Mitochondria

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Mitochondria:

Mitochondria is an organelle which is commonly found in large numbers in most cells, in which the biochemical processes of respiration and energy production occur. It has a double membrane, the inner layer being folded inward to form layers.

Characteristics:

The term mitochondrion is coined by C. Benda. These are energy converting organelles, which are present in virtually all eukaryotic cells.

Mitochondria Structures
 

Mitochondria characteristics:

  • They are site of aerobic respiration.
  • They produce cellular energy in the form of ATP, hence they are called power houses of the cell.
  • These are membrane bound, mobile as well as plastic organelle.
  • Each mitochondria is a double membrane; bound structure with outer and inner membranes.
Structure of mitochondrion

Structure of Mitochondria:

The mitochondria are covered with two membranes:

Outer membrane:

The outer membrane is fairly smooth. It has special proteins embedded into the membrane called porins which are intrinsic proteins responsible for the transport of molecules across the membrane. The outer membrane protects the organelle, and porins allow free passage for various molecules into the inner-mitochondrial space of the mitochondria.
Mitochondrial porins are voltage dependent anion-selective channels that allow the passage of small molecules across the mitochondrial outer membrane.

Inner membrane:

Inner membrane is highly convoluted; forming folds called cristae. The inner membrane is also very impermeable to many solutes due to the high content of a phospholipid called cardiolipin. The cristae greatly increase the inner membrane’s surface area.

The two faces of this membrane are referred to as the matrix side and the cytosolic side. Inner membrane contains enzymes complex called ATP synthase (also called F0-F1 complex or oxysomes) that make ATP.

Inner membrane

Mitochondrial matrix:

The matrix contains several identical copies of the dsDNA, mitochondrial ribosomes (ranging from 55S-75S),tRNA and various proteins.

Mitochondrial DNA:

Mitochondrial dsDNA is mostly circular. The size of mitochondrial DNA also varies greatly among different species.

Organisms                                                                    Size (kb)
Human                                                                                 16.6
Xenopus (frog)                                                                    18.4
Drosophila                                                                           18.4
Saccharomyces (yeast)                                                      75.0
Arabidopsis                                                                          367.0

Endosymbiotic theory:

Mitochondria are semi-autonomous organelle and divide by binary fission just like bacteria. These similarities with the prokaryotic characters, suggest that mitochondria evolved from bacteria. According to endosymbiotic theory (proposed by Lynn Margulls), mitochondria are supposed have evolved in eukaryotes from endosymbiotic association of purple photosynthetic bacteria about 1.5* 109 years ago.

The captured cell was then reduced to a functional organelle bound by two membranes, and was transmitted cytoplasmically to subsequent generations. In lower eukaryotes such as yeast, both parents contribute equal amount of mitochondria to the zygote. Thus, mitochondrial inheritance in yeast is therefore bi-parental.

Endosymbiotic theory

Hydrogenosome and mitosomes:

Some primitive protists such as Trichomonas vaginalis lack mitochondria. These protists contain a specialized organelle, involved in synthesis of ATP abd hydrogen, called the hydrogenosome. These are the site of fermentative oxidation of pyruvate, coupled with ATP production via substrate level phosphorylation.

Like mitochondria, hydrogenosome are surrounded by a double membrane and produce ATP. In contrast to mitochondria, hydrogenosomes produce molecules hydrogen through fermentations, lack cytochromes and usually lack DNA.

Mitosomes, organelles of mitochondrial origin, are double membrane bound organelles found in some unicellular eukaryotes, including Entamoeba histolytica and microsporidia. The mitosome has been detected only in anaerobic organisms that do not have mitochondria.

Functions of mitochondria:

  • Mitochondria has an important function is to harvest energy. The simpler compounds obtained from nutrition are carried to the mitochondria where they are further processed and converted to charged molecules. These charged molecules again chain with oxygen and produce ATP molecules. This process is referred as oxidative phosphorylation.
  • Mitochondria aid the cells to sustain proper concentration of calcium ions within the partitions of the cell.
  • The mitochondria also supports in building certain fragments of blood and hormones like testosterone and estrogen.
  • The mitochondria in liver cells have enzymes for detoxifying ammonia.
  • The mitochondria also play an important role apoptosis (also called as programmed cell death). Irregular death of cells due to the deformity in mitochondria can affect the function of organ.

Targeting of mitochondrial proteins:

Mitochondrial proteins are synthesized by cytosolic as well as matrix ribosomes. About 99% of mitochondrial proteins are encoded by nuclear genes and are synthesized as precursors on cytosolic ribosomes. Proteins synthesized by cytosolic ribosomes are translocated into mitochondria post- translationally. Proteins imported into mitochondria may be located into outer membrane, the intermembrane space, the inner membrane or the matrix.

Mitochondrial protein import requires membrane receptors and translocons. Import is initiated by the binding of a mitochondrial targeting sequence to an import receptor in the outer mitochondrial membrane. Two distinct translocation complexes that mediate translocation are situated in the outer and inner mitochondrial membranes, referred to as TOM (Translocase of the outer membrane) complex and TIM (Translocase of inner membrane) complex.

Both translocases have to cooperate tightly to promote protein translocation. The TOM complex consists of seven different subunits. The receptors Tom20, Tom22, Tom70 recognize precursor proteins and transfer them to the central component, the channel forming Tom40. Three small Tom proteins Tom5, Tom6, and Tom7, are tangled in the association and dynamics of the TOM complex.

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