Mitochondria




MITOCHONDRIA ( Chondriosome):

The mitochondria are visible under a light microscope. However, their detailed structure has been revealed by electron microscope. 
Discovery: The mitochondria were first seen in 1880 by Kolliker,who isolated them from insect muscle cells. They were named mitochondria by Benda in 1898.They were recognized as the site of respiration by Hoogeboom and coworkers in 1948.

Mitochondria are called powerhouses of aerobic  eukaryotes  because  they take part in oxidative phosphorylation and Krebs cycle of aerobic respiration and are the major centres of release of energy in the aerobic respiration.They are stained differentially by Janus Green and can be seen under light microscope, but their ultrastructure is identifiable only under the electron microscope. 

   Mitochondria are absent in prokaryotes and anaerobic  eukaryotes  and are secondarily lost in the red blood corpuscles of mammals. Their number which depends upon cellular  activities  varies from one in some algae ( e.g., Microasterias, Chlorella), 500_ 1000 in liver cell, 30,000 in some oocytes, 50,000 in giant amoeba named Chaos chaos and 500,000   in flight muscle cells. Cells of dormant seeds have very  few mitochondria  while those of germinating seeds have several mitochondria. In general green plant cells contain less number of mitochondria as compared to nongreen plant cells and animals cells. 

In a cell the position of mitochondria depends upon the requirements of energy  and amino acids being randomly  distributed  throughout  the  cytoplasm in unspecialised cells and lie in the peripheral  cytoplasm in absorptive and secretory  cells. During nuclear division, more of mitochondria come to lie around the spindle. Mitochondria  are more abundant  at the bases of cilia or flagella while in muscle fibres they occur in rows in the regions of light bands in between  the contractile  elements. 




What is the  Location of Mitochondria?

The mitochondria are found in a aerobic  eukaryotic cells. They are lacking in certain unusual anaerobic protozoans. They are absent in the mature red blood corpuscles, although they are present before these cells mature. They tend to slowly disappear in the old cells.  Prokaryotic cells are without mitochondria. Their   cell membrane bears respiratory  enzymes.

         The mitochondria are often concentrated in the more active regions of the cells where energy_ requiring processes occur. For example, in the muscle cells, they lie in rows sandwiched between the contractile elements; in the sperms, they surround the longitudinal fibrils that causes movement of the tail; in the gland cells, they are near the secretory surface; in the intestinal epithelial cells, they are near the absorptive surface; in a dividing cells, they come to aggregate around the spindle; and in cilia_ bearing cells, they are aggregate at the base of the cilia. These Locations of mitochondria quickly deliver   ATP for cell activities. The cell in the growing root tips have many mitochondria. 

What is the Structure of  Mitochondria? 

Form, size and number of mitochondria vary but are characteristic for each cell type.

(a) Form: The mitochondria are usually sausage_ shaped, but may be spherical, oval, cylindrical, filamentous, or even branched.

 (b) Size:The spherical mitochondria are 1_ 5 ųm in diameter.The cyclindrical mitochondria are usually 1_ 4 ųm long and 0.2 __ 1 ųm thick.

(c)Number: There may be a few to over 1,000 mitochondria per cell. Growing, dividing and actively synthesizing cells of dormant seeds.

    Green plant cells, in general, have fewer mitochondria than the non green plant cells and animal cells because their chloroplasts also form ATP.

     There may be over 5,000 mitochondria in each insect flight muscle cell, which is a unit of the most mechanically active tissue in nature. A liver cell, which is a busy biochemical factory, may contain about 2500 mitochondria. The alga Chlamydomonas has a single mitochondria. A sperm ce has about 25, a kidney cell about 300 _ 400, certain oocytes about 30,000, and the giant amoeba,  Chaos Chaos upto 50,00p mitochondria. 




(d) Components of mitochondria:

Under the light microscope, the mitochondria are seen as small spheres or short rods, hence their name ( G. mito= thread, chondrion=granule). The electron microscope shows them as vesicle bounded by an envelop of two unit membranes and filled with a fluid matrix.



(i) Membranes : 

Both the membranes resemble the cell membrane in structure. However, they are slightly thinner than the cell membrane.  Each membrane is 60 to 70 A° thick and composed of two layers of phospholipid molecules sandwiched between two layers of  protein  molecules. However, the two membranes differ in the kinds of proteins and lipids they have and also in their properties. Both the membranes contain specific pumps, or channels, for the transport of molecules through them. The two membranes may be connected at adhesion sites. Protein  are transferred from the outer to the inner membrane through the adhesion sites. The two membrane are separated from each other by a narrow space called inter membrane space, or outer chamber.It contains a clear, homogeneous fluid.



( a) Outer Membrane:

The outer membrane is smooth, freely permeable to most small molecules, contains fewer enzymes and is poor in proteins.  It has porin proteins which form channels for the passage of molecules through it. It allows uptake of substrates and release of  ATP. 

   It is mooth and permeable to number of metabolites due to the presence  of porins or minute pores. Its composition  is similar  to  that of E.R. A few  enzymes are located in the membrane.  Lipids  in outer  membrane are cholesterol and other lipids. Cardiolipins do not occur in outer membrane. Outer membrane has no electron transport system.It is poorer in  proteins as compared to inner membrane and contains more phospholipids and cholesterol  than the inner membrane. 

( b) Inner Membrane:

The inner membrane is semipermeable and regulates the passage of materials into and out of the mitochondrion. It is rich in enzymes and carrier proteins ( permeases). It is usually produced into numerous infolds called cristae ( singular crista) meant for increasing  the physiologically active area of the inner membrane. The  greatest concentration  of cristae is found in the flight muscles of insects. The cristae are generally arranged  like baffles. They enclose a space that is continuation of the outer chamber. The density of cristae indicates the intensity of respiration. The side of inner membrane facing the matrix side is called M_ side, while the side facing the outer chamber is called the C_ side.

( ii) Cristae: 

The cristae extend inward to varying degrees and may fuse with those from the opposite side, dividing the mitochondrion into compartments.The cristae are  arranged in characteristic ways in different cells. They may be simple or branched, straight or zigzag, lamellar ( most animals) or tubular ( most plants). The cristae vary in number also. The active cells may have very numerous cristae whereas the inactive cells may have only a few. The cristae have in them a narrow intracristal space.It is continuous with the inter membrane space.

       The cristae greatly increase the inner surface area of the mitochondria to hold a variety of enzymes.

(iii) Oxysomes:

The inner mitochondrial membrane bears minute regularly spaced lollipop_ shaped, about 8.5 nm wide particles known as the inner membrane subunits, or elementary particles (EP), or oxysomes. An oxysome consists of 3 parts __ a rounded head piece, or F1 subunits, joined by a short stalk to a base piece, or F0 subunit,located in the inner membrane. There may be 100,000 to 1,000,000 oxysomes in a single mitochondrion.The oxysome, also called  F0__ F1 complex, represent adseosine triphosphatase, or ATPase,  or ATP  synthetase,  enzyme and in thus concerned with ATP formation. The rest of the inner mitochondrial membrane contains the electron carrier molecules ( coenzymes) of the electron transport chain.



Outer  chamber ( peri_ mitochondrial Space):It is the  space lying between  the outer and inner membrane of the mitochondrial envelope. It extends into the spaces of the cristae  and contains  a fluid having  a few enzymes.


( iv) Matrix:

The wide space between the cristae is called the  inner chamber.It is filled with a dense fluid termed the mitochondrial matrix.The matrix contains proteins, lipids, some ribosomes,  RNA, one or two  DNA molecules and certain ions, fibres, crystals and granules. Major portion of the proteins consists of  enzymes concerned with mitochondrion's energy _ producing activities ( Krebs cycle reactions). The ribosomes are 70S in size and have contents similar to those of the prokaryotic cells. They are called mitoribosomes in contrast to the 80S cytoribosomes that occur in the cytoplasm. The DNA  molecules are circular, short and with few proteins again as in the prokaryotic cells.All the three types of RNAs  ( rRNA, tRNA and mRNA) are present in the mitochondrial matrix.The ions include Ca++ and Mg++ .These are necessary for the functioning of mitochondrial enzymes. 


Origin:

Mitochondria are self _ duplicating organelles.  New mitochondria arise by the division of existing ones. This is another  prokaryotic  feature. Cells cannot form raw materials. Mitochondria are continually renewed. In rat liver cells, they are replaced in about 10 days.








 Functions of Mitochondria:

The mitochondria have the following functions ___ 

(i) Mitochondria are the main seat of cell respiration. They bring about stepwise oxidation of food __ stuffs or " low _ grade" fuel of the cell and transfer the energy, so released, to the energy carrier ATP, the " high_ grade" fuel of the cell. ATP is used to bring about energy _ requiring activities in the cell. On this account, the mitochondria are often described as the "power _ houses",or " storage batteries", or " ATP mills"  of the cell.

( ii) Mitochondria provide intermediates for the synthesis of important biomolecules, such as chlorophyll, cytochromes, steroids, etc.

(iii) Some  amino acids are also formed in mitochondria. 

( iv) Mitochondria regulate the calcium ion concentration in the cell by storing and releasing Ca²+as needed .The calcium ions, in turn, regulate many biochemical activities in the cell.

( v) Matrix contains enzymes for the synthesis of fatty acids. 

(vi) Mitochondria act as miniature biochemical factories where food stuffs or respiratory substrates are completely oxidised and form energy rich ATP.

(vii) They take part in maternal inheritance as an organisms generally  receives mitochondria from its mother.

What is the Autonomy of Mitochondria ?

Mitochondria are semiautonomous organelles.The are capable of self _ duplication ( replication). For replication, they have
 ( i) genetic  information ( DNA), 

( ii) Self transcribed RNAs ( rRNA, tRNAs and mRNA), 

(iii) Protein _ making machinery ( ribosomes),

(iv) Energy _ producing mechanism ( respiratory enzymes), 

(v) Synthesze membrane material and also structural proteins and enzymes for their use.However, they can manufacture only some of their proteins. They get other proteins from  cytoplasm formed under the directions of nuclear DNA . During cell  division, each daughter cell receives some mitochondria from the mother cell. These mitochondria later replicate to restore the normal number in the  cell. 




Evolution:

Since mitochondria are capable of replicating, some workers think that they were once independent organisms ( bacteria).They got entrapped in larger   eukaryotic cells and became symbionts and then changed into their useful inseparable components in due course of time. This view is called symbiotic, or prokaryotic  hypothesis of mitochondrial origin. 

Inheritance:

A unique feature of mitochondria is that they are inherited from one's mother. The middle piece of a sperm, that contains mitochondria,may not enter the egg during fertilization. 

Differences Between Outer and Inner Mitochondrial Membranes? 

Outer  Membranes

1: It is smooths,having much less surface area.

2: It is freely permeable unlike biological membranes. 

3: It consists of 50% proteins and 50% lipids.

4: Its lipids include a large amount of cholesterol and no cardiolipins.

5: Its proteins include porins that form channels for free movement of molecules across the outer membrane. 

6: It is poor in enzymes. 

7: It lacks electron transport system. 

8: It is in contact with the cytoplasm. 

 Inner Membrane 

1: It is unfolded to form cristae, hence having very large surface area. 

2: It is semipermeable like most biological membranes. 

3: It consists of 80% proteins and 20% lipids.

4: Its lipids include cardiolipins and virtually no cholesterol. 

5: Its proteins include permeases that actively transport desired molecules across the inner membrane. 

6: It is rich in enzymes.

7: It bears electron transport system.

8: It is in contact with the matrix.


































































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