Mitochondria how many

Typically, a sperm carries mitochondria in its tail as an energy source for its long journey to the egg. When the sperm attaches to the egg during fertilization, the tail falls off. Consequently, the only mitochondria the new organism usually gets are from the egg its mother provided. Therefore, unlike nuclear DNA, mitochondrial DNA doesn't get shuffled every generation, so it is presumed to change at a slower rate, which is useful for the study of human evolution.

Mitochondrial DNA is also used in forensic science as a tool for identifying corpses or body parts, and has been implicated in a number of genetic diseases, such as Alzheimer's disease and diabetes. License Info.

Image Use. Custom Photos. Site Info. Contact Us. The Galleries:. Photo Gallery. Silicon Zoo. Chip Shots. DNA Gallery. Amino Acids. Religion Collection. Cocktail Collection. Screen Savers. Win Wallpaper. Mac Wallpaper. These can only get across with the aid of specific membrane transport proteins, each of which is selective for a particular ion or molecule.

As a result of its ion selectivity, an electrochemical membrane potential of about mV builds up across the inner mitochondrial membrane. The inner membrane is where oxidative phosphorylation takes place in a suite of membrane protein complexes that create the electrochemical gradient across the inner membrane, or use it for ATP synthesis.

Membrane compartments in the mitochondrion. The outer membrane separates mitochondria from the cytoplasm. It surrounds the inner membrane, which separates the inter-membrane space from the protein-dense central matrix. The inner membrane is differentiated into the inner boundary membrane and the cristae. The two regions are continuous at the crista junctions. The cristae extend more or less deeply into the matrix and are the main sites of mitochondrial energy conversion.

The shallow proton gradient between the inter-membrane space pH 7. Adapted from Figure 14—8 C in Alberts B. Molecular Biology of the Cell. The inner and outer membranes of mitochondria define three compartments within the organelle, each with its distinct role and corresponding protein components.

The innermost compartment, surrounded by the inner membrane, is the mitochondrial matrix. It is the equivalent of the bacterial cytoplasm, from which it is distinguished by a pH of 7. The high pH of the mitochondrial matrix creates the trans-membrane electrochemical gradient that drives ATP synthesis see below.

The mitochondrial matrix is the site of organellar DNA replication, transcription, protein biosynthesis and numerous enzymatic reactions. Mitochondrial DNA is compacted by the mitochondrial transcription factor TFAM into supramolecular assemblies called nucleoids, of which there are about per cell [ 21 ].

Mitochondrial ribosomes are membrane-attached, as their only products in human cells are hydrophobic membrane protein subunits, which integrate directly into the inner membrane upon translation. The biosynthetic reactions that happen in the matrix include those of the citric acid cycle. For cryo-ET of intact organelles, the high matrix density has the disadvantage of obscuring internal detail.

The equivalent of the periplasm in the bacterial ancestors of mitochondria is the intermembrane space. All matrix proteins imported into the mitochondrion from the cytoplasm must pass through the outer and inner membrane and therefore also through the intermembrane space. Conventional EM of thin plastic sections suggested sites of direct contact between the lipid bilayers of the inner and outer membrane [ 24 , 25 ], but these seem to be artifacts of fixation and dehydration. The inner boundary membrane must contain large numbers of the carrier proteins that shuttle ions, ATP, ADP and small metabolites between the cytoplasm and the matrix.

The inner membrane forms invaginations, called cristae, that extend deeply into the matrix. The cristae define the third mitochondrial compartment, the crista lumen. The crista membranes contain most, if not all, of the fully assembled complexes of the electron transport chain and the ATP synthase Fig. The crista lumen contains large amounts of the small soluble electron carrier protein cytochrome c. The mitochondrial cristae are thus the main site of biological energy conversion in all non-photosynthetic eukaryotes.

Membrane protein complexes of the respiratory chain. UQ ubiquinol. Cristae were first discovered by electron microscopy in thin sections of plastic-embedded cells and tissues [ 28 , 29 ]. They are disk-like lamellar, tubular or bag-like extensions of the inner boundary membrane, and are continuous with it at the crista junctions. In cells that divide frequently, such as yeasts, the crista junctions tend to form 25 nm slits in the boundary membrane that are up to a few nm long [ 26 , 30 ].

In mitochondria of all organisms, the mitochondrial contact site and cristae organizing MICOS system [ 31 ], an assembly of one soluble and five membrane proteins, anchors the cristae to the outer membrane.

Cristae of yeast strains where particular MICOS components have been knocked out look like concentric onion rings and have few if any junctions [ 32 ]. It is thought that the MICOS complex forms a diffusion barrier within the inner membrane at the crista junctions to account for the apparent lateral segregation of proteins between the cristae and boundary membranes. Evidence of such differences comes primarily from electron microscopy, because it has not been possible to separate cristae and boundary membranes biochemically.

Immuno-staining of thin plastic sections has shown that respiratory chain complexes reside in the cristae rather than in the boundary membrane, whereas components of the protein translocases are found in the boundary and outer membranes [ 33 ].

Cryo-ET of intact mitochondria, which resolves large membrane protein complexes in situ, did not reveal any such assemblies for example, the ATP synthase or complex I in the boundary membrane [ 17 ], suggesting that the protein complexes of the inner membrane are indeed laterally segregated.

In tissues with a high energy demand, such as skeletal or heart muscle, the cristae are closely stacked disk-like lamellae that take up most of the mitochondrial volume Fig. In animal tissues with lower energy demand, such as liver or kidney, the cristae are less closely stacked, leaving more room for the matrix with its biosynthetic enzymes.

In all mitochondria, the cristae account for most of the inner membrane surface, highlighting their importance for cellular physiology. Tomographic volume of mouse heart mitochondrion. The outer membrane grey envelops the inner membrane light blue. The inner membrane is highly folded into lamellar cristae, which criss-cross the matrix. The dense matrix, which contains most of the mitochondrial protein, appears dark in the electron microscope, whereas the intermembrane space and crista lumen appear light because of their lower protein content.

The inner membrane turns sharply at the crista junctions, where the cristae join the inner boundary membrane. Courtesy of Tobias Brandt. The ATP synthase is an ancient nanomachine that uses the electrochemical proton gradient across the inner mitochondrial membrane to produce ATP by rotatory catalysis [ 34 ].

Protons moving through the F o complex in the membrane drive a rotor ring composed of 8 in mammals [ 35 ] or 10 in yeast [ 36 ] c -subunits. The central stalk propagates the c -ring torque to the catalytic F 1 head, where ATP is generated from ADP and phosphate through a sequence of conformational changes. The peripheral stalk prevents unproductive rotation of the F 1 head against the F o complex. For many years it was assumed that the ATP synthase and other energy-converting complexes are randomly distributed over the inner membrane.

The first hint that this is not the case came from deep-etch freeze-fracture electron microscopy, which revealed double rows of macromolecular complexes in the tubular cristae of the single-cell ciliate Paramecium [ 37 ].

The double rows were thought to be linear arrays of mitochondrial ATP synthase. This is indeed what they are, but it could only be shown unambiguously more than 20 years later by cryo-ET [ 30 , 38 ], which revealed rows of ATP synthase dimers in mitochondria of all species investigated [ 30 ] Fig. Until then, the rows were thought to be peculiar to Paramecium. Double rows of ATP synthase in seven different species. Insets show side views of the dimers in the membrane. Yellow arrowheads indicate F 1 heads of one dimer.

Bottom row : Surface representations of subtomogram averages. Adapted from [ 17 ]. The linear arrays of ATP synthase dimers are a ubiquitous and fundamental attribute of all mitochondria. They are always found along the most tightly curved regions along the crista ridges Additional file 2 , or around narrow tubular cristae.

Subtomogram averages indicate that dimers from fungi and mammals are indistinguishable at low resolution, whereas those from plants, algae and protists differ in dimer angle or position of the peripheral stalk relative to the catalytic F 1 head Fig.

However, the basic assembly of ATP synthase complexes into dimers and their association into long rows along the crista ridges are conserved. Coarse-grained molecular dynamics simulations indicate that the dimers bend the lipid bilayer and, as a result, self-associate into rows [ 30 ]. Accordingly, row formation does not require specific protein interactions, but is driven by the energy of elastic membrane deformation.

Therefore, it is in fact the dimers that bend the membrane, not the other way round. The long helices appear to play a central role in this process, as they are preserved in all rotary ATPases [ 40 ]. Together with the fitted high-resolution X-ray structures of the catalytic F 1 head and the rotor ring in the membrane, the cryo-EM map provides the first complete picture of this pivotal mitochondrial membrane complex Fig.

Structure of the mitochondrial ATP synthase dimer from Polytomella sp. Side view of the two mitochondrial ATP synthase in the V-shaped dimer. Adapted from [ 40 ]. The ubiquitous nature of the dimer rows raises the question as to the biological significance of this striking, conserved arrangement.

In yeast the two protomers are linked by the dimer-specific ATP synthase subunits e and g. If either subunit is knocked out, only monomeric ATP synthase is found in the inner membrane [ 30 ] and the usual lamellar or tubular cristae do not form [ 30 , 41 ]. ATP synthase dimers and dimer rows are thus a prerequisite for proper cristae formation. Although the loss of the dimer-specific subunits is not lethal, it is a serious disadvantage.

This raises the further question about the role of the cristae, and hence the dimer rows, in cellular physiology and fitness. Most likely the invaginations prevent protons that are pumped into the crista lumen by the respiratory chain from escaping rapidly to the inter-membrane space and the cytoplasm, so that they can be harnessed more efficiently for ATP production.

In this way, the cristae, and hence the dimer rows, would contribute to effective ATP synthesis. Prokaryotic ATP synthases lack the dimer-specific subunits, and ATP synthase dimers or dimer rows have not been found in bacterial or archaeal inner membranes, which also do not have cristae.

The cristae and dimer rows may thus be an adaptation that enables mitochondria to satisfy the high energy demand of eukaryotic cells with the available, shallow proton gradient of around 0. ATP synthase dimers have recently been implicated in the formation of the permeability transition pore [ 43 ] that triggers apoptosis.

On the basis of the structure of the mitochondrial ATP synthase dimer [ 39 ] or the dimer rows [ 30 ], however, it is difficult to see how they might form a membrane pore. Complex I feeds electrons from the soluble carrier molecule NADH into the respiratory chain and transfers them to a quinol in the membrane. The energy released in the electron transfer reaction is utilized for pumping four protons from the matrix into the crista lumen. Complex III takes the electrons from the reduced quinol and transfers them to the small, soluble electron carrier protein cytochrome c , pumping one proton in the process.

Finally, complex IV transfers the electrons from cytochrome c to molecular oxygen and contributes to the proton gradient by using up four protons per consumed oxygen molecule to make water. Complex II succinate dehydrogenase transfers electrons from succinate directly to quinol and does not contribute to the proton gradient. The respiratory chain complexes have been studied in great detail for decades. The mitochondrial complex has about three times as many protein subunits as its bacterial ancestor.

Most functions of the extra subunits are unknown, but many of them are likely to work in assembly or the regulation of complex I function.

However, the way in which electron transfer from NADH to ubiquinone in complex I is coupled to proton translocation is still unknown, and much else remains to be discovered. Cryo-EM structure of bovine heart complex I. The matrix arm contains a row of eight iron-sulfur clusters red that conduct electrons from NADH to ubiquinol at the junction of the matrix and membrane arms Fig. The membrane arm consists of 78 trans-membrane helices, including three proton-pumping modules.

Adapted from [ 51 ]; EMDB code Respiratory chain supercomplexes were first postulated on the basis of blue-native gels of yeast and bovine heart mitochondria solubilized in the mild detergent digitonin [ 49 ].

Negative-stain electron microscopy [ 50 ] and single-particle cryo-EM [ 51 ] of the 1. X-ray structures of the component complexes were fitted to the 3D map Fig. Genetic evidence provides strong support for the existence of respirasomes in vivo [ 52 ], but they were long thought to be artifacts of detergent solubilization, notwithstanding their well defined structure. Saccharomyces cerevisiae , which lacks complex I, nevertheless has a respiratory chain supercomplex consisting of complex III and IV [ 53 ].

Far from being randomly distributed in the membrane, the ATP synthase and electron transport complexes of the respiratory chain thus form supramolecular assemblies in the cristae, in a way that is essentially conserved from yeast to humans Fig. A clear functional role of mitochondrial supercomplexes has not yet been established.

They may make electron transfer to and from ubiquinone in complexes I and III more efficient, as the relative positions and orientations of the two complexes are precisely aligned rather than random. However, there is no direct evidence that this makes a difference.

The supercomplexes may simply help to avoid random, unfavorable protein—protein interactions in the packed environment of the inner mitochondrial membrane [ 54 ].

Alternatively, they may control the ratio of respiratory chain complexes in the membrane, or aid their long-term stability. Cryo-EM structure of the 1. Cofactors active in electron transport are marked in yellow FMN , orange iron—sulfur clusters , dark blue quinols , red hemes , and green copper atoms. Figure 2: The electrochemical proton gradient and ATP synthase. At the inner mitochondrial membrane, a high energy electron is passed along an electron transport chain.

Is the Mitochondrial Genome Still Functional? Figure 3: Protein import into a mitochondrion. A signal sequence at the tip of a protein blue recognizes a receptor protein pink on the outer mitochondrial membrane and sticks to it. Logically, mitochondria multiply when a the energy needs of a cell increase. Therefore, power-hungry cells have more mitochondria than cells with lower energy needs.

For example, repeatedly stimulating a muscle cell will spur the production of more mitochondria in that cell, to keep up with energy demand. Mitochondria, the so-called "powerhouses" of cells, are unusual organelles in that they are surrounded by a double membrane and retain their own small genome. They also divide independently of the cell cycle by simple fission. Mitochondrial division is stimulated by energy demand, so cells with an increased need for energy contain greater numbers of these organelles than cells with lower energy needs.

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