Mitochondria STRUCTURE AND FUNCTION

INTRODUCTION

Mitochondria are cell organelles of aerobic eukaryotes which take part in oxidative phosphorylation and Krebs cycle of aerobic respiration. They are called power houses of cell because they are the major centres of release of energy in the aerobic respiration.

They were first observed by Kolliker in 1850. Benda (1897) gave the present name of mitochondria (Gk. mitos- thread, chondrion- grain) to the organelles. Mitochondria can be stained differentially with Janus Green and are easily distinguishable under light microscope though ultrastructure can be studied only under electron microscope.

Mitochondria are absent in prokaryotes and anaerobic eukaryotes. Mitochondria are secondarily lost in the red blood corpuscles of mammals. Their number varies from one to several. The number depends upon cellular activities.

Cells of dormant seeds have very few mitochondria. Those of germinating seeds have several mitochondria. In general green plant cells contain less number of mitochondria as compared to non-green plant cells and animal cells.

The position of mitochondria in a cell depends upon the requirement of energy and amino acids. In unspecialized cells they are randomly distributed throughout the cytoplasm. In absorptive and secretory cells, they lie in the peripheral cytoplasm.

During nuclear division, more of mitochondria come to lie around the spindle. Mitochondria are more abundant at the bases of cilia or flagella to provide them energy for movements. In muscle fibres they occur in rows in the regions of light bands in between the contractile elements.

Shape and Size of Mitochondria

Commonly mitochondria are cylindrical in outline. The size of the mitochondria is variable. Normally, they have a length of 1.0-4.1 µm and a diameter of 0.2-1.0 µm (average 0.5 µm). Chemical Composition. Proteins. 60-70%, Lipids 25-35%, RNA 5-7%, DNA. Small quantity. Minerals. Traces, Granules Manganese and Calcium phosphate.

Structure of Mitochondria

A mitochondrion contains two membranes and p,g g 34 structure of a mitochondrion, two chambers, outer and inner (Fig. ). 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.

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.

The inner membrane as well as its cristae possess small tennis-racket like particles called elementary particles, F₀ – F₁ particles or oxysomes (= oxisomes).

A mitochondrion contains 1 x 10⁴ – 1 x 10⁵ elementary particles (Fig.). Each elementary particle, F₀ –F₁ particle or oxysome has a head, a stalk and a base (Fig. 8.35 B). The base (F₀ subunit) is about 11nm long and 1.5 nm in thickness. The stalk is 5 nm long and 3.5 nm broad.

The head (F₁ subunit) has diameter of 8.5 nm. Elementary particles function as ATP-ase. They are, therefore, the centres of ATP synthesis during oxidative phosphorylation. Both head and stalk constitute F₁. F₀ or base has a roter and a stator.

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 (Fig). The chamber contains a fluid having a few enzymes.

Inner Chamber

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.

Autonomy of Mitochondria

Mitochondria show a large degree of autonomy or independence in their functioning.

1. Mitochondria have their own DNA which can replicate independently.
2. Mitochondrial DNA produces its own mRNA, tRNA and rRNA.
3. The organelles possess their own ribosomes.
4. Mitochondria synthesise some of their own structural proteins. However, most of the mitochondrial proteins are synthesised under instructions from cell nucleus.
5. The organelles synthesise some of the enzymes required for their functioning.
6. They grow internally.
7. New mitochondria develop by division/binary fission of pre-existing mitochondria.

However, mitochondria are not fully autonomous. Both their structure and functioning are partially controlled by nucleus of the cell and availability of materials from cytoplasm. Mitochondria are believed to be symbionts in the eukaryotic cells which became associated with them quite early in the evolution.

Functions of Mitochondria

• 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.

• Mitochondria provide important intermediates for the synthesis of several biochemicals like chlorophyll, cytochromes, pyrimidine’s, steroids, alkaloids, etc.
• 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.
• Synthesis of many amino acids occurs in the mitochondria. The first formed amino acids are glutamic acid and aspartic acid. They are synthesized from aketoglutaric 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.
• Mitochondria may store and release Calcium when required.
• An organism generally receives mitochondria from its mother (maternal inheritance).

Ultrastructure of Mitochondria

In 1953, Palade and Sjostrand independently described the ultrastructure of mitochondria. Mitochondria are bounded by an envelope consisting of two concentric membranes, the outer and inner membranes. The space between the two membranes is called inter-membrane space.

A number of invaginations occur in the inner membrane; they are called cristae (Fig. 2.21). The space on the interior of the inner membrane is called matrix.

Outer Membrane

The outer mitochondrial membrane has high permeability to molecules such as sugars, salts, coenzymes and nucleotides etc. It has many similarities with the ER but differs from it in some respects, e.g., mono-amine-oxidase is present in the mitochondrial outer membrane but not in ER.

On the other hand, the enzyme glucose-6-phosphatase is absent from the mitochondrial outer membrane but is present in ER. The mitochondrial outer membrane contains a number of enzymes and proteins (Table).

Inter-Membrane Space

The inter-membrane space is divided into two regions.

1. Peripheral space
2. Peripheral space

Large flattened cristae are connected to the inner membrane by small tubes called peduculi cristae which are few nanometers in diameter. The inter-membrane space has several enzymes of which “adenylate kinase” is the chief one (Table 2). This enzyme transfers one phosphate group from ATP to AMP to produce two molecules of ADP.

Inner Membrane

The inner mitochondrial membrane invaginates inside the matrix; the invaginations are called cristae (Fig.). This membrane has a high ratio of protein to lipid. “Knobs” or “spheres” of 8-9 nm diameter are spaced 10 nm apart on the cristae membranes. These knobs contain F₁ proteins and ATPase responsible for phosphorylation. They are joined to the cristae by 3 nm long stalks called “F₀“. The F₀-F₁ ATPase complex” is called ATP synthase.

The inner membrane contains large number of proteins which are involved in electron transfer (respiratory chain) and oxidative phosphorylation (Table 2.6). The respiratory chain is located within the inner membrane, and consists of pyridine nucleotides, within the inner membrane, and consists of pyridine nucleotides, flavoproteins, cytochromes, ironsulphur proteins and quinones.

Besides its role in electron transfer, and phosphorylation, the inner membrane is also the site for certain other enzymatic pathways, such as, steroid (hormone) metabolism.

Matrix

The interior of mitochondrion is called matrix (Fig.). It has granular appearance in electron micrographs. Some large granules ranging from 30 nm to several hundred nanometers in diameter are also present in the matrix. The matrix contains enzymes and factors for Krebs cycle, pyruvate dehydrogenase and the enzymes involved in β-oxidation of fatty acids. (Table ).

However, succinate dehydrogenase is present in the inner membrane instead of matrix; this enzyme catalyses the direct transfer of electrons from succinate to the electron transfer chain.

The enzyme pyruvate dehydrogenase converts pyruvate to acetyl-Coenzyme A (acetyl-CoA) which enters the Krebs cycle. Besides above, matrix also contains DNA, RNA, ribosomes and proteins involved in protein and nucleic acid syntheses.

Function of Mitochondria

Mitochondria is regarded as the power house of the cell as it is the site of respiration. The general formula for glucose oxidation is,

C₆H₁₂O₆ + 6O₂ ———-> 6CO₂ + 6H₂O + 686 kcal

Glucose is degraded into two pyruvate molecules through glycolysis which occurs in the cell sap (cytosol). Further steps in oxidation of pyruvate take place in the mitochondria. Pyruvate is converted to acetyl-Coenzyme A (acetyl-CoA) which is then metabolised through the Krebs cycle, also called the citric acid cycle or tricarboxylic acid cycle.

In this cycle, energy is liberated and CO₂ is produced. Some of the released energy is used to produce ATP, while a major part is conserved in the form of reduced coenzymes NADH and FADH₂ (FAD = flavinadenine dinucleotide). The energy conserved in NADH and FADH₂ is released by re-oxidizing them into NAD⁺ and FAD, respectively; the energy so obtained is utilized to produce ATP (oxidative phosphorylation).

This process occurs in different steps in a strict sequence called electron transfer chain or respiratory chain located in the cristae. The electrons are finally transferred to oxygen, and H₂O is produced at the end of this chain. The carriers of electrons are organized into three complexes, viz., I, III, and IV, and the sequence of electron transfer is as follows.

COMPLEX I (NADH ——> FMN group of NADH dehydrogenase ——> ironsulphurcentre ——> ubiquinone) ——> COMPLEX III (ubiquinone ——> cytochrome b ——> cytochrome c₁ ——> cytochrome C) ——> COMPLEX IV (cytochrome C——> cytochrome a ——> cytochrome a₃) ——> Oxygen.

There is another complex (Complex II) which transfers electrons from succinate (produced by Krebs cycle) to ubiquinone. At last O₂ is reduced to water, as the following reaction.

O₂ + 4e^– + 4H⁺ —> 2H₂O…(2.4)

In complete oxidation of one glucose molecule, 6 molecules of oxygen are utilized resulting in the production of 6 carbon dioxide and 6 water molecules; in addition, energy is released (see formula 2-3). The maximum number of ATP molecules produced during complete oxidation of one glucose molecule is 36

Reproduction in Mitochondria

Mitochondria originate by growth and division of pre-existing mitochondria. Their development requires the presence of oxygen. In the absence of O₂, yeast mitochondria are replaced by “pro-mitochondria” which are double-membrane vesicles without cristae. In the presence of O₂, cristae and other components of mitochondria develop so that pro-mitochondria convert into mitochondria.