Plastids are the cytoplasmic organelles, which are found in most of the plant cells and photosynthetic protists.

Plastids are of three types namely leucoplasts, chromoplasts and chloroplasts.

Leucoplasts are colourless due to the absence of pigments.

On the basis of the material stored, leucoplasts are classified into three types namely amyloplasts, elaioplasts and proteinoplasts.

Chromoplast is a coloured plastid due to the presence of carotenoids and other pigments, which are yellow, orange or red in colour. Chromoplasts give bright colour to fruits, which thereby attracts animals for their dispersal.

Chromoplasts are further classified into phaeoplasts and rhodoplasts.

Phaeoplasts contain fucoxanthin which absorbs light.

Rhodoplasts contain phycoerythrin pigment which absorbs light

Chloroplasts are present in green algae and higher plants and contain chlorophyll a and chlorophyll b. Chloroplasts contain their own genetic material (DNA and RNA).

Each chloroplast is surrounded by a double layered membrane, i.e. the inner membrane and the outer membrane. Inside the membrane it contains stroma and grana. Stroma is denser, colourless and granular substance while grana are made up of flattened membranous sacks called thylakoids which are arranged like stacks of coins. The inner surface of the thylakoid is granular due to the presence of the quantasomes.

The evolution of chloroplast can be explained by the theory of endosymbiosis.

The chloroplast is composed of carbohydrates, lipids, proteins, chlorophylls, carotenoids (carotene and xanthophylls), DNA, RNA, certain enzymes and coenzymes. Some of the major functions of chloroplast are photosynthesis, oxygen supply, starch synthesis, carbon dioxide balance and chromoplast conversion.

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