Mitochondria: Morphology, structure and Function

Mitochondria: Morphology, structure and Function

Mitochondria (Gr: mito = thread; chondrion = granule) (Singular: mitochondrion) are most essential cell-organelles found distributed in the cytoplasm of plant and animal cells and  protozoa.

However, they are not found in bacteria and  red blood cells of multicellular animals. Mitochondria are characterized by a number of morphological, biochemical and functional properties like, size,shape, special staining properties, the specific structural organization, lipoprotein composition, and presence of certain specific enzymes and coenzymes.

From the physiological viewpoint, mitochondria are known as "Biochemical machines “because they recover the energy contained in the food stuffs through Kerb's Cycle and the respiratory chain. The energy is produced in the form of ATP (adenosine-triphosphate) by the process  phosphorylation. ATP possesses the high energy phosphate bond. Most commonly mitochondria are also known as Power-House of the cell because they produce the energy necessary for many cellular functions.

• HISTORY

Mitochondria were first observed by Kollicker in 1850. Flemming (1892) named them 'fila', Later on Altmann (1894) described them as bioblasts. Benda (1897) named them asmitochondria. Michaelis (1900) stained them supravitally with janus green. Regard (1908) established their chemical nature. Kingsbury (1912) suggested that mitochondria were the sites of cellularoxidation.

 Lewis and Lewis (1914) observed their sensitivity to metabolic conditions. Mever (1918) described their transformation into various other type of cells. Keilin (1923) demonstrated the effects of cyanide and carbon monoxide on respiration. Hogeboom, et al. in (1948) finally confirmed that mitochondria were indeed the sites of cellular respiration. Very recently it has confirmed that mitochondria contain a specific type of DNA different from that of nuclear DNA.

They perform the process of protein synthesis in them and also take part in the genetical phenomenon of "Plasma inheritance". This plasma-inheritance where the characters are transmitted through the mitochondria of female, is known as mitochondrial inheritance.

• MORPHOLOGY:

shape:The shape of mitochondria varies according to the type of cell, In general, they are sausage shaped or filamentous and rod-shaped organelles that can be considered the power generators of the cell,

Depending upon the physiological conditions they may be club-shaped, tennis recket-shaped, rod-shaped or vescular in shape. Such shapes are only for a short period and after 48 hours these changes cease and the mitochondria regain their original form.

SIZE:

       The size of mitochondria is also variable and chiefly depends upon the functional stage of the cell. The length of mitochondria varies from 1.5 u to 7 u and the width is relatively constant being 0.5 u. Very thin mitochondria are about 0.2 u and thick about 2 u. In fixed preparations their size and shape depend upon osmotic pressure and pH of the fixative. In acidic pH they tend to fragment and to become vescicular. Mitochondria of rat-liver are 3.3. u in length; of mammalian exocrine pancrease about 10 u in length; and of amphibian cocytes about 20 to 40 u in length.

NUMBER:

         Like shape and size, number of mitochondria also varies according to the type and functional activities of the cell. Generally, a cell has 200 to 300 mitochondria but their number may reach from a few to 1000 or more. Some algal cells have been reported to have only one mitochondria. A normal liver cell possesses about 1000 to 16,000 mitochondria; eggs of sea urchin 14,000 to 150,000; ovocytes 300,000; and Chaos about 500,000 mitochondria. The animal cells posses greater number of mitochondria than green plant cells. Mitochondria are rod-shaped organelles that can be considered the power generators of the cell.

• LOCATION:

  In general the mitochondria are found uniformly distributed in the cytoplasm. However, they may exceptionally found around the nucleus and towards the periphery of the cytoplasm. Such exceptions are frequent in pathological conditions but sometimes may be over-loading with inclusions like glycogen and fat.Mitochondria are usually observed in large number in that area of the cell where the metabolic activities are at their maximum. During mitosis, they are found concentrated near the spindle.Mitochondria either move freely in the cytoplasm or are permanently placed near the regions of the cell which need more energy.

• ORIENTATION:

             Mitochondria possess a more of less definite orientation in some cells. For example, in cylindrical cells, their orientation is in the basal-apical direction, parallel to the main axis. In leukocytes, their arrangement is redical with respect to the centrioles. The orientations of mitochondria depend upon the direction of the diffusion currents within cells (Pollister, 1941) and are also related to the sub-microscopic organisation of the cytoplasmic matrix and vacuolar system.

Structure of Mitochondria:

A mitochondrion contains two membranes and p,g g 34 structure of a mitochondrion, two chambers, outer and inner. The A mitochondrion partly cut open to show two membranes form the envelope of the mitointernal and external structure, chondrion. Each of them is 60-75A in thickness.

Outer Membrane:

The membrane is smooth. It is permeable to a number of metabo­lites. It is due to presence of protein channels called porins or minute pores. A few enzymes connected with lipid synthesis are located in the membrane. It is poorer in proteins as compared to inner membrane.

Inner Membrane:

It is permeable to only some metabolites. It is rich in double phos­pholipid called cardiolipin (having four fatty acids) which makes the membrane impermeable to ions. Protein content is also high, being 70—75% of total components. The inner membrane is in-folded variously to form involutions called cristae. They are meant for increasing the physiologically active area of the inner membrane.

The cristae are generally arranged like baffles, at right angles to the longitudinal axis of the mitochondrion. They are tubular (most plant cells) or plate like (most animal cells) or vesicle-like (e.g., Euglena). A crista encloses a space that is continuation of the outer chamber. The density of cristae indicates the intensity of respiration.

A chan­nel occurs between roter and stator for passage of protons (H+). Stator is connected to head region by an arm. Enzymes of electron transport are located in the inner membrane in contact with elementary particles.

At places, outer and inner mitochondrial membranes come in contact. They are called adhesion sites. Adhesion sites are special permeation regions of the mitochondrion for transfer of materials from outside to inside and vice versa.

Outer Chamber (Peri-mitochondrial Space):

The chamber is the space that lies be­tween the outer and inner membrane of the mitochondrial envelope. Usually, it is 60-100 A wide. It extends into the spaces of the cristae.The chamber contains a fluid having a few enzymes.

Inner Chamber:

 It forms the core of the mitochondrion. The inner chamber contains a semi-fluid matrix. The matrix has protein particles, ribosomes, RNA, DNA (mitochondrial or mDNA), enzymes of Krebs or TCA cycle (except succinate dehydrogense which is membrane based), amino acid synthesis and fatty acid metabolism, crystals of calcium phosphate and manganese.

Mitochondrial ribosomes are 55 S to 70 S in nature. They thus resemble the ribosomes of prokaryotes. DNA is naked. It is commonly circular but can be linear. DNA makes the mitochondrion semi-autonomous.

Functions of Mitochondria:

1. Mitochondria are miniature biochemical factories where food stuffs or respiratory substrates are completely oxidized to carbon dioxide and water. The energy liberated in the process is initially stored in the form of reduced coenzymes and reduced prosthetic groups.

The latter soon undergo oxidation and form energy rich ATR ATP comes out of mitochondria and helps perform various energy requiring processes of the cell like muscle contraction, nerve impulse conduction, biosynthesis, membrane transport, cell division, movement, etc. Because of the formation of ATP, the mitochondria are called power houses of the cell.

2. Mitochondria provide important intermediates for the synthesis of several bio-chemicals like chlorophyll, cytochromes, pyrimidine’s, steroids, alkaloids, etc.

3. The matrix or inner chamber of the mitochondria has enzymes for the synthesis of fatty acids. Enzymes required for the elongation of fatty acids have been reported in the outer mitochondrial chamber.

4. Synthesis of many amino acids occurs in the mitochondria. The first formed amino acids are glutamic acid and aspartic acid. They are synthesized from a-ketoglutaric acid and oxaloacetic acid respectively. Other amino acids are produced by transformation and tran­samination or transfer of amino group (—NH₂) from glutamic acid and aspartic acid.

5. Mitochondria may store and release Calcium when required.

6. An organism generally receives mitochondria from its mother (maternal inheritance).

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