Mitochondria: Meaning, Structure and Functions

In this article we will discuss about:- 1. Meaning of Mitochondria 2. Shape and Size of Mitochondria 3. Structure 4. Autonomy 5. Functions.

Meaning of Mitochondria:

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. 8.34). 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.

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. 8.35 A). 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. 8.35 A). 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.

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:

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).