Cellular Biology

Mitochondrial Disorders Oxysomes Matrix Cristae Atp Synthesis Mitochondria Function Structure



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Mitochondria are sausage-shaped, double membraned cytoplasmic bodies usually called "Power House Of Cell". These are responsible for ATP synthesis through cellular respiration. Mitochondria are present in almost all eukaryotic cell with the exception of protist Chaos (Pelomyxa) carolinensis.They show constant motion within the cell and tend to aggregate at places where energy requiring process is going on or at the parts of cell associated with active processes e.g in flagellated protozoas mitochondria are present around the base of flagella and in cardiac muscle, they surround the contractile elements. One of the richest sources of mitochondria is Hummingbird flight muscle.

Mitochondrian, 0.5 to 1.0 micrometer in diamete are bounded by outer and inner membrane, composed of phospholipid bilayers and proteins. Both membranes differ in their structure and function. Outer membrane is smooth and elastic helps in its extension & contraction. Porins (integral protein) present in outer membrane form channels through which small ions, nutrient molecules, ATP etc can pass easily. Any disruption in outer membrane allows proteins in the intermembrane space to leak into the cytosol (cytoplasmic matrix) which can lead to cell death.

Inner membrane shows folded structure, forms invaginations into the mitochondrial cavity to increase the surface area for cellular respiration. These invaginations are called cristae which can be branched or unbranched. Cristae are the sites for ATP synthesis. Inner membrane is permeable only to oxygen, carbon dioxide, and water etc. About 1/5 of the total protein in a mitochondrion is present here.

Inter Cellular Space is the space between outer and inner membrane play important role in oxidative phosphorylation. Due to the permeablility of outer membrane it is rich in protein especially cytochrome C and other small molecules.

Matrix is the fluid or substance fill the mitochondrial cavity inside the inner membrane. It is rich in ribosomes, DNA, proteins and manganese. Ribosomes (of 70S type) present here are different from cytoplasmic ribosomes. Its DNA is of circular shape called mitochondrial DNA. About 2/3 of the total protein of mitochondrion is present in matrix. The matrix contains highly-concentrated mixture of enzymes. Enzymes for Kreb s cycle are also present here. The matrix play important role in ATP synthesis with the help of the ATP synthase present in inner membrane.

Oxysomes are knobs like structures present on the outer surface of outer membrane and on the inner surface on inner membrane. Oxysomes present on outer side are stalkless while those attached to inner surface have small stalk for attachment, this stalk is called F0 particles and rounded part is called F1 particle. Both have enzyme ATP-ase and are related to ATP synthesis. These are proteinaceous in nature. Oxysomes act as electron carrier and oxygen released results in the formation of water and ATP molecule

Main function of mitochondria is energy production through ATP synthesis. It makes impossible for multicellular organisms to exist without mitochondria. They provide energy through the break down of respiratory substances for ATP synthesis used in cell metabolism.Through oxidative phosphoryation mitochondria make efficient use of nutrient molecules. Mitochondria are also involved in cell signalling (communication system to control basic cellular activities), cellular differentiation, apoptosis (PCD-programmed cell death) and in cell cycle and cell growth as well. Certain amount of protein synthesis also takes place here. Mitochondrial perform specific function in specific types of cells e.g mitochondria present in liver cells contain enzymes that allow them to detoxify ammonia (waste product of protein metabolism). Additionally mitochondria also regulates membrane potential, cellular proliferation and certain heme and steroid synthesis reactions. Mitochondria can also store calcium. There is a significant interplay between the mitochondrion and ER (significant storage site of calcium) regarding calcium storage.

Any damage to mitochondria can cause a wide range of disorders which mostly present as neurological disorders. Its improper functioning lead to energy break down which causes poor coordinations, sensory poblems and reduced mental activity etc. Errors in cellular information processing are responsible for diseases such as cancer, autoimmunity and diabetes.

Diseases caused by mutation in the mtDNA include Kearns-Sayre syndrome and MELAS syndrome. Mostly these diseases are transmitted by mother as the zygote derives its mitochondria and its mtDNA from the ovum. Large-scale mtDNA mutations can cause Kearns-Sayre syndrome, Pearson's syndrome and progressive external ophthalmoplegia while MELAS syndrome, Leber's hereditary optic neuropathy, myoclonic epilepsy with ragged red fibers are the outcome of small-scale or point mutations in mtDNA. Environmental factors may also influence mitochondrial disease. Influence of pesticide exposure on Parkinson's disease is an example of environmental influences.

Aging process also lead to a number of changes in mitochondria. Tissues of elderly patients show a decreased enzymatic activity. Even large deletions in the mitochondrial genome can lead to high levels of oxidative stress and neuronal death in Parkinson's disease. Hypothesized links between aging and oxidative stress show their symptoms after the age of 50 years.

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