Mitochondria are the most important cell organelle of virtually all eukaryotic organisms.

The term “Mitochondria” is coined by C. Benda. Where the Greek word “Mito” means tread and “chondrion” means grain.

This organelle is participating in oxidative phosphorylation and aerobic respiration.

They can convert energy, produce ATP, for this reason, Mitochondria are called ‘the power houses’ of eukaryotic cells.

There is some other name of Mitochondria, such as bioplast, chondriosome, plasmosome.

Features of Mitochondria

Origin: Normally Mitochondria originated from cell membrane and nuclear membrane. But Mitochondria can also produce Mitochondria.

Site: It is present in cytoplasm of ideal plant cell and animal cell. It is not present in red blood cell of mammals.

Mitochondria: The number of mitochondria present in different cells and organisms is written in the table below;

Size:  Normally the length of mitochondria is 1.5 micrometre – 10 micrometre and the width is 0.5 micrometre – 1 micrometre.

Structure of Mitochondria

Mitochondria are double membrane bound organelle. The membrane in the inner side or which is facing the cytoplasm is called the inner mitochondrial membrane.

The membrane in the outside is called outer mitochondrial membrane. The matrix side of membrane is called N – side or negative site and the cytosolic side of membrane is called P – side or positive site.

The space between inner and outer mitochondrial membranes is called intermembrane space.

Outer Mitochondrial Membrane

This membrane is permeable type. The thickness of this membrane is 60 – 75 Armstrong. This membrane protects the organelle and helps in transport.

There are some pores present on the surface of this membrane, these pores are called porins.

This membrane contains many lipid synthesis enzymes. The porin proteins allow free passage for various molecules into intermembrane space.

Small molecules can pass through Porins, or voltage-dependent anion selective channels or VDAC.

Inner Mitochondrial Membrane

The inner membrane has folds structure. These structures are called cristae. And the outer mitochondrial membrane is fairly smooth.

This membrane is very impermeable. Many solutes can’t pass this membrane due to very high content of cardiolipin.

Cardiolipin is a phospholipid molecule present in inner mitochondrial membrane. Oxisome or F0-F1 particle is present on the cristae.

The cristae increase the surface area of inner membrane. The main enzyme of this membrane is ATP synthase or ATPase or F0-F1 ATPase or oxysome. It makes ATP.

Components in Mitochondria

The mitochondrial matrix contain dsDNA, mitochondrial ribosomes, tRNAs and various proteins. Mitochondrial dsDNA is normally circular and has similarities with bacteria. The ribosomes are also like bacteria, ranging from 55S-75S.

F0-F1 Particle or ATP Synthase

ATP synthase is located in the inner membrane. It has two parts- FO and F1. The FO has hydrophobic regions.

This part is embedded in the inner membrane. It causes rotation of F1. The F1 contains 5 polypeptides 3α, 3β, 1γ, 1ε and 1δ subunits. The F1 contains a water- soluble part, which hydrolyze ATP.

Mitochondria Function

o Aerobic respiration and energy production: In aerobic respiration 3 major processes are occurred; these are glycolysis, citric acid cycle and oxidative phosphorylation.

Glycolysis occurs in cytosol and the citric acid cycle and oxidative phosphorylation occurs in mitochondria (for eukaryotic cells).

Citric acid cycle occurs in matrix but the oxidative phosphorylation in inner membrane. In oxidative phosphorylation, ATP synthase takes ADT and Pi and synthesis ATP.

o Storage of calcium ions: In Mitochondria Ca2+ is stored. The free calcium concentrations in cell can regulate many reactions and important for signal transduction.

On the inner membrane Ca2+ uniporter is present. By which calcium enter inside the matrix and maintains cytoplasmic concentration of calcium.

o Synthesis of amino acids: TCA occurs in mitochondria, which produced many amino acids, such as glutamic acids, aspartic acid etc.

Evolutionary Origin of Mitochondria

It is thought that mitochondria developed from symbiotic bacteria. These bacteria are engulfed by endosymbiotic process by large cell. But they are not destroyed by cell; they stay inside the large cell and form mutualism relationship.

In this relationship both of them taking benefit from each other. Mitochondria are semiautonomous, because they can produce some protein by their own.

They divide by binary fission (like bacteria). It has their own genetic materials and ribosome. But their DNA are bacterial type of DNA molecule specifically cyanobacteria and alpha-proteobacteria.

This also suggests that they evolved from bacteria. Lynn Margulis proposed the endosymbiotic theory. According to this theory about 1.5×109 years ago mitochondria have evolved in eukaryotic organism from endosymbiotic association of purple photosynthetic bacteria.

The endosymbiont (which is the captured cell) then slowly loses its ability to survive fully autonomously.

Inheritance of Mitochondria

Mitochondria is present in cytoplasm and so the genome of mitochondria is inherited by only mother in higher eukaryotic cells.

That means in the time of zygote formation, egg get only nucleated genome from sperm. Sperm does not transfer any cytoplasm to egg. So the organelle present in cytoplasm is also not transferred by male.

So in offspring all DNA of cytoplasmic organelle (mitochondria) are coming from mother. For this reason if any change or mutation occurs in mother’s mitochondrial genome then there is no chance of replacement.

All offspring will be infected if diseased genes will come from mother’s mitochondria. But for lower eukaryotes (yeasts) both male and female parents contribute same amounts of mitochondria to zygote. So for lower eukaryotes like yeasts, mitochondrial inheritance is biparental.

Endosymbiotic Theory of Mitochondria

o It has bacterial type of DNA molecules.

o It is self-replicating bodies like bacteria and divide in a manner resembling binary fission in bacteria.

o It has 70s ribosomes (bacterial type).

o t has two membranes and the composition of innermost membranes is highly similar to the bacteria.

o The Mitochondrial enzymes, ribosomes and transport systems are very similar to bacteria.

o The size of Mitochondria is approximately same as bacteria.

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Respiratory Quotient: Definition, Equation, and Calculation

Amphibolic Pathway: Definition, Examples, and Facts

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Citations

• Mitochondria are not captive bacteria. J Theor Biol . 2017 Dec 7;434:88-98.
• Cell Biology of the Mitochondrion. Genetics . 2017 Nov;207(3):843-871.
• Ultrastructure of the mitochondrion and its bearing on function and bioenergetics. Antioxid Redox Signal . 2008 Aug;10(8):1313-42.

Almost all eukaryotic cells required mitochondria for the production of energy in the form of adenosine triphosphate (ATP) an energy-rich molecule that powers basic cell processes.

Therefore, known as the cell’s powerhouse. Mitochondria is a rod shaped membrane bound organelle that range in size from 0.5 to 10 m.

Mitochondria are highly dynamic and their number in a cell is determined by the metabolic needs of that cell i.e. Red blood cells in humans lacks mitochondria, whereas thousands of mitochondria are found in liver cells and muscle cells.

The oxymonad Monocercomonoides species is the only known eukaryotic species without mitochondria.

Evolution of Mitochondria

The evolution of mitochondria is supported by endo-symbiotic theory. It is assumed that evolution of mitochondria took place from free living bacteria in a long process of millions of years and now mitochondria has become inseparable part of cell.

Structure of Mitochondria

Mitochondria is separated by double-membrane system by the cytoplasm consisting of outer and inner mitochondrial membranes separated by an inter-membrane gap.

Outer membrane is porous and made up of protein and phospholipids. Small molecules, ions, uncharged molecules can pass through the outer mitochondrial membrane through pore forming membrane proteins (porins).

The inner membrane is selectively permeable, it also have especial enzyme called ATP synthase which play significant role in ATP synthesis.

Inner membrane of mitochondria possesses several tubular invagination folds called cristae that extend into the organelle’s interior (matrix) and allowing only very small molecules, ions via specific membrane transport proteins and bears electron transport chain.

The mitochondrial matrix is the space filled with viscous fluid within inner membrane it comprises mitochondrial DNA, enzymes for kerbs cycle (TCA) cycle, ribosomes, proteins, inorganic ions, organic compounds, nucleotide cofactors compounds.

Each of these components serves a specific purpose. A synchronisation between compartments in mitochondria generate ATP following a complex multistep process.

Vernalization: Definition, Hormone, and Mechanism

Mitochondrial DNA and its Inheritance

A surprising feature of mitochondria is that it has its own circular DNA which independently reproduces.

Mitochondria DNA have 37 genes and almost 16,500 DNA base pair in humans, required for proper mitochondrial function.

Maternal lineage is the primary source of mitochondrial inheritance in most animal species, although paternal passage of mitochondrial inheritance in very few cases is also confirmed is by some latest research.

Mitochondria is found in a sperm’s tail as a energy reservoir for the sperm’s long journey to the egg.

The tail of the sperm slips off when it connects to the egg during fertilisation. As a result, the zygotes only source of mitochondria is usually the egg delivered by its mother.

Unlike nuclear DNA shuffling of mitochondrial DNA doesn’t take place in every generation so slower rate of change can be anticipated, which make it important for human evolution studies

Function of Mitochondria

Mitochondria synthesize ATP through aerobic cellular respiration.

In mitochondria matrix glucose is oxidised in pyruvate and form acetyl co-enzyme and produced reduced NADH through citric acid cycle which later on transfer electron to inner membrane embedded protein system (ETC) used in the process of oxidative-phosphorylation and synthesize ATP via ATP synthase enzyme present in mitochondrial inner membrane.

Besides energy production mitochondria also perform other important roles for eg. calcium storage for cell signalling, oxidative stress control, cellular metabolism, heat generation, cell growth, programme cell death.

Susceptibility of Mitochondria for Toxic Insult

The DNA within mitochondria is more vulnerable to damage than the rest of the genome. During ATP synthesis the generation of free radical can damage DNA.

Although most of the mitochondrial malfunction are caused by changes in nuclear DNA that effects mitochondrial functions.

These mutations can be inherited or occur naturally. Any damage to mitochondria results in energy-depleted cells which eventually leads to cell death.

In general, cells that require the most energy, such as heart muscle cells and nerve cells, are the most harmed by defective mitochondria.

Mitochondrial Malfunction Disease

Parkinson’s disease (PD), bipolar illness, schizophrenia, Alzheimer’s disease (AD), Huntington’s diabetes, autism.

Mitochondrial DNA is also employed in forensic science to identify bodies or sections of bodies.

Mitochondrial Function in Aging

Researchers have been looking into a link between mitochondrial malfunction and aging in recent years.

There are many ideas about aging, and the mitochondrial free radical theory of aging has gained popularity in recent years.

Reactive oxygen species (ROS) are thought to be created as a by-product of energy synthesis in mitochondria. DNA, lipids, and proteins are all harmed by these highly charged particles.

The functioning portions of mitochondria are damaged as a result of ROS damage. When the mitochondria can no longer operate properly, more ROS is created, which exacerbates the damage.

Although there has been evidence of a link between mitochondrial activity and aging, not all experts have reached the same findings. It’s yet unclear what role they play in the aging process.

Mitochondria Citations

• Our (Mother’s) Mitochondria and Our Mind. Perspect Psychol Sci . 2018 Jan;13(1):88-100.
• Mitochondria: in sickness and in health. Cell . 2012 Mar 16;148(6):1145-59.