Mitochondria are double membrane-bound cytoplasmic organelles found in all eukaryotic cells. They respire to generate an energy-rich molecule, adenosine triphosphate (ATP), through the breakdown of carbohydrates and fatty acids using various enzymes and electron transport systems. ATP molecules are crucial to fulfilling cellular functions.
What does mitochondria do in an animal cell? and Why are the mitochondria the most important organelle?
- Cellular respiration is the main mechanism by which mitochondria in the cell use glucose as fuel and produce energy in the form of chemical bonds known as ATP molecules, hence the mechanism called cellular respiration. Overall, three main steps are involved in this process, such as glycolysis, the Krebs or citric acid cycle, and ATP synthesis. The produced ATP is readily available for breakdown by other cellular organelles to use energy for their activities.
- Mitochondria can also produce heat in brown fat tissue by a process called thermogenesis.
- It accumulates iron-containing pigments (heme ferritin) and ions, Ca2+ and HPO42-.
- involved in cellular respiration or oxidation (not in photosynthesis).
- involved in the conversion of pyruvate to acetyl co. A.
- involved in the Krebs cycle, which occurs in the mitochondrial matrix.
- The inner membrane of the mitochondria is involved in electron transport and oxidative phosphorylation.
- involved in the regulation of the redox status of the cell.
- involved in the beta-oxidation of fats.
- Some steps in the biosynthesis of steroid hormones (aldosterone and cortisol) occur in mitochondria.
- Mitochondria detoxify ammonia into urea.
- involved in a few steps of heme biosynthesis.
- involved in the detoxification of ammonia in the liver.
- It is involved in the degradation of neurotransmitters, which is a major function of the outer membrane.
- involved in the degradation of tryptophan.
- involved in the phosphorylation of nucleotides.
- involved in the process of non-shivering thermogenesis in neonates of hibernating animals.
- Mitochondria are involved in PCD or Apoptosis.
- The release of cytochrome -C from the mitochondria leads to PCD.
- The brown adipose tissue contains a specialized mitochondrion, which contains a natural uncoupler called Thermogenin or Uncoupling protein-I (UCP-I).
- Cytochrome-C is a marker protein of mitochondria.
- Monoamine oxidase is a very important enzyme present in the outer membrane, hence this enzyme has been considered a biomarker for the outer membrane.
- Cytochrome oxidase is a marker enzyme for the mitochondrial inner membrane.
- Malate dehydrogenase is a marker enzyme of the mitochondrial outer membrane.
- Adenylate kinase is a marker enzyme of the intermembrane space.
- The mitochondrial matrix contains several enzymes that are very important in the process of beta-oxidation of fats.
- Cardiolipin (di-phosphatidyl glycerol) is restricted to the inner mitochondrial membrane and the bacterial plasma membrane.
- Mitochondria are considered semi-autonomous organelles since they can synthesize 10% of their proteins using their own genome.
- The mitochondrial membrane has a specialized function, through which phosphorylated compounds (e.g., ADP and ATP) can penetrate, but this feature is not found in any other membranes (plasma membranes or other organelle membranes).
- Cytochrome-C is a molecular chronometer because it is employed in evolutionary studies to determine the timing of evolutionary events.
Introduction to Mitochondria
Mainly, two cellular organelles of eukaryotes, such as mitochondria and chloroplasts, are originated by symbiogenesis as bacterial endosymbionts.
The mitochondrion is considered the powerhouse of the eukaryotic cells, and it was first discovered by Kolliler in the sarcoplasm of striated muscle cells of insects, hence they were named acrosomes. However, various scientists called mitochondria by different names; Altman called them bioplast, Fleming called them flia, and finally, Cristian coined the term “mitochondria” (Mito means thread and Chondria means body).
Mitochondria contain various types of enzymes, which are mainly involved in respiration and energy transduction. It is present in all types of eukaryotic cells except red blood cells (erythrocytes), and primitive anaerobic eukaryotic cells of Microsporidia and Giardia.
In the evolutionary course, mitochondria gave rise to two organelles.
a). Mitosome/Crypton/krypton (Mitochondrion-derived organelle) was identified in Entamoeba histolytica
b). Hydrogenosomes are found in trichomonads, anaerobic ciliates, and fungi. They can produce molecular hydrogen and ATP molecules.
The Genome of Mitochondria
The mitochondrial genome is a multiple copy number. The mitochondrial DNA molecules will form a cluster called the mitochondrial nucleoid.
The mitochondrial genome is circular in configuration. However, some mitochondrial genomes are linear in configuration.
Example: Mitochondria genome of Plasmodium, Tetrahymena, Amoebedium, and Chlamydomonas.
In trypanosomes, there is a disc-shaped mass of circular DNA molecules called a kinetoplast. The mitochondrial genome size is different in some organisms.
At least 6 kb of the mitochondrial genome is present in the species of the genus Plasmodium (the smallest mitochondrial genome).
Humans have a 16.8 kb mitochondrial genome.
Animals have less than 30 kb, yeast cells have 78 kb, and plants have 100 kb to 2 Mb of mitochondrial genome.
Most of the mitochondrial genome in plants is composed of junk DNA. 90% of the genome has repeated sequences and also contains introns, pseudogenes, insertion elements, viral genes, chloroplast genes, and nuclear genes.
Interestingly, the fungal mitochondrial genome contains introns but the mammalian mitochondrial genome does not contain introns.
Laboratory techniques for the mitochondrial study
Microscopic observation of mitochondria Michaelis was the first person to stain mitochondria using a dye, Janus green-B.
Mitochondria can be observed using a phase-contrast microscope and an interference microscope.
Rhodamine is a fluorescent dye that is more sensitive than other dyes, and it has been used in mitochondrial isolation from intact cultured cells. These types of stains are more appropriate for in-situ mitochondrial metabolic studies.
Mitochondria can also be observed by green fluorescence protein (GFP)-tagging methods and by immune fluorescence techniques employing a mitochondrial-specific dye called DIOC-6.
The dynamic nature of the mitochondrial reticulum can be observed using time-lapse fluorescence microscopy.
Digitanin is a detergent that can be used to separate the outer membrane of the mitochondria from the remaining part called the mitoplast (inner membrane and matrix).
Cytochemical Marking of Mitochondrial Enzymes
Mitochondria have distinct marker enzymes in their different compartments for their histochemical markings, such as monoamine oxidase for the outer membrane, cytochrome oxidase for the inner membrane, adenylate kinase for the outer chamber, and malate dehydrogenase for the matrix.
Mitochondria can easily be isolated from cell fractions using differential centrifugation.
Homogeneous fractions of mitochondria can be obtained from the heart, liver, skeletal muscle, and some other tissues. In differential centrifugation, mitochondria will be sedimented at 5000 to 24000 g; while in living cells, by ultra-centrifugation (20,000 to 400,000 g), the intact mitochondria will be deposited at the centrifugal pole.
Scientific tips for mitochondrial membrane proteins:
Mostly, detergents such as lubrol or some other organic solvents may be used in the lab to get inner membrane fragments into vesicles, called submitochondrial vesicles.
These vesicles are produced in an inverted position, hence F0-F1 particles are seen on the outer surface of the vesicles.
In most of these vesicles, the complexes of electron transport and ATP synthase are arranged in reverse orientation. Hence, ATP synthase exhibits ATPase activity in this orientation.
These vesicles are isolated mainly to manipulate the positions of small molecules such as NADH and FADH2 and to study the cellular activity of the electron transport system, which is not possible in the intact mitochondrion.
The Cell: A Molecular Approach. 2nd edition.Sunderland (MA): Sinauer Associates; 2000.
Berardi MJ1, Shih WM, Harrison SC, Chou JJ. Mitochondrial uncoupling protein 2 structure determined by NMR molecular fragment searching. Nature. 2011 Jul 24;476(7358):109-13. doi: 10.1038/nature10257.
Kunji ER, Aleksandrova A, King MS, Majd H, Ashton VL, Cerson E, Springett R, Kibalchenko M, Tavoulari S, Crichton PG, Ruprecht JJ.The transport mechanism of the mitochondrial ADP/ATP carrier.Biochim Biophys Acta. 2016 Oct;1863(10):2379-93. doi: 10.1016/j.bbamcr.2016.03.015. Epub 2016 Mar 19.
Alberts B, Johnson A, Lewis J, et al.New York: Garland Science; 2002.Molecular Biology of the Cell. 4th edition.
Jonathan R. Friedman and Jodi Nunnari. Mitochondrial form and function. Nature. 2014 Jan 16; 505(7483): 335–343. doi: [10.1038/nature12985]