Mitochondrion

Mitochondrion
Structure
Property
Outer Membrane
Inner Membrane
Matrix
Inheritance

Mitochondrion is the singular form of mitochondria. Although mitochondria convert energy derived from chemical fuels wheras chloroplasts convert energy derived from sunlight, the two types of oranelles are organized similarly; moreover, both produce large amounts of ATP by the same mechanism. This striking conclusion emerged from painstaking studies carried out over the past 30 years.

Structure

Mitochondria occupy a substantial portion of the cytoplasmic volume of eucaryotic cells, and they have been essential for the evolution of complex animals. They are usually depicted as stiff, elongated cylinders with a diameter of 0.5 to 1 micrometer, resembling bacteria.

Mitochondria have membrane-bounded organelle, about the size of a bacterium, that carries out oxidative phosphorylation and produces most of the ATP in eucaryotic cells. They have outer and inner membrane, and matrix space inside inner membrane; The matrix space contains a concentrated solution of many different enzymes.

Property

Mitochondrion changes their shape rapidly and even fuse with one another and then separate again. As they move about in the cytoplasm, they often appear to be associated with microtubules, which may determine the unique orientation and distribution of mitochondria in different type of cells. Thus the mitochondria in some cells form long moving filaments or chains, while in others they remain fixed in one position where they provide ATP directly to a site of unusually high ATP consumption.

Outer Membrane

Because it contains a large channel-forming protein (called porin), the outer membrane is permeable to all molecules of 5000 daltons or less. Other proteins in this membrane include enzymes involved in mitochondrial lipid synthesis and enzymes that convert lipid substrates into forms that are subsquently metabolized in the matrix.

Inner Membrane

This space contains several enzymes that use the ATP passing out of the matrix to phosphorylate other nucleotides.

Matrix

The matrix contains a highly concentrated mixture of hundreds of enzymes, including those required for the oxidation of pyruvate and fatty acids and for the citric acid cycle. It also contains several identical copies of the mtDNA genome, special mitochondrial ribosomes, tRNAs, and various enzymes required for expression of the mitochondrial genes.

Inheritance

Many organelle DNA molecules are about the same size as typical viral DNAs. In mammals, for example, the mitochondrial genome is a DNA circle of about 16,500 base pairs (less than 1/100,000 times the size of the nuclear genome). All mitochondria and chloroplasts contain multiple copies of the mtDNA. The molecules are usually distributed in several clusters in the matrix of the mitochondrion, where they are thought to be attached to the inner membrane. Although it is not known how the DNA is pckaged, the genome structure is likely to resemble that in bacteria rather than eucaryotic chromatin. As in bacteria, for example, there are no hitones.

Despite the small number of proteins encoded in their genomes, mitochondria carry out their own DNA replication, DNA transcription, and protein synthesis. These processes take place in the matrix space. Although the proteins that mediate these genetic processes are unique to the organelle, most of them are encoded in the nuclear genome.