Mitochondria are found in all eukaryotic cells. They may be lost in the later stages of the development (red blood cells and phloem sieve tubes). These are granular or filamentous organelles having characteristic morphological, biochemical and functional properties.
Mitochondria were first observed at the end of the 19th century by Altmann (1894). He described them as bioplasts. Benda (1897) called these structures “mitochondria”. Altmann predicted the relationship between mitochondria and cellular respiration.
Hogeboom and coworkers ultimately demonstrated this in 1948. In recent years, a lot has been known about its ultrastructural organisation with the aid of the electron microscope. For many years, it was believed that the mitochondrion was completely closed and autonomous. However, it is now suggested to be dynamic, interacting extensively with other cellular structures.
The shape of mitochondria is variable. Generally, these organelles are filamentous or granular but in certain cases, the appearance is club shaped or hollowed out to take the form of a tennis racket. At other times, the mitochondria may appear vesicular.
The size of mitochondria is also variable. In most cells, however, the width is constant (0.05 pm) and the length is variable reaching a maximum of 7.0 pm.
A mitochondrion, as seen under a high resolution microscope consists of two membranes and two compartments. An outer, 6.0 nm thick limiting membrane, surrounds it.
The inner membrane is separated from the outer by a space of about 6.0 to 8.0 nm. These projects into the mitochondria cavity in the form of folds called cristae (crests).
The inner membrane divides the mitochondrion into two compartments. The outer chamber occupying the space between two membranes and the inner chamber filled with relatively dense proteinaceous material usually called mitochondria matrix. This is generally homogeneous but in some cases contains highly dense granules. These granules contain phospholipids.
The mitochondrial cristae show incomplete separations. This makes the inner chamber, the matrix continuous. The membrane of the crista is seen to have particles, each with a head, a stalk and a base. These are called elementary particles / Ft particles / oxysomes. Each particle is attached to the inner membrane with its base. The particles are regularly spaced at intervals of 10 nm. Each particle has a spherical head called F subunit, nearly 10 nm in diameter
and connected to the base by F0 subunit which is 3.5 to 5 nm in length. These particles correspo to a special ATPase involved in the coupling of oxidation and phosphorylation
The outer and inner membranes and crests may be considered as fluid and com molecular structures. The matrix is gel like and contains a variety of soluble proteins. Coe other smaller molecules are also obtained from the matrix. Several ribosomes and a circ DNA are situated in it. Besides, it contains enzymes for citric acid cycle, lipid and amino metabolism, protein synthesis and respiratory chain.
Mitochondria are the main energy transforming organelles of the cells. The breakd of the substrates of respiration to pyruvic acid takes place in the cytoplasm. But the m energy transforming process as such as Krebs cycle and electron transport system take pi
inside the mitochondria. Since the mitochondria are responsible for supplying energy, these are collectively called the power house of the cell.