Mitochondria

Mitochondria, also known as the powerhouse of cell, is a double membrane bound organelle present in all eukaryotic cells. The word is derived from the Greek word, ‘mitos’ means thread and ‘chondrion’ means granule or grain like. Generation of ATP (adenosine triphosphate) by electron transport and oxidative phosphorylation and the activity of tricarboxylic acid (TCA) cycle takes place here. Major ATP requirement of a cell is fulfilled by mitochondria and hence it is known as powerhouse of cell. Mitochondria also play a central role in calcium signaling and apoptosis i.e. programmed cell death.

Structurally, mitochondria is cylindrical in shape and about 0.75 to 3.0 μm in diameter. They vary considerably in number; a cell contain as many as 1000 mitochondria or a single mitochondria (in some yeasts, trypanosome protozoa etc.). The ribosome of mitochondria is smaller in size than that of cytoplasmic ribosomes. The mitochondrial ribosomes resembles that of bacterial one in size and composition. Also, mitochondrial DNA is like that of bacterial DNA i.e. closed circle and they reproduce by binary fission. They synthesize their own proteins with the help of their DNA and ribosome. Since mitochondria has its own independent genome and as it resembles the bacterial genome to some extent, it is generally believed that they arose from symbiotic associations between bacteria and larger cells. 

The two membranes of mitochondria are outer mitochondrial membrane and inner mitochondrial membrane. The two membranes are mainly composed of phospholipid bilayer and proteins. The space between these two membrane is known as intermembrane space and the space within the inner membrane is called the matrix, containing mainly ribosomes, DNA and calcium phosphate granules. The special infoldings of the inner membrane which increases its surface area is known as cristae (singular crista). Cristae varies in shape from species to species. For instance: in fungi, they are plate like, amoebae possesses vesicles shaped cristae, tubular shaped cristae are found in many eukaryotes and so on. Mitochondria that has stripped of its outer membrane is known as mitoplasts. Mitoplasts have intact inner membrane.

Although mitochondria is the site for Krebs cycle, electron transport and oxidative phosphorylation, they occurs in the different mitochondrial compartments. Electron transport and oxidative phosphorylation takes place in inner membrane as the enzymes and electron carriers involved in these processes are present in the inner membrane. TCA cycle and oxidation pathway for fatty acids takes place in matrix as it contains their associated enzymes. F₁ particles are attached as small spheres on the inner surface of inner membrane and synthesizes ATP during cellular respiration.