Mitochondria | NEET BIOLOGY

 STD-11     UNIT-3     CHA-8

CELL : STRUCTURE AND FUNCTIONS

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 1880. 
• 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 nongreen plant cells and animal cells.
• The position of mitochondria in a cell depends upon the requirement of energy and amino acids.
• In unspecialised 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 

• Commonly mitochondria are cylindrical in outline.
• The size of the mitochondria is variable.
• Normally, they have a length of 1.0-4.1 um and a diameter of 0.2-1.0 um (average 0.5 um).

Chemical al Composition 

• Proteins . 60-70 %, Lipids 25–35 %, RNA 5-7 %, DNA, Small quantity Minerals, Traces, Granules Manganese and Calcium phosphate. 

Ultrastructure 

• A mitochondrion contains two membranes and two chambers, outer and inner.
• The two membranes form the envelope of the mito chondrion.
• Each of them is 60-75Å in thickness.

• Outer Membrane 
• The membrane is smooth
• It is permeable to a number of metabo lites 
• 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 phos pholipid 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 infolded 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 respirationThe inner membrane as well as its cristae possess small tennis - racket like particles called elementary particles, Fo - F1, particles or oxysomes ( = oxisomes ). 
• Elementary particles function as ATP- ase.
• They are, therefore, the centres of ATP synthesis during oxidative phosphorylation. 
• Both head and stalk constitute F1. Fo or base has a roter and a stator.
• A chan nel 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 transfer of materials from outside to inside and vice versa adhesion sites. Adhesion sites are special permeation regions of the mitochondrion for transfer of material 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 Å 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 cominonly circular but can be linear.
• DNA makes the mitochondrion semi - autonomous.

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