NEET 2020 Study Notes: Cell Structure - Mitochondria

Mitochondria is a  semi-autonomous organelle as it contains its own DNA, RNA and ribosome. Produces energy i.e., ATP. This formation of energy is known as oxidative phosphorylation. They are therefore known as the powerhouse of a cell.

Important Information

• Has enzymes like dehydrogenase, synthetases etc. that help in oxidative deamination of amino acids.
• Kolliker in 1880 observed mitochondria in muscle cells of insects.
• Also known as chondriosome, sarcosome and plastochondria.
• Chondriome- a term used for collective mitochondria of a cell.
• They are stained with Janus green.
• Found in all eukaryotes except RBC’s of mammals. Absent in prokaryotes.
• Found in the cytoplasm of all cells that require oxygen to respire.
• Higher the metabolic activity of a cell, higher the no. of mitochondria found.
• Normal liver cell contains 10 to 1500 mitochondria.
• 30 % of the total content of cells in the liver is of mitochondria. Kidney cells have 20%.
• Number increases in muscles as many times thyroxine is given to the organism and also during hyperthyroidism.
• Sperm has fewest mitochondria 20-25. Oocytes may have as many as 19,000-20,000.
• 12,000 to 13,000 in large sea urchin’s eggs.
• Algae cells may have only one mitochondrion.

Distribution

• Evenly distributed in the cytoplasm
• Between the myofibrils in skeletal muscles.
• Clefts b/w the myofibrils of cardiac muscles have mitochondria in association with lipids.
• In sperms, they fuse with one another and are found in the mid piece.
• Found arranged radially in leucocytes.
• Each fibril of insect flight muscle has mitochondria.
• PCT of kidney contains mitochondria in cell’s basal region.

Size and Shape

• Length varies from cell to cell, found up to 6microns. Width about 0.5 microns.
• Thinnest mitochondrial- 0.2 microns. Thickest (rod-like)- 2 microns.
• Smallest size found in yeast cells ( 1micrometere cube) and largest size in the oocyte of amphibians. (20 to 40micrometre)
• 3 microns length of mitochondria found in rat liver.
• The shape may be Filamentous or granular.
• Also seen in the club-shaped, shape of a tennis racket, vesicular, rod-shaped, etc.

Structure

• Bounded by 2 membranes i.e., outer and inner, separated by inter membranous space or also known as peri-mitochondrial space.
• This space is filled with watery fluid in it.
• Integral proteins called porins are present on the outer membrane. Porins make it permeable.
• Inner membrane space has matrix filled in it and is selectively permeable. The matrix is composed of ribosomes, granules (insoluble Organic salts) and DNA. Matrix also has enzymes that are used in Kreb’s cycle.
• The side of the inner membrane that faces the matrix is M side whereas the side that faces the outer membrane is the C side. The side facing inter membranous space is O face.
• The inner membrane is thrown up into folds that form the cristae of mitochondria.
• These cristae have cavities in them known as an inter-cristae space, and are continuous with the inter-membrane space.

Mitochondrial particles

• The side of the inner membrane that faces the matrix i.e., M side have small stalked particles called subunits of Fernandez-Moran particles/Elementary particle/electron transport particles/FI particles or the oxysomes. Also called F0 - F1
• Oxysomes have a base, a stalk/pedicel and a head and there may be up to 105 oxysomes in a mitochondrion. The particles contain the enzyme ATP synthetase so they are associated with ATP production during cellular respiration.

Chemical Composition

• According to Lindberg and Ernster, proteins are 70 to 75%, Lipids 25-30%, DNA 2% and RNA 5% of the dry weight in mitochondria.
• They are rich in Manganese.
• Enzymes- 70 Enzymes and 12 coenzymes are found in mitochondria.

• Proteins- Outer membrane – 10%; Inner membrane- 60%
• Lipids- Outer membrane – 40%; Inner membrane- 20%
• DNA- depending on the size of mitochondria, DNA may be one or more. Approx= 2-5%
• RNA- 5-7%
• Ribosome found have a sedimentation coefficient of 70S.
• Semi-autonomous nature- it contains DNA, RNA, and 70S ribosome.

Mitochondrial DNA

• closed circular DNA with high G-C ratio. This DNA is 1% of the total DNA of the cell.
• Presence of DNA, ribosomes make the mitochondria independent for the production of their own proteins in presence of ATP.

Functions of mitochondria

• Produces energy i.e., ATP. This formation of energy is known as oxidative phosphorylation. They are therefore known as powerhouse of a cell.
• It has enzymes like dehydrogenase, synthetases etc. that help in oxidative deamination of amino acids.
• Oxidize fatty acids and use them to produce energy while starvation of cell.
• In a growing ovum, it helps in forming yolk.
• Have respiratory chain complexes which perform the electron transport system.
• Help in haem synthesis in rat liver cells and avian red blood corpuscles.
• Forms the middle piece of the sperm below the acrosome.
• Mitochondrial DNAhase the gene for male sterility in maize plant.

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