Krebs cycle, fatty acid cycle, electron transport system etc. take place in mitochondria (sing: mitochondrion). All energy producing process occurs in mitochondria for which mitochondria are compared to the powerhouse of the cell. Number of mitochondria may vary according to the species. Normally each cell contains average 300-400 mitochondria. Their shape may be globular, rod, thread, star-shaped or ring-shaped.
The outside of mitochondria is surrounded by a bi-layered membrane. The membrane is made by lipoprotein, which is enriched with lipid and protein. The outer membrane is smooth but the inner one has a series of enfolding into the inner cavity of mitochondria. These enfolding are called cristae. Small stalked granular bodies are arranged on the cristae and they are called auxisome.