Write short notes on

(a) Mitochondria (b) Plastids.

(a) Mitochondria

Mitochondria are tiny bodies of varying shapes and size, found in the cytoplasm. Each mitochondrion is covered by a double-membrane envelope. Outer membrane is smooth and porous while inner membrane is projected with finger-like structures, the cristae. These cristae create a large surface area for ATP-generating chemical reactions. The cristae as well as inner membrane possess small tent like particles called F1 particles or oxysomes.

The cavity of mitochondria is filled with matrix. The mitochondrial matrix contains lipids, proteins, circular DNA and RNA. Mitochondria is the site of cellular respiration or oxidation of food in a cell.

(b) Plastids: Plastids occur in plant cells and are absent in animal cells. They have their own genetic material and ribosomes. There are three types of plastids:

(i) Chloroplasts: These are green coloured plastids because of the presence of chlorophyll. Chloroplasts occur abundantly in green leaves and also to some extent in green parts of the plant. These are known as the kitchen of the cell.

(ii) Chromoplasts: These contain pigment other than chlorophyll. They impart colour to flowers and fruits to attract insects and other animals for pollination and fruit dispersal.

(iii) Leucoplasts: The leucoplasts are the colourless plastids of varied shapes and sizes. They store starch, proteins or lipids in the cell.