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The cell. 6. Non vesicular.

MITOCHONDRIA

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Mitochondria are organelles present in nearly all the eukaryotic cells.

Mitochondria are dynamic organelles. Their shape and number are diverse in different cell types. These organelles can grow, increase in number by fission, and two mitochondria can fuse together.

Mitochondria are composed of outer membrane, intermembrane space, inner membrane, and mitochondrial matrix. DNA, ribosomes, and all the molecules needed for protein synthesis are found in the mitochondrial matrix.

Mitochondria produce ATP thanks to a proton gradient generated in the inner membrane by oxidation-reduction reactions.

The proton gradient is used by the enzyme ATP synthase to produce ATP.

Mitochondria are almost ubiquitous organelles in the eukaryotic cells. However, they have not been found in Archezoa, probably because these organelles were lost during evolution. Mitochondria was regarded as a key element for eukaryotic cells in the late XIX century. Altmann (1890), by using fuchsine staining, observed tiny cellular structures that he named as bioblasts. Bioblasts were later identified in all eukaryotic cells. In 1914 it was already known that mitochondria (bioblasts) were able to adopt many forms, such as rods, long cords, and reticular structures. With the help of the electron microscope, it was observed that they were delimited by two membranes, inner and outer. In 1962 it was proposed that mitochondria can grow and then split by fission in two new mitochondria. Thus, the morphology and number are dynamic. Currently, mitochondria dynamics can be studied in vivo by using fluorescent molecules, and it has been uncovered the ability of mitochondria to fuse between each other.

The shape of mitochondria is variable, from branched to small ellipsoids. Two mitochondria can fuse between each other and mix their ADN molecules. If two cells are fused between each other, their mitochondria become a homogeneous population in 8 hours. Fusion and fission of mitochondria are complex mechanisms because membranes are involved and must be open and sealed correctly. The number of mitochondria in a cell may change because of this behaviour. However, it has been measured the total mitochondrial volume of some cellular types, and it has been observed that the increase in mitochondrial volume is related to the increase in the cell volume.

Mitochondria show high motility through the cytoplasm and are concentrated in those places under energy demand or calcium need (see below). The distance traveled by mitochondria may be very long. For example, in neurons, mitochondria move from the soma toward the distant dendrites and axons and come back to the soma. The movement is not continuous but saltatory. Long distance movements are ruled by microtubules, whereas local movements depend more on actin filaments. Sometimes both types of cytoskeleton filaments cooperate to either move mitochondria or keep them motionless. In the axons, the speed of the mitochondrial movement along microtubules is around 0.1 to 1.4 µm/s. However, 50 µm/s has been observed in growing axons.

Mitochondria are made up of outer membrane, inner membrane, intermembrane space and mitochondrial matrix enclosed by the inner membrane.

 Mitochondria

Mitochondria show a wide variety of forms, from long branched to short unbranched. At ultrastructural level, mitochondria have outer membrane, intermembrane space, inner membrane and mitochondrial matrix. The inner membrane has many invaginations known as mitochondrial cristae. The mitochondrial matrix contains the DNA and the enzymes that carry out many metabolic processes.

The outer mitochondrial membrane is highly permeable and contains many proteins known as aquaporins, which are aqueous channels. This membrane is like a kind of net allowing the pass of all those molecules smaller than 5000 daltons, including small proteins.

On the other hand, the inner mitochondrial membrane is impermeable to ions and small molecules. Thus, mitochondrial matrix content, which is quite different from that of the cytosol, is determined by the inner membrane. The inner membrane is highly folded with many invaginations known as mitochondrial cristae, which are morphologically diverse: discoidal, tubular and flattened. Cristae form a compartment different from the rest of the inner membrane because they have their own protein repertoire. The number and morphology of cristae are supposed to reflect the mitochondrial activity, and hence the cellular activity too.

 Mitochondria

Transmission electron microscopy images. A) Mitochondria from hepatocytes. White arrow indicates mitochondrial cristae. It can be observed that mitochondria, as well as cristae, show a highly variable morphology. B) Higher magnification image showing the continuity of the inner mitochondrial membrane with the mitochondrial cristae (white arrows). Black arrow points to the outer mitochondrial membrane. C) The black arrow indicates a long mitochondria located inside a dendrite of a neuron. Bars: A and C: 0.4 µm, and C: 50 nm.

DNA, ribosomes, and many enzymes are located in the mitochondrial matrix, where several metabolic processes take place, such as beta-oxidation. Mitochondrial DNA is located in places referred as nucleoids, Some nucleoids may contain more than one DNA molecule. Mitochondrial DNA is around 15400 pb with about 27 genes that code for 13 proteins of the respiratory chain, 2 ribosomal proteins, and tRNA for protein synthesis.

One of the most important functions of mitochondria is the synthesis of ATP, the fuel for most of the cellular processes. Another one is beta-oxidation, which is part of the fatty acids metabolism. They also work as compartments for calcium storing. Mitochondria take part in apoptosis, cancer, aging, and pathologies like Parkinson disease and diabetes. Furthermore, comparative studies of the mitochondrial DNA are very useful in anthropology for deciphering genealogies and in other phylogenetic studies because mitochondrial DNA is only inherited by maternal lineage and recombination does not occur in mitochondrial DNA during sexual reproduction.

ATP synthesis

Most of the ATP in eukaryotic cells (non photosynthetic eukaryotes) is synthesized in mitochondria. The citric acid metabolic cycle uses Acetyl-CoA to produce CO2 and NADH. Electrons from NADH pass to the electron transport chain of proteins located in the inner mitochondrial membrane. Electrons are finally transferred to oxygen to give water. The transport process of electrons through these chain of proteins is able to move protons across the inner mitochondrial membrane, from mitochondrial matrix to the intermembrane space. A gradient of protons is created, which is used by the ATP synthase to produce ATP. ATP synthase is a transmembrane protein located in the inner mitochondrial membrane. This metabolic process is referred as oxidative phosphorylation because it chemically joins inorganic phosphorus and ADP into ATP. In anaerobic bacteria, which do not have mitochondria, this process takes place in the cell membrane.

 Synthesis of ATP

The production of energy in mitochondria is a two steps process: first, a proton gradient across the inner mitochondrial membrane is produced by the electron transport chain of proteins, and second, ATP synthase uses the proton gradient to synthesize ATP. Both steps take place in the inner membrane, more precisely in the mitochondrial cristae.

Electron transport chain proteins and ATP synthases are located in the cristae, the invaginations of the inner mitochondrial membrane. These foldings are a way to increase the amount of membrane because they can include more proteins and increase the ATP production. In hepatic cells, the inner mitochondrial membrane may be up to 1/3 of the total cellular membranes. There are so many copies of these proteins that they are around 80% of the total weight of the inner mitochondrial membrane.

Electron transport chain. The set of proteins that carries out the transport of electrons from NADH to water is known as electron transport chain, but also respiratory chain. It contains around 40 proteins, 15 of them are for electron transport. All these proteins are mainly grouped in 3 complexes: NADH dehydrogenase complex, b-c1 cytochrome complex, and cytochrome oxidase complex. All of them have mechanisms to extrude protons from the cytosol to the intermembrane space through the inner mitochondrial membrane, process coupled to the transport of electrons.

The journey of electrons begins when a hydride ion is donated by NADH. The ion is split into two electrons and one proton. This process happens in the NADH dehydrogenase complex that also gets the two electrons. By means of an ubiquinone, the two electrons are transferred to the b-c1 cytochrome oxidase complex. The delivering of electrons by the two complexes is coupled to the extrusion across the inner mitochondrial membrane of two protons, one per complex, from the mitochondrial matrix to the intermembrane space. From the b-c1 cytochrome complex, the two electrons are taken by cytochrome C and transferred to the cytochrome C complex. In this third complex, another proton is moved through the inner mitochondrial membrane when the electrons are finally captured by oxygen molecules.

The electron transfer is like in batteries, where electrons move from one side with lower affinity to other with higher affinity. This movement releases energy that is used to transport protons against concentration gradient. Weakly attached electrons jump from NADH to the NADH complex, and from this complex to the next because electrons are attracted more strongly. Without the protein complexes of the respiratory chain, the electrons are lost and the energy is released as heat energy. Protein complexes are able to create a concentration of protons ten times higher in the intermembrane space than in the mitochondrial matrix. In this way, protons are pushed by gradient concentration to enter the mitochondrial matrix, and this movement is used by ATPase to synthesize ATP.

ATP sinthase. ATP synthase has a passage that allows protons to cross the inner mitochondrial membrane to balance the concentration gradient created by the electron transport chain. This flux of protons toward the mitochondrial matrix is coupled to the synthesis of ATP. ATP synthase is a well-preserved enzyme during evolution so that it is present in bacteria, chloroplasts and in mitochondria. It is a large protein made up of many subunits. The molecular mechanism for ATP synthesis is not well-known but every synthesized ATP molecule needs 3 protons passing through the ATP synthase. It has the ability to synthesize more than 100 ATPs per second. It is interesting that this enzyme is able to carry out the opposite process, i.e. uses ATP to pump protons from the matrix to the intermembrane space.

ATP synthesis is not the only purpose of the proton gradient. Other electrically charged molecules such as pyruvate, ADP, and inorganic phosphorus must be pumped into the mitochondrial matrix, whereas ATP, which is synthesized in the inner membrane toward the mitochondrial matrix, must be transferred to the cytosol. Pyruvate and inorganic phosphorus enter coupled to the inward flux of protons. However, ADP enters coupled to the ATP exit, by antiport transport.

Lipid metabolism

An important amount of lipid synthesis happens in mitochondria. They synthesize lysophosphatidic acid, from which triglycerides are synthesized. Phosphatidic acid and phosphatidylglycerol are also synthesized in mitochondria. Phosphatidylglycerol is needed for the synthesis of cardiolipin and phosphatidylethanolamine.

Protein import

Mitochondria have a very low number of genes compared to the large number of different types of proteins they contain. A yeast mitochondria contains around 1000 different proteins, whereas in humans is around 1500. Only a few proteins are coded by mitochondrial DNA, so that most of mitochondrial proteins must be imported from the cytosol. It is a complex process since there are several compartments in the mitochondria where proteins can be targeted: external membrane, inner membrane and mitochondrial matrix. The imported proteins contain amino acid sequences that are like addressing posts, that inform to mitochondrial transporters where these proteins must be targeted.

Bibliography

Kiefel BR, Gilson PR, Beech PL. 2006. Cell biology of mitochondrial dynamics. International review of cytology. 254: 151-213.

MacAskill AF, Kittler JT. 2010. Control of mitochondrial transport and localization in neurons. Trends in cell biology. 20: 102-112


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Updated: 2016-08-31. 13:17