What are Mitochondria?
Mitochondria (Mito = thread; chondrion = granule) are complex structures resembling threads and granules, most commonly seen in eukaryotic organisms without which the cells starve to death hence, it is also termed as the “Power house/ Power generators” of the cells.
They are situated in the cytoplasm and have a double membrane bounded organelles.
It was uncovered and explained in the year 1980 by Richard Altman who was a German Pathologists.
Mitochondria’s transform oxygen and nutrients into adenosine triphosphate (ATP) which ultimately provides energy to the cell through mechanism known as aerobic respiration.
Aerobic respirations allow the animals to use oxygen for their respiration.
Mitochondria are known to generate ATPs which are fifteen times more higher than compared to normal cell and this helps human beings to survive.
Although their prime function is to produce energy through adenosine triphosphate (ATP) but they are also responsible for generating heat, piling calcium for signallling activities as well as controlling growth and fate of cell.
The shape of mitochondria is oval and the number of mitochondria in cell differs in number some may have many whereas others may have none like human RBC’s.
Origin of Mitochondria
There are two suggested hypotheses explaining the origin:
1) Endosymbiotic:
It states that mitochondria were originally prokaryotic cells, having capacity of initiating oxidative mechanisms which was impossible for eukaryotic cells; and hence became endosymbionts living inside the eukaryote.
2) Autogenous:
This hypothesis suggests that mitochondria were born by breaching off a part of DNA from the eukaryotic cell nucleus at the time of parting with the prokaryotes, because of which the DNA part must have been entrapped by membranes, that could not be crossed by proteins.
However, the most widely accepted theory is of endosymbiotic as it has many characteristics similar to the bacteria.
Structure of Mitochondria
The detailed structure of a mitochondrion is particularly important as they are related to the functioning of an organelle.
There are 2 main membranes that surfaces every mitochondrion situated in a cell, dividing the organelle into a narrow intermembrane space and an internal matrix, comprising of highly specialized proteins.
a) The outer membrane – having the protein porin and which is similar to a sieve, aiding in filtration of molecules that are too big.
b) The inner membrane, which is highly convoluted having large number of infoldings called cristae that helps only certain molecules to pass through it & is selective as compared to the outer membrane.
Following is the basic anatomical and morphological structure of Mitochondria:
• It is double in membrane and has a shape of rod observed in plant as well animal cell.
• Oblong/oval in shape.
• Size in length varies from 1 and 10 micrometre depending on the metabolic activity of the cell.
• It can be classified into an outer membrane, an inner membrane which are composed of proteins and phospholipid layers divided by an intermembrane space, and a gel like viscous matrix.
• The outer membrane surfaces the mitochondrion and comprises of a special protein termed as porins.
• It allows free movement of ions, nutrient molecules, ADP as well as ATP molecules.
Cristae
The anatomical as well as morphological features of the inner membrane of mitochondria is complex and has ample number of folds to form a layered structure commonly known as cristae.
The surface area inside the organelle is increased many folds because of this infoldings.
The production of ATP molecules is aided by the cristae and the proteins of the inner membrane.
The inner membrane is stringently permeable only to oxygen and to ATP molecules where a number of chemical reactions occur inside them.
Mitochondrial Matrix
The mitochondrial matrix is a gel like viscous fluid comprising of enzymes and proteins, ribosomes, inorganic ions, mitochondrial DNA, nucleotide cofactors, and organic molecules.
In the matrix there are various enzymes which are present that has a major role in the synthesis of ATP molecules.
The locomotion of the organelles through the network within the cell occur because of microtubules and motor protein.
Likely, mitochondria pack themselves tightly into comparatively stable group, for long travelling chains, based on the need of the cell.
Role of Mitochondria
Apart from the major function of generating entropy through the mechanism of oxidative phosphorylation, it also displays many other functions that are given as follows:
1. It keeps a check on the normal metabolic activity of the cell
2. It aids in growth of new cells and cell multiplication
3. It supports the liver cells while detoxifying ammonia.
4. Plays an important role in programmed cell death
5. Plays crucial role in the formation of the blood cells and numerous hormones like testosterone and oestrogen.
6. It regulates the minimal concentration of calcium ions inside the cell compartment.
7. It is helpful for cellular activity at various levels like cell signalling and differentiation, cell senescence, regulating the cell cycle and growth of the cell.
ATP Production in Mitochondria
The ATP production occurs through 3 processes that includes glycolysis, the tricarboxylic acid cycle (TCA), and oxidative phosphorylation.
In the cells of eukaryotes, the two processes tricarboxylic acid cycle (TCA), and oxidative phosphorylation occur inside the mitochondria.
Electrons that move from the ETC (electron transport chain) finally produces free energy having the potential of aiding the phosphorylation of ADP.
Chemical Composition
The chemical composition of mitochondrion was given by Cohn:
It is composed of 65 – 70% of proteins, 25 – 30% of lipids (90% phospholipids, 10% cholesterol, Vit. E., etc.).
RNA is present approximately 5 – 7%, whereas DNA comprises of 2 – 5%.
The matrix also comprises of various catabolic enzymes like cytochrome oxidase and reductases, fatty acid oxidase, transaminase, and many more.
Mitochondrial Enzymes
| Structural Location | Enzymes |
| Outer membrane | Monoamine oxidase, glycerophosphatase, acyltransferase, phospholipase A. |
| Inner membrane | Cytochrome b, c1, c, a, NADH, dehydrogenase, succinate dehydrogenase, ubiquinone, flavoprotein, ATPase. |
| Perimitochondrial space | Adenylate kinase, nucleoside diphosphokinase. |
| Inner matrix | Pyruvate dehydrogenase, citrate synthase, aconitase, isocitrate dehydrogenase, fumarase, α− Ketoglutarate dehydrogenase, malate dehydrogenase. |
Mitochondrial DNA
mtDNA has high susceptibility towards mutations, as it is devoid of the robust DNA repair mechanisms which is a common phenomenon in DNA.
It also highly produces free radicals – reactive oxygen species (ROS) because of its increased tendency for irregular release of free electrons.
Although various antioxidant proteins present inside the mitochondria engulfs and counterbalances these molecules, there might be some ROS that may cause harm to mtDNA.
Chemicals and infectious agents, alcohol abuse, high ethanol intake can damage mtDNA by letting highly reactive electrons to leak from the inner membrane into the cytoplasm or matrix, forming various radicals especially in case of high ethanol intake.