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Introduction
Mitochondria are organelles present in aerobic eukaryotes and are absent in prokaryotes and RBCs. They are also known as ‘powerhouse of cells’ as they take part in cellular respiration and energy generation. These structures were first discovered by Kolliker in 1857 while the name ‘mitochondria’ was given by Benda (1898). The word ‘mito’ means ‘thread’ and ‘chondrion’ means ‘granules like’. These are double membrane bound structures, found in the cytoplasm and function as a ‘cells digestive system’. They are semi-autonomous in nature as they have their own DNA and can replicate independently to produce their own mRNA, tRNA, to synthesise some of their proteins. Every new mitochondria is produced by pre-existing mitochondria (through fission).
Structure of Mitochondria
These are cylindrical or sausage shaped double membranous structures, about length of 1.0- 4.17 micrometre and a diameter of 0.2-1.0 micrometre
Each mitochondrion has an outer membrane and an inner membrane, both of which dividing the lumen into two distinct aqueous compartments
The outer compartment
it forms a continuous limiting boundary of the organelle. It covers the surface of mitochondria and possesses a large number of special proteins known as porins.
The inner compartment
it forms a number of infoldings known as cristae. A dense homogeneous substance known as matrix fills the inner compartment. This membrane also possesses mushroom-like projections known as elementary particles (F0 – F1 particles) or oxysomes. Inner mitochondrial membrane consists of enzymes of electron transport chain in contact with elementary particles.
These two compartments are made up of phospholipid layers that are separated by the intermembrane space.
Cristae
The inner compartment of mitochondria forms a number of infoldings these are referred to as cristae. These structures increase the surface area inside the organelle. The cristae along with the proteins of mitochondria helps in the production of ATP molecules. A wide variety of chemical reactions takes place inside the inner compartment. Inner compartment is permeable only to oxygen and ATP molecules.
Mitochondrial Matrix
It is a homogeneous, viscous fluid containing a mixture of enzymes and proteins. It includes ribosomes, mitochondrial DNA, inorganic ions, nucleotides, cofactors and organic molecules. This also helps in the synthesis of ATP molecules.
Functions of Mitochondria
The most vital function of mitochondria is the production of energy via the process of oxidative phosphorylation
Mitochondria helps in growing new cells and cell multiplication
Helps in regulating the metabolic activities of cells
Promotes detoxification of ammonia in liver cells
It has a vital role in apoptosis
It maintains the concentration of calcium ions in the compartments of the cell
The Role of Mitochondria in Disease
It is largely due to the fact that mitochondrial DNA (mtDNA) lacks the robust DNA repair mechanisms found in nuclear DNA that it is highly susceptible to mutations. The mitochondrion, in addition, is a major site for the production of reactive oxygen species (ROS; also known as free radicals) as a result of its high proclivity for the uncontrolled release of free electrons. However, while several different antioxidant proteins within the mitochondria scavenge and neutralise these molecules, some ROS have been shown to cause damage to mitochondrial DNA (mtDNA). Aside from that, exposure to certain chemicals and infectious agents, as well as alcohol abuse, can harm mitochondrial DNA (mtDNA). As an example, when a person consumes too much ethanol, the body’s detoxification enzymes become overworked, allowing highly reactive electrons to leak from the inner membrane and into the cytoplasm or mitochondrial matrix, where they combine with other molecules to form a large number of radicals.
Contribution to the Production of Energy
In addition to containing special channels capable of transporting large molecules, the outer mitochondrial membrane is also permeable to small molecules. As compared to the outer membrane, the inner membrane is significantly less permeable, allowing only very small molecules to cross into the gel-like matrix that forms the centre of the organelle’s central mass. TCA cycle enzymes (also known as the citric acid cycle or the Krebs cycle) are found in this matrix, which metabolises nutrients into by-products that the mitochondrion can use for energy production. The mitochondrial genome is encoded by deoxyribonucleic acid (DNA), which is found in the matrix.
Count of Mitochondria in a Cell
Mitochondria cannot be created “from scratch” because they require gene products from both mitochondrial and nuclear sources. Asexual cell division is used by bacteria to reproduce their organelles, and this process is similar to the simple, asexual form of cell division used by bacteria. Video microscopy reveals that mitochondria are extremely dynamic structures. They are constantly dividing, fusing, and morphing into different forms. In fact, a single mitochondrion may contain multiple copies of its genome at any given time, depending on the circumstances.
Logic dictates that mitochondria multiply when the energy demands of a cell increase, and the reverse is true. As a result, cells with high energy demands have more mitochondria than cells with low energy requirements. In the case of muscle cells, for example, repeatedly stimulating them will cause them to produce more mitochondria in order to keep up with the demand for energy.
Disorders Associated with Mitochondria
Abnormalities in the functioning of mitochondria, directly affects human health. Its symptoms vary from person to person. Mitochondrial disorders are often severe, in few cases it may also lead to organ failure.
Conclusion
Mitochondria divide independently of cell cycle via simple fusion. Division of mitochondria is stimulated by huge energy demand because of which the cells of increased need for energy contain greater numbers of these organelles than the cells with lower energy needs.
