What is Mitochondria?
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;
|Number of Mitochondria
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.
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.