Thylakoid is the structural unit of the chloroplast. They are a membranous sac-like structure which often remain group together in a bundle called granum (plural grana). There are many grana inside chloroplast and they remain interconnected through intergranal thylakoid. The thylakoids contain photosynthetic pigment, chlorophylls, carotenoids and help in the light reaction of photosynthesis.
Semi-autonomous nature of Chloroplasts
Due to the presence of DNA, ribosomes and the capacity of energy generation they can synthesis some of the required protein but for other protein, they depend on upon nucleus and ribosomes of the cell organelles. Hence, they are called a semi-autonomous nature of chloroplast.
Functions of chloroplast