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Mitochondria – Definition, Structure, Functions, Origin

Mitochondria Definition All eukaryotic cells contain membrane-bound organelles called mitochondria, which generate adenosine triphosphate (ATP), the cell’s primary source of energy. History of Mitochondria Distribution ...

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Mitochondria Definition

All eukaryotic cells contain membrane-bound organelles called mitochondria, which generate adenosine triphosphate (ATP), the cell’s primary source of energy.

  • Mitochondria (Greek:, mito=thread,, chondrion=gran ule) are cytoplasmic granules or filaments found in all aerobic cells of higher animals and plants as well as some microorganisms including algae, protozoa, and fungi.
  • This is because bacterial cells do not have them. Lipoprotein framework in mitochondria comprises several enzymes and coenzymes necessary for energy metabolism.
  • They also have ribosomes, which are responsible for protein synthesis, and a specialised form of DNA for cytoplasmic inheritance.
  • Mitochondria (singular: mitochondrion) are double-membraned organelles that perform cellular respiration and are found in nearly all eukaryotic species.
  • They can be located in the cytoplasm and serve as the “digestive system” of the cell.
  • Their work is crucial in the cell’s production of molecules rich in energy.
  • The mitochondria are responsible for many of the metabolic events that occur during cellular respiration.
  • The name “mitochondrion” comes from the Greek for “thread” and “granules-like,” “mitos” and “chondrion,” respectively. In 1890, a German pathologist called Richard Altmann was the first to describe it.

History of Mitochondria

  • Kolliker discovered mitochondria in 1850, noticing them as granular granules in striated muscles.
  • In 1888, he demonstrated that the isolated mitochondria from insect muscles (which include multiple slab-like mitochondria) expanded in water and are surrounded by a membrane. Flemming gave them the name “fila” in 1882.
  • With the goal of highlighting mitochondrial function, Richard Altmann (1890) created a stain with high specificity. The bioblast is the name he gave to this cell component. Bioblasts, as postulated by Altmann, are the cellular equivalent of free-floating elementary living particles that alter cellular genetics and metabolism.
  • Currently known as mitochondria, this word was coined by Benda (1899-1978). He used alizarin and crystal violet to dye the mitochondria.
  • Using the supravital dye Janus green, Michaelis (1900) proved that mitochondria were cellular centres of oxidation and reduction. Kingsbury hypothesised in 1912 that the oxidation reactions mediated by mitochondria were typical cellular procedures.
  • In 1910, Otto Warburg (1883-1970), often regarded as the “father of respirometry,” extracted mitochondria (“big granules”) via low-speed centrifugation of tissues disturbed by grinding.
  • These granules, he demonstrated, house enzymes that catalyse oxidative cellular processes. German biochemists Gustaf G. Embden (1874-1933) and Otto F. Meyerhof (1884-1951) independently discovered the several phases of glycolysis.
  • Both Meyerhof and the English biophysicist A.V. Hill won the Nobel Prize in 1922 for their work on the metabolism of lactic acid in muscle and the discovery of oxygen (i.e., production of heat in muscle).
  • Muscle ATP was first identified by Lohmann (1931). German biochemist in the United States named Lipmann (born in 1899) developed coenzyme A and demonstrated its role in intermediate metabolism.
  • High-energy phosphates and phosphate bonds (i.e., ATP) were first proposed by him in the field of bioenergetics in 1941. Warburg established a connection between the oxidation of glyceraldehyde phosphate and the occurrence of ATP production.
  • Meyerhof proved that ATP could be synthesised from phosphopyruvate, and Kalckar connected oxidative phosphorylation to aerobic respiration. German-born British biochemist Sir Hans Adolph Krebs (1900) figured out the citric acid cycle’s different processes in 1937. (or tricarboxylic acid or TCA cycle).
  • Radioactively labelled chemicals had not previously been made available for use in biological studies, and the precise locations of the reactions within cells had not been established, making his contribution all the more impressive.
  • The citric acid cycle was discovered by Krebs and Lipmann in 1953, and they shared the Nobel Prize for their work.
  • mitochondrial citric acid cycle (Krebs cycle), mitochondrial oxidative phosphorylation, and mitochondrial fatty acid oxidation were all demonstrated by Kennedy and Lehninger (1948–1950).
  • As Lehninger showed in 1951, electron transport is essential for oxidative phosphorylation. Warburg, Szent-Gyorgyi, and Kuhn are just few of the early ETS researchers who went on to win Nobel Prizes for their groundbreaking work.
  • Mitchell put up the now widely accepted “chemiosmotic-coupling theory” in 1961 to explain how mitochondria generate ATP. To honour his work on this approach, the Nobel Prize in Economics was bestowed upon him in 1978. In 1954, Palade outlined the hyper structure of cristale.
  • DNA fibres were first seen in the mitochondrial matrix of embryonic cells by Nass and Nass in 1963. Mitochondrial 70S ribosomes were first discovered by Attardi, Attardi, and Aloni (1971).
  • Fuchsinophilic granules, parabasal bodies, plasmosomes, plastosomes, fila, vermicules, bioblasts, and chondriosomes are just a few of the names that have been used to describe mitochondria over the years.

Distribution Or Localization 

  • Mitochondria may move independently in the cytoplasm, therefore they are often dispersed uniformly throughout the cytoplasm. However, in many cells, their mobility is severely impeded.
  • Most mitochondria (including mitochondrial cristae) are organised into clusters, and the presence or absence of these clusters can be used to infer the cell’s function.
  • Mitochondria with many cristae are typically associated with mechanical and osmotic work situations with constant needs for ATP and limited available space, such as between muscle fibres, in the basal infolding of kidney tubule cells, and in a portion of the inner segment of rod and cone cells of the retina.
  • Many large mitochondria, known as sarcosomes, can be seen within myocardial muscle cells.
  • Hepatic cells perform their biosynthetic and degradative tasks in distinct cellular locations. Having several “low key” sources of ATP synthesis spread out may be more efficient for these cells.
  • In the oocyte of Thyone briaeus, for instance, rows of mitochondria are intimately associated with RER membranes, indicating that ATP is necessary for protein synthesis at the location.
  • The numerous mitochondria in unmyelinated axons, each of which has only a single crista, are inefficient ATP producers. areas where osmotic work is done by ATP, such as the brush border of proximal tubules in the kidney, the infolding of the plasma membrane in dogfish salt glands and Malpighian tubules in insects, and the contractile vacuoles of some protozoans (Paramecium).
  • The monoamine oxidase enzyme in the outer mitochondrial membrane oxidatively deamininates monoamines, including those found in neurotransmitters (acetylcholine).

Orientation of Mitochondria

  • The orientation of mitochondria is specific. In cylindrical cells, for instance, mitochondria are often oriented in a basal-apical direction and lie parallel to the major axis.
  • In leucocytes, mitochondria continue to be organised radially relative to the centrioles. As they move about in the mitochondria, they form long moving filaments or chains, whereas in other cells they remain fixed in one position and provide ATP directly to a site of high ATP consumption, such as between adjacent myofibrils in cardiac muscle cells or tightly wrapped around the flagellum of sperm.

Morphology of Mitochondria

Number

  • The quantity of mitochondria in a cell is determined by the cell’s type and functional condition. It differs from cell to cell and between species.
  • Certain cells contain an extraordinarily high number of mitochondria, such as the Amoeba and Chaos chaos, which contain 50,000; the eggs of sea urchins contain between 140,000 and 150,000; and the oocytes of amphibians contain 300,000 mitochondria.
  • Certain cells, such as rat liver cells, have between 500 and 1600 mitochondria. Compared to animal cells, green plant cells contain less mitochondria due to chloroplasts taking over the mitochondria’s function.
  • Certain algal cells may possess a single mitochondrion.

Shape

  • Mitochondria can be filamentous or granular, depending on the physiological state of the cell, and can transition from one form to another.
  • Consequently, they may be club-, racket-, vesicular-, ring-, or round-shaped. Mitochondria in primary spermatocytes or rat cells are granular, however in liver cells they are club-shaped.
  • The time-lapse microcinematography of living cells reveals that mitochondria are remarkably dynamic and changeable organelles, altering their structure continuously.
  • They occasionally fuse together and then separate again. In particular euglenoid cells, for instance, the mitochondria fuse into a reticulate structure during the day and dissociate at night.
  • Similar modifications have reportedly occurred in yeast species in response to culture conditions.

Size

  • Mitochondria typically range in size from 0.5 m to 2.0 m and are consequently not distinguishable under a light microscope.
  • Occasionally, their length may approach 7 m.

Mitochondria Structure

  • Each mitochondrion is held together by two highly specialised membranes that are essential to its function.
  • Each mitochondrial membrane has a thickness of 6 nm and a fluidmosaic ultrastructure.
  • Numerous copies of a transport protein called porin generate huge water channels across the lipid bilayer of the outer membrane, which is quite smooth.
  • This membrane is permeable to any molecules with a molecular weight of 10,000 daltons or less, including tiny proteins. Inside and separated by a 6–8 nm-wide gap from the outer membrane is the inner membrane.
  • The inner membrane is not smooth but highly convoluted and impenetrable, generating a sequence of infoldings called cristae in the matrix space.
  • The inner membrane splits the mitochondrial space into two different chambers.
    • The outer compartment, perimitochondrial space, or space between the outer membrane and inner membrane. This gap continues into the crests’ or cristae’s core.
    • The inner compartment, inner chamber, or matrix space, which is comprised of a dense, uniform, gel-like proteinaceous material known as mitochondrial matrix.
  • The mitochondrial matrix is composed of lipids, proteins, circular DNA molecules, 55S ribosomes, and specific granules associated with the mitochondria’s capacity to accumulate ions.
  • Granules are abundant in the mitochondria of cells involved in the transport of ions and water, such as kidney tubule cells, small intestine epithelial cells, and bone-forming osteoblasts.
  • Additionally, the inner membrane has an outer cytosol or C face facing the perimitochondrial space and an inner matrix or M face facing the matrix.
  • In general, the cristae of plant mitochondria are tubular, whereas those of animal mitochondria are lamellar or plate-like (Hall, Flowers, and Roberts, 1974); however, in many Protozoa and steroid-synthesizing tissues, such as the adrenal cortex and corpus luteum, they are found as regularly packed tubules (Tyler, 1973).
  • In liver cell mitochondria, the area of the cristae membrane is 3–4 times bigger than the area of the outer membrane, as a result of the cristae increasing the area of the inner membrane significantly.
  • Some mitochondria, especially those found in the heart, kidney, and skeletal muscles, have a more widespread arrangement of cristae than liver mitochondria. Other mitochondria (such as those from fibroblasts, nerve axons, and the majority of plant tissues) have considerably fewer cristae.
  • Mitochondria in epithelial cells of carotid bodies (or glomus carotica, which are chemoreceptors sensitive to changes in blood chemistry and are located near the bifurcation of carotid arteries) have only four to five cristae, whereas mitochondria from non-myelinated axons of rabbit brain have only one crista.
  • Multiple units of stalked particles, known as elementary particles, inner membrane subunits, or oxysomes, are attached to the M face of the inner mitochondrial membrane.
  • They are also known as F1 or F0-F1 particles and are essential for ATP synthesis (phosphorylation) and ATP oxidation (i.e., acting as ATP synthetase and ATPase).
  • On the inner surface of inner mitochondrial membrane, F0-F1 particles are consistently spaced at 10 nm intervals. According to some estimations, each mitochondrion contains between 104 and 105 elementary particles.
  • When the mitochondrial cristae are ruptured by sonic vibrations or detergent activity, they generate inverted submitochondrial vesicles. Attached to the outside surface of these vesicles are F0-F1 particles.
  • These submitochondrial vesicles are capable of phosphorylation of the respiratory chain. However, in the absence of F0 -F1 particles, these vesicles lose their phosphorylation capacity, as demonstrated by resolution (i.e., removal by urea or trypsin treatment) and reconstitution.
Mitochondria Structure
Mitochondria Structure

Mitochondria Origin

  • Mitochondria may have evolved from early bacteria that were so symbiotic with their hosts, eukaryotic cells, that they became indispensable energy-producing organelles within eukaryotic cells (endosymbiotic theory).
  • Nonetheless, there are eukaryotes without mitochondria.
  • Monocercomonoides is the first eukaryotic species without mitochondria to be discovered.
  • It obtains energy by metabolising environment-absorbed nutrients.

1. Endosymbiotic hypothesis

  • This hypothesis is by far the most frequently accepted. According to this theory, mitochondria derive from a free-living aerobic prokaryote, possibly a -proteobacteria.
  • A symbiotic relationship established between the ancestral pre-eukaryotic cell and the bacterium that survived endocytosis.
  • In the process of evolution, the ingested bacterium lost its cell wall and the majority of its DNA, both of which were useless to the host cell.
  • Similarities between bacteria and mitochondria bolster the validity of the idea.
  • Several remarkable parallels between mitochondria and bacteria:
    • Circular DNA is present in both bacteria and mitochondria.
    • The size and structure of the 70S ribosomes of mitochondria and bacteria are comparable.
    • The outer mitochondrial membrane and bacterial cell membrane include porins.
    • Cardiolipin is present in the inner membrane of mitochondria and bacterial cells.
    • Mitochondria can only be created through binary fission and not by the cell itself.
Endosymbiotic hypothesis
Endosymbiotic hypothesis

2. Autogenous hypothesis

  • According to this view, the mitochondria of today were produced by the functional grouping of DNA from the main genome and the subsequent compartmentalization of the cell membrane.

Components of Mitochondria

Mitochondrion is a microscopic organelle that is normally spherical to oval and 0.75-3 μm² in size. It is an organelle with two membranes found in the cytoplasm of most eukaryotic cells. Mitochondria are similar to other organelles, such as the nucleus and plastids, due to their dual membranes. Depending on the sort of cell, the number and form may vary substantially.

There are five major components that constitute the structure of mitochondria. The following list outlines the many components of mitochondria.

1. Outer membrane

  • The outer membrane is the mitochondrion’s outermost coat. It isolates the intermembrane space’s contents from the cytoplasm.
  • Its composition is similar to that of the plasma membrane, since both are formed of phospholipid bilayer and include numerous embedded proteins and enzymes.
  • It is permeable to molecules 1000 kDa in size.
  • It plays a crucial role in regulating programmed cell death. List of significant outer membrane components of mitochondria and their significance:
  • Porins: Porins are specialised protein structures that allow ions, nucleotides, metabolites, and tiny proteins to flow across a membrane.
  • Translocase: Translocases facilitate the transfer of certain big proteins.
  • Metabolic enzymes: Monoamine oxidase, Kynurenine hydroxylase, Fatty-acid CoA ligase, and NADH-Cyt-c reductase are metabolic enzymes.
  • MAM (Mitochondria-associated ER-membrane): MAM are the domains that connect mitochondria with ER. They are important for lipid transport, calcium homeostasis, autophagy, and apoptosis.

2. Intermembrane space

  • The intermembrane space, also known as the peri mitochondrial space, exists between the outer and inner mitochondrial membranes.
  • It is essential for the transport of proteins and ions, the assembly of inner membrane proteins, and the respiration of cells.
  • It contains the protons that are pushed from the mitochondrial matrix into the intermembrane space by the electron transport chain’s redox processes.

3. Inner mitochondrial membrane

  • The inner mitochondrial membrane is larger than the outer membrane. Under an electron microscope, it seems wrinkled because it makes multiple folds to conform itself within the outer membrane.
  • It is the major location for oxidative phosphorylation because it includes the electron transport chain complexes.
  • The inner mitochondrial membrane, unlike its outer counterpart, is highly impermeable and devoid of porins.
  • This allows the inner mitochondrial membrane to maintain the proton gradient necessary for ATP production.

4. Cristae

  • Cristae are tiny compartments formed by the mitochondrial inner membrane’s many folds.
  • These cristae are packed with ETC proteins and aid in increasing available surface area for oxidative phosphorylation reactions.
  • More cristae can be found in the mitochondria of cells that need a lot of ATP/energy.

5. Matrix

  • The mitochondrial matrix is the area within the limits of the inner mitochondrial membrane.
  • It stores mitochondrial DNA and a wide variety of metabolic enzymes.
  • The Krebs cycle, commonly known as the tricarboxylic acid (TCA) cycle, is an essential metabolic cycle that takes place in the mitochondrial matrix.

Isolation of Mitochondria

There have been three main approaches to the study of mitochondria:

1. Direct Observation of Mitochondria

  • This low refractive index makes it challenging to study mitochondria in living cells.
  • However, they are readily visible in cells grown in a petri dish, especially when seen with a phase contrast or darkfield microscope.
  • Coloration with the vital stain Janus green, which stains living mitochondria greenish blue due to its action with the cytochrome oxidase system found in the mitochondria, has considerably aided such an examination.
  • This method keeps the necessary dye in its oxidised (coloured) form. The stain is degraded to a colourless leukobase in the surrounding cytoplasm.
  • In both isolated mitochondria and intact grown cells, fluorescent dyes (such as rhodamine 123) have been utilised because of their increased sensitivity. This type of dye is preferable for studying mitochondrial metabolism in vivo.

2. Cytochemical Marking of Mitochondrial Enzymes 

  • Histochemical markers are enzymes specific to different regions of mitochondria, such as cytochrome oxidase for the inner membrane, monoamine oxidase for the outer membrane, malate dehydrogenase for the matrix, and adenylate kinase for the outer chamber.

3. Isolation 

  • Through differential centrifugation, mitochondria can be separated from other cell components.
  • Mitochondria have been isolated in homogenous fractions from several tissues, including liver, skeletal muscle, heart, and others.
  • Mitochondria sediment at a density of 5000–24000 g during differential centrifugation, but at a density of 20,000–400,000 g during ultracentrifugation in living cells, they are deposited whole at the centrifugal pole.
  • Density gradient centrifugation has been used to partition the mitochondrial inner and outer membranes.
  • Breaking the outer membrane and then allowing the inner membrane and matrix to contract causes a swelling that separates the two layers.
  • Some common detergents used for this function include digitonin and lubrol. Because the outer membrane is significantly lighter and weaker than the inner, centrifugal force is required to separate them.
  • To create the so-called mitoplast, the outer membrane is stripped away by digitonin. Mitoplasts have an inner membrane that contains unfolded cristae and a matrix.
  • Oxidative phosphorylation is known to occur in the mitochondria. Negative staining reveals the “foldedbag” look of the isolated outer membrane.
  • The separation of these two membranes and compartments has made it possible to pinpoint certain mitochondrial enzyme systems.

Chemical Composition of Mitochondria

Mitochondria of various animal and plant cells have diverse overall chemical compositions. Mitochondria, on the other hand, are reported to be composed of 65%- 70% proteins, 25%-30% lipids, 0.5-1% RNA, and a trace quantity of DNA. The mitochondrial lipid content is made up of phospholipids (lecithin and cephalin) at a 90% clip, cholesterol at a rate of 5% or less, and free fatty acids and triglycerides at a rate of 5%. Cardiolipin, a specific type of phospholipid abundant in the inner membrane, acts as a physical barrier to the passage of many different types of ions and small molecules (including Na+, K+, Cl—-, NAD+, AMP, GTP, CoA, and so on). The ‘unit membrane’ of the mitochondrial outer membrane typically consists of 50% proteins and 50% phospholipids. In contrast, it has a higher proportion of healthy unsaturated fats and a lower cholesterol content. It has been calculated that in liver mitochondria 67% of the total mitochondrial protein is placed in the matrix, 21% is found in the inner membrane, 6% is in the outer membrane, and 6% is in the outer chamber. These four sub-regions of mitochondria each house a unique set of proteins that mediate certain cellular processes:

1. Enzymes of outer membrane

  • Additional proteins of this membrane include those that catalyse the synthesis of mitochondrial lipids and those that convert lipid substrates into forms that are subsequently processed in the matrix, such as porin.
  • Enzymes such as monoamine oxidase, rotenone-insensitive NADH-cytochrome-C-reductase, kynurenine hydroxyalase, and fatty acid CoA ligase are located in this membrane and play critical roles in cellular metabolism.

2. Enzymes of intermembrane space

  • Multiple enzymes are located in this region, and they phosphorylate additional nucleotides with the ATP that leaves the matrix.
  • Adenylate kinase and nucleoside diphosphate kinase are the two most important enzymes in this section.

3. Enzymes of inner membrane

  • The respiratory chain’s oxidation processes are carried out by proteins in this membrane that have three distinct functions.
  • matrix-based ATP-producing enzyme complex, also known as ATP synthase.
  • certain transport proteins that control how metabolites enter and exit the matrix. The respiratory chain creates an electrochemical gradient across this membrane, which drives ATP synthase, hence it is crucial that this membrane be permeable only to large ions.
  • ATP synthase, succinate dehydrogenase, -hydroxybutyrate dehydrogenase, ubiquinone or coenzyme Q10, non-heme copper and iron, and enzymes involved in electron transport pathways are all essential components of the inner membrane.

4. Enzymes of mitochondrial matrix

  • Hundreds of enzymes, including those needed for pyruvate and fatty acid oxidation and the citric acid cycle or Krebs cycle, are concentrated in the mitochondrial matrix.
  • Multiple identical copies of mitochondrial DNA, as well as the specialised 55S mitochondrial ribosomes, transfer RNAs (tRNAs), and numerous enzymes necessary for mitochondrial gene expression, are found in the matrix.
  • Malate dehydrogenase, isocitrate dehydrogenase, fumarase, aconitase, citrate synthase, -keto acid dehydrogenase, and -oxidation enzymes are all found in the mitochondrial matrix.
  • In addition, the mitochondrial matrix has inorganic electrolytes including K+, HPO4 -, Mg++, Cl -, and SO4 -, as well as nucleotides and nucleotide coenzymes.

Dysfunction and Disease

  • At this point, you should understand why mitochondria are essential for the survival and operation of cells. Consequently, any mitochondrial abnormality can also result in illness.
  • Mitochondrial dysfunction may be caused by abnormalities in the mitochondrial genome, changes in the nuclear genome that encode key mitochondrial components, or the side effects of specific medications.

Mitochondrial diseases

The disorders caused by accumulating mutations in the mitochondrial genome are listed below.

  • Hereditary optic neuropathy of Leber
  • Leigh’s syndrome
  • NARP syndrome
  • Kearns Sayre syndrome
  • CPEO: progressive chronic external ophthalmoplegia
  • MELAS syndrome is the most prevalent mitochondrial disease.
  • Pearson’s disorder

These disorders are entirely maternally inherited. Typically, mitochondrial illnesses impact the brain, the eyes, and the skeletal muscles. This is because these tissues are highly dependent on mitochondria for their energy needs.

Mitochondrial replacement therapy

  • On April 6, 2016, the first male kid born using mitochondrial replacement treatment was born in Mexico to a Jordanian couple whose mother had Leigh’s syndrome.
  • Mitochondrial replacement treatment is a revolutionary IVF technique used when a woman with mitochondrial DNA abnormalities want to have a genetic kid.
  • In this instance, the cytoplasm containing healthy mitochondrial DNA from a donor’s ovum is preserved, while the nuclear DNA from the intended mother is transferred, leaving the damaged mitochondrial genome behind.
  • The hybrid ovum is then fertilised by the father’s sperm. This procedure produces embryos with healthy mitochondrial DNA from a donor mother and nuclear DNA from the intended parents.
Mitochondrial replacement therapy
Mitochondrial replacement therapy

Relationships to aging

The link between mitochondria and ageing is too intricate and incompletely understood. However, the majority of the responsibility for the production of reactive oxygen species (ROS), the accumulation of mitochondrial mutations, and the decline in functional capacity lies with the ageing process. Several potential pathways are described below.

  • Generation of ROS – Mitochondria are the principal source of reactive oxygen species (ROS) production (ROS). These ROS are able to cause damage to macromolecules, such as DNA. The oxidative damage to macromolecules has been linked to the ageing process. Mutations generated in the DNA cause mitochondrial dysfunction, which exacerbates the generation of reactive oxygen species (ROS).
  • Decreased respiratory capacity – With time, the respiratory activity of mitochondria declines. This modifies cellular metabolism and contributes to the ageing process. In addition, the diminished functional capacity is responsible for the increased production of ROS.
  • Mitophagy – Mitophagy is the mitochondrial version of autophagy. Mitophagy assists the cell in controlling the quantity of defective mitochondria. Mitophagy declines as we age, and the number of defective mitochondria grows.
  • Mitochondrial mutations — Since it lacks as efficient DNA repair mechanisms as nuclear DNA, mitochondrial DNA is more susceptible to mutations. These mutations can accumulate over time, leading to a reduction in mitochondrial function.
  • Population Decline of Stem Cells – Stem cells are vital for maintaining the health of tissues. It is known that mitochondria influence the function of stem cells. With advancing age, the stem cell population declines.
  • Cellular senescence – Mitochondria control cellular ageing by altering the cell’s metabolic profile. Significant alterations in the cellular metabolome are biologically linked to the senescence of the cell.
  • Mitochondrial unfolded protein response (UPRmt) – UPRmt is a cellular stress response activated by mitochondrial stress that results in the misfolding of mitochondrial proteins. The nucleus detects this and increases the expression of proteases and chaperones that eliminate misfolded proteins. Aging and age-related illnesses, such as cardiovascular disease and diabetes, are connected with reduced protease levels.
  • DAMPs (damage-associated molecular patterns) – DAMPs (damage-associated molecular patterns) are molecules that are released during times of stress. Mitochondrial stress results in the release of mitochondrial DNA, one of the DAMPs. The quantities of mitochondrial DNA in the blood rise with age and are connected with a low-grade chronic inflammatory state associated with ageing.
  • Mitochondrial derived peptides – Mitochondrial peptides, such as humanin and MOTS-c, help protect Alzheimer’s disease and age-related insulin resistance, respectively. They are also related with a longer lifespan.
  • Mitochondrial metabolism – Mitochondria create NAD, which sirtuins utilise as a substrate. Sirtuins are proteins associated with lifespan. The cellular concentrations of NAD continue to decrease as age advances.

Mitochondria Function

1. Synthesis of ATP

  • ATP production is the mitochondrion’s most essential activity. ATP is the cell’s energy currency that drives metabolic operations.
  • The production of ATP molecules is termed oxidative phosphorylation.
  • Oxidative phosphorylation is a series of complex redox processes in the electron transport chain that oxidises nutritional metabolites in order to liberate energy for the phosphorylation of ADP to ATP.
  • The electron transport chain is positioned within the inner membrane of mitochondria. The electron transport chain is a succession of electron acceptor complexes arranged in ascending order of redox potential.
  • NADH and FADH2 provide electrons to the complexes of the electron transport chain through redox processes.
  • These redox reactions are associated with proton pumping into the intermembrane space, thereby producing an electrochemical gradient across the inner mitochondrial membrane. The FOF1 ATP synthase complex uses this gradient to generate ATP.

2. Production of heat

  • Utilizing the mitochondrial electrochemical gradient established by the electron transport chain, ADP is converted to ATP.
  • This phenomenon is called chemiosmotic coupling. Therefore, the energy produced during the electron transfer is trapped in phosphate bonds with high energy.
  • Nonetheless, tissues like brown fat lack chemiosmotic coupling. Consequently, the liberated energy is released as heat.
  • This heat energy assists little toddlers in maintaining their body temperature in the absence of other mechanisms, such as shivering.

3. Homeostasis of calcium 

  • Mitochondria play a crucial function in calcium homeostasis. The principal intracellular calcium storage sites are mitochondria and the endoplasmic reticulum.
  • MAM are the domains between mitochondria and endoplasmic reticulum that strictly regulate cellular calcium levels, which are vital to the survival and function of the cell.

4. Regulation of Immune function

  • Immune cell activation, survival, and differentiation all involve mitochondria. It can do so via modifying the metabolism, stimulating the inflammatory response, affecting mitochondrial dynamics through fission and fusion, and signalling at the mitochondrial-ER junction.

5. Apoptosis or programmed cell death

  • In the intrinsic pro-apoptotic pathway, mitochondria play a well-established role. The intrinsic apoptotic process is activated by cues including DNA damage.
  • It occurs prior to an imbalance of pro-apoptotic and anti-apoptotic proteins.
  • This imbalance results in increased cytochrome-c (cyt-c) leakage from the outer mitochondrial membrane into the cytoplasm.
  • Cyt-c forms a complex with the apoptosis activating factor APAF-1 (apoptosis activating factor), which attracts and activates caspase-9 (initiator protease).
  • Caspases-3, 6, and 7 are further activated by caspase-9 (executioner protease). This causes cell death and effective phagocytosis of the cellular debris.

6. Stem Cell Regulation

  • Mitochondrial metabolism is essential for the tight regulation of somatic stem cell homeostasis (SCC).
  • SCC homeostasis is disturbed due to mitochondrial malfunction caused by mitochondrial DNA mutations.
  • This is associated with accelerated tissue deterioration and ageing.

7. Synthesis of biochemicals

  • Mitochondria supply intermediates for the production of a variety of chemicals, including amino acids, chlorophyll, cytochromes, steroids, alkaloids, and pyrimidines, among others.
  • Certain amino acids are synthesised using the TCA cycle’s intermediates as precursors.
  • The antecedents of aspartic acid and glutamic acid are oxaloacetate and -ketoglutaric acid, respectively.
Mitochondria Function
Mitochondria Function

References

  • Cooper GM. The Cell: A Molecular Approach. 2nd edition. Sunderland (MA): Sinauer Associates; 2000. Mitochondria. Available from: https://www.ncbi.nlm.nih.gov/books/NBK9896/
  • Gray, M. W. (2013). Mitochondria. Brenner’s Encyclopedia of Genetics, 430–432. doi:10.1016/b978-0-12-374984-0.00957-8
  • Gray, M. W. (2001). Mitochondria. Encyclopedia of Genetics, 1215–1217. doi:10.1006/rwgn.2001.0835
  • Manoli, I., Alesci, S., & Chrousos, G. P. (2007). Mitochondria. Encyclopedia of Stress, 754–761. doi:10.1016/b978-012373947-6.00559-6 
  • Liao, P.-C., Bergamini, C., Fato, R., Pon, L. A., & Pallotti, F. (2019). Isolation of mitochondria from cells and tissues. Methods in Cell Biology. doi:10.1016/bs.mcb.2019.10.002 
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Citation

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MN Editors. (December 2, 2022).Mitochondria – Definition, Structure, Functions, Origin. Retrieved from https://microbiologynote.com/mitochondria/

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MN Editors. "Mitochondria – Definition, Structure, Functions, Origin." Microbiology Note, Microbiologynote.com, December 2, 2022.

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