mitochondria, history and origin of mitochondria, function of mitochondria,

 

  MITOCHONDRIA  

Introduction: - 

Mitochondria (Gr., mito, thread; chondrion, granule) are thread like or granular structures of eukaryotic cells. These may assume rod-like shape called chondriosomes which may enlarge or aggregate to form massive spheroidal bodies called chondriospheres. These are not present in bacterial cells. Mitochondria are the 'power plants' which by oxidation release the energy contained in the fuel molecules or nutrients and make other forms of chemical energy. The main function of mitochondria is oxidative phosphorylation, which is an exergonic reaction, meaning that it releases energy. In prokaryotes, oxidation of organic material is carried out by plasma membrane enzymes. 

 History: - 

Kölliker (1880) was the first who observed the mitochondria in insects muscle cells. He called them as 'sarcosomes'. Flemming (1882) named the mitochondria as 'fila'. Altmannin 1894 observed them and named them Altmann's granules or bioblasts. The term 'mitochondria' was applied by Benda (1897-98). They were recognized as the sites of respiration by Hogeboom and his coworkers in 1948. Lehninger and Kennedy (1948) reported that the mitochondria catalyze all the reactions of the citric acid cycle, fatty acid oxidation and coupled phosphorylation.

Morphology of Mitochondria: - 

 Morphologically mitochondria may be in the form of filaments or small granules. These may assume rod-like shape called chondriosomes which may enlarge or aggregate to form massive spheroid bodies called chondriospheres.

1. Position- Mitochondria lie freely in cytoplasm, possessing power of independent movement and may take the form of filaments. In some cells they can move freely, carrying ATP where needed, but in others they are located permanently near the region of the cell where more energy is needed. E.g., in the rod and cone cells of retina mitochondria are located in the inner segment, in cells of kidney tubules they occur in the folds of basal regions near plasma membrane, in neurons they are located in the transmitting region of impulse, in certain muscle cells (e.g., diaphragm), mitochondria are grouped like rings or bracers around the I-band of myofibril. During cell division they get concentrated around the spindle. 

2. Number- The number of mitochondria varies a good deal from cell to cell and from species to species. A few algae and some protozoans have only single mitochondria. Their number is related to the activity, age and type of the cell. Growing, dividing and actively synthesizing cells contain more mitochondria than the other cells. In Amoeba (Chaos chaos), there may be as many as 50,000 mitochondria. In rat liver cells, these are few in number, about 1000 to 1600. Some Oocytes contain as many as 3, 00,000 mitochondria. 

3. Size- The average size of mitochondria is 0.5-1.0µ in diameter and about 2-8 µ in length. In exocrine cells of mammalian pancreas, they are about 10 µ long and in oocytes of amphibian Rana pipiens are 20-40µ long. Yeast cells have the smallest mitochondria.

 Ultra structure of Mitochondria: - 

The electron microscope shows the mitochondrion as the vesicles bounded by an envelope of two unit membranes and filled with a fluid matrix. Granule Inner Membrane, Outer Chamber F1 Particle DNA Matrix Cristae Ribosome Outer Membrane 

1. Membranes- Both the inner and the outer mitochondrial membranes resemble the plasma membrane in molecular structure. Each of them is 60-70Å, tri lamellar and composed of two layers of phospholipid molecules sandwiched between two layers of protein molecules. However, the two membranes differ in the kinds of protein and lipids they have and also in their properties. Both the outer and the inner membranes contain specific pumps or channels, for the transport of molecules through them. The membranes may be connected at adhesion sites through which proteins are transferred from the outer to the inner membrane. The outer and the inner membrane are separated from each other by a narrow space called the inter-membrane space or outer chamber or peri-mitochondrial space. It is about 80Å wide. It contains a clear homogeneous fluid. 

(i) Outer Membrane- The outer membrane is smooth permeable to most small molecules, having trans-membrane channels formed by the protein 'porin'. It consists of about 50% lipid, including a large amount of cholesterol. It contains some enzymes but is poor in protein.

(ii) Inner Membrane- The inner membrane is selectively permeable and regulates the movement of materials into and out of the mitochondrion. It is rich in enzymes and carrier proteins permease. It has a very high protein/lipid ratio (about 4:1 by weight). It lacks cholesterol. Cardiolipin is closely associated with certain integral proteins and is apparently required for their activity. 

2. Matrix- The space between the cristae called the inner chamber is filled with a gel like material termed the mitochondrial matrix. It contains proteins, lipids, some ribosomes, RNA, one or two DNA molecules and certain fibrils, crystals and dense granules. 

3. Cristae- The inner mitochondrial membrane bears plate like infoldings called the cristae. They extend inwards to varying degrees, and may fuse with those from the opposite side, dividing the mitochondrion into compartments. They are arranged in a characteristic manner in different cells. Normally they run at right angles to the long axis of the rod- shaped mitochondria. In cells of the proximal parts of the kidney tubules, the cristae are longitudinal folds parallel to the long axis of mitochondrion. In many protozoans, in insect flight muscles cells and in adrenal endocrine cells the cristae are tubular. Cristae are lamellar in hepatocytes. In heart muscle cells cristae are zigzag.  They also vary in number. The active cells may have numerous cristae whereas the inactive cells may have only a few. The cristae have in them a narrow intra-crista space. It is continuous with the inter-membrane space. The cristae greatly increase the inner surface of the mitochondrion to provide enough space for housing enzyme assemblies. The cristae also allow for expansion or swelling of mitochondria under different metabolic and environmental conditions.

4. Oxysomes- The inner mitochondrial membrane bears minute regularly spaced particles known as the inner membrane subunits or elementary particles (EP) or oxysomes. An oxysome consists of three parts- a rounded head piece or F1 subunit joined by a short stalk to a base piece or F0 subunit located in the inner membrane. There may be 100,000 to 1000,000 oxysomes in a single mitochondrion.

Biogenesis of Mitochondria: - 

The formation of new mitochondria has been explained with the following hypothesis. 

1. De Novo Synthesis- According to this hypothesis mitochondria arises de novo from precursors in the cytoplasm. 

2. Origin from membrane- This hypothesis proposes that the mitochondria arises from the invaginations of plasma membrane, endoplasmic reticulum, Golgi apparatus or nuclear envelop. The membrane invaginates and extends into the cytoplasm as a tubular structure. It gradually becomes curved and folded and forms a double walled structure, the mitochondrion. 

3. Develop from Micro bodies- It is held that they mitochondria are developed by the accumulation of micro bodies in the cytoplasm. A micro body consists of a single outer membrane and a dense matrix with a few cristae which eventually develops into fully formed mitochondria. 

4. Prokaryotic Origin- It is believed that mitochondria are originated from bacteria. It is supported by many evidences. 

(i) First is the localization of enzymes of respiratory chain, which in case of bacteria, are localized in plasma membrane which can be compared with the inner membrane of the mitochondrion. 

(ii) In some bacteria, plasma membrane forms membranous projections (called mesosomes) like cristae of mitochondria. These mesosomes possess respiratory chain enzymes. 

(iii) The mitochondrial DNA is circular as it is in bacteria. Replication process of mitochondria is similar to bacteria. 

(iv) Ribosomes in mitochondria are smaller and similar in size to that of bacterial ribosomes. Chloramphenicol inhibits the synthesis of protein in mitochondria as well as in bacteria. Furthermore, in the process of protein synthesis, mitochondria depend partially on mitochondrial matrix and DNA and partially on nucleus and cytoplasm of the eukaryotic cells. It exhibits the symbiotic nature of mitochondria. These evidences support the prokaryotic origin of mitochondria. 

5. Replication- It is held that mitochondria are self-replicating organelles. New mitochondria arise by some type of splitting process from pre-existing mitochondria. The last hypothesis seems probable. Since the mitochondria have their own DNA and ribosomes, they can replicate new mitochondria. However, there is a nuclear control over the process as the mitochondria synthesize some of their proteins themselves and get others from the cytoplasm of the cell formed under the direction of the nuclear DNA

Functions of Mitochondria: - 

Mitochondria perform the following functions: - 

1. Cell respiration takes place in mitochondria and so they are known as the 'powerhouse' of the cell. 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 the energy-requiring activities in the cells, namely, biosynthesis, active transport, transmission of nerve impulse, muscle contraction, cell growth and division and bioluminescence. 

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

3. Some amino acids are also formed in the mitochondria. 

4. Mitochondria actively accumulate calcium ions as calcium phosphate precipitate. They regulate the calcium ions concentration in the cytoplasm by storing and releasing Ca+ The calcium ions regulate numerous biochemical activities in the cell

 Electron Transport Mechanism: - 

In the electron transport chain electrons are transferred from a donor molecule to an acceptor molecule, thus, it consists of a several electron receptors. Molecular oxygen is the final hydrogen acceptor. The respiratory chain is located in the inner mitochondrial membrane. In the respiratory chain, the electron transfer is done in stepwise fashion in which the electron pairs are passed from one acceptor to another, thus, delivering energy more gradually. Flow of electrons in mitochondria occurs as follows: 

Self-Assessment Questions and Possible Answers: - 

Multiple Choice Questions: - 

1. Cell's power houses are its: 

 (a) Lysosomes (b) Mitochondria (c) Ribosomes (d) Golgi apparatus 

2. Mitochondrion is bounded by: 

 (a) A single unit membrane (b) Two-unit membranes (c) No membranes (d) Plasma membranes 

3. New mitochondria arise: 

 (a) De novo (b) By replication (c) From plasma membrane (d) from nuclear envelop 

4. The ATPase enzyme is located in the mitochondria in: 

 (a) Oxysomes (b) Outer membrane (c) Inner membranes (d) Matrix 

5. The name mitochondria were given by: 

 (a) Altman (b) Flemming (c) Benda (d) Kollikar 

6. ETS is located in: 

 (a) Outer mitochondrial membrane (b) Inter membrane space(c) Inner mitochondrial membrane   (d)mitochondrial matrix 

ANSWERS 

1. (b) 2. (b) 3. (b) 4. (a) 5. (c) 6. (c)

Very short questions: 

1. Where are ETS enzymes located in mitochondria? 

2. Give the function of mitochondria. 

3. What are cristae? 

4. What type of DNA do mitochondria have? 

5. Mention three parts of oxysome. 

6. Who named mitochondria? 

7. What kind of enzymes is present in the mitochondria? 

8. Name the enzymes oxysomes represent. 

9. Which is the most common energy carrier in cells? 

10. Give alternative names of oxysomes. 

ANSWERS: -

1. Inner membrane 

 2. ATP formation 

 3. Infolds of inner mitochondrial membrane 

 4. Circular, single molecule and double stranded 

 5. Head piece, stalk and base piece (FO & F1) 

 6. Benda 

 7. Respiratory enzymes 

 8. ATPase (ATP Synthetase) 

 9. ATP 

 10. Elementary particles, inner membrane subunits, F0-F1 Complex.