11th Class Biology Cell - The Unit Of Life Mitochondria

Mitochondria (Gk. Mito = thread ; chondrion = granule) are semi autonomous having hollow sac like structures present in all eukaryotes except mature RBCs of mammals and sieve tubes of phloem. Mesosomes of prokaryotes (bacteria) is analogous to mitochondrion in eukaryotes.

Mitochondria are also called chondriosome, chondrioplast, plasmosomes, plastosomes and plastochondriane.

Discoveries

(1) These were first observed in striated muscles (Voluntary) of insects as granules by Kolliker (1880), he called them “sarcosomes”.

(2) Flemming (1882) called them “fila” for thread like structure.

(3) Altman (1890) called them “bioplast”.

(4) C. Benda (1897) gave the term mitochondria.

(5) F. Meves (1904) observed mitochondria in plant (Nymphaea).

(6) Michaelis (1898) demonstrated that mitochondria play a significant role in respiration.

(7) Bensley and Hoerr (1934) isolated mitochondria from liver cells.

(8) Seekevitz called them “Power house of the cell”.

(9) Nass and Afzelius (1965) observed first DNA in mitochondria.

Number of mitochondria : Presence of mitochondria depends upon the metabolic activity of the cell. Higher is the metabolic activity, higher is the number e.g., in germinating seeds.

(1) Minimum number of mitochondria is one in Microasterias, Trypanosoma, Chlorella, Chlamydomonas (green alga) and Micromonas. Maximum numbers are found (up to 500000) in flight muscle cell, (up to 50000) in giant Amoeba called Chaos – Chaos. These are 25 in human sperm, 300 – 400 in kidney cells and 1000 – 1600 in liver cells.

(2) Mitochondria of a cell are collectively called chondriome.

Size of mitochondria : Average size is \[0.51.00\,\,\mu \,m\] and length up to \[110\,\,\mu \,m.\] Smallest sized mitochondria in yeast cells \[(1\,\mu \,{{m}^{3}}).\] and largest sized are found in oocytes of Rana pipiens and are \[2040\,\,\mu \,m.\]

Ultrastructure : Mitochondrion is bounded by two unit membranes separated by perimitochondrial space (6 – 10nm wide). The outer membrane is specially permeable because of presence of integral proteins called porins. The inner membrane is selective permeable. The inner membrane is folded or convoluted to form mitochondrial crests. In animals these are called cristae and in plants these folding are called tubuli or microvili.

The matrix facing face is called ‘M’ face and face towards perimitochondrial space is called ‘C’ face. The ‘M’ face have some small stalked particles called oxysomes or \[{{F}_{1}}\] particle or elementory particle or Fernandez – Moran Particles (\[{{10}^{4}}{{10}^{5}}\] per mitochondria). Each particle is made up of base, stalk and head and is about 10nm in length.

Oxysomes have ATPase enzyme molecule (Packer, 1967) and therefore, responsible for ATP synthesis. These elementary particles are also called \[{{F}_{0}}\text{ }{{F}_{1}}\] particles. The \[{{F}_{1}}\]  particle is made up of five types of subunits namely \[\alpha ,\,\beta ,\,\gamma ,\,\delta \] and \[\varepsilon .\] of these \[\alpha \] is heaviest and \[\varepsilon \] is lightest. \[{{F}_{0}}\] particles synthesize all the enzymes required to operate Kreb’s cycle.

Semi-autonomous nature of mitochondrion : Mitochondria contain all requirements of protein synthesis :

(1) 70 S ribosomes.

(2) DNA molecules (rich in G-C ratio) to form mRNA and also replicate.

(3) ATP molecules to provide energy.

The mitochondria can form some of the required proteins but for most of proteins, these are dependent upon nuclear DNA and cytoplasmic ribosomes, so the mitochondria are called semi-autonomous organelles.

According to endosymbiotic origin of mitochondria by Kirns Altman, mitochondria were intially a free living, aerobic bacteria which during to the process of evolution entered an anaerobic cell and become established as mitochondria. This theory is supported by many similarities which exist between bacteria and mitochondria.

Chemical composition : Cohn gave the chemical composition of mitochondrion :

Proteins = 65 – 70% ; Lipids = 25 – 30% (90% phospholipids and 10% cholesterol, Vit. E., etc.) ; RNA = 5 – 7%. Some amount of DNA 2 – 5%.

The mitochondrial matrix has many catabolic enzymes like cytochrome oxidase and reductases, fatty acid oxidase, transaminase, etc.

Enzymes of Mitochondria

(1) Outer membrane : Monoamine oxidase, glycerophosphatase, acyltransferase, phospholipase A.

(2) Inner membrane : Cytochrome b,c₁,c,a, (cyt.b, cyt.c₁, cyt.c, cyt.a, cyt.\[{{a}_{3}}\]) NADH, dehydrogenase, succinate dehydrogenase, ubiquinone, flavoprotein, ATPase.

(3) Perimitochondrial space : Adenylate kinase, nucleoside diphosphokinase.

(4) Inner matrix : Pyruvate dehydrogenase, citrate synthase, aconitase, isocitrate dehydrogenase, fumarase, \[\alpha -\] Ketoglutarate dehydrogenase, malate dehydrogenase.

Origin : Mitochondria are self-duplicating organelles due to presence of DNA molecules so new mitochondria are always formed by growth and division of pre-existing mitochondria by binary fission.

Functions

(1) Mitochondria are called power house or storage batteries or ATP mills formation of ATP is called oxidative phosphorylation.

(2) Intermediate products of cell respiration are used in the formation of steroids, cytochromes, chlorophyll, etc.

(3) These are also seat of some amino acid biosynthesis.

(4) Mitochondria also regulate the calcium ion concentration inside the cell.

(5) Site of thermiogenesis.

(6) Yolk nucleus (a mitochondrial cloud and golgi bodies) controls vitellogenesis.

(7) Mitochondria of spermatid form nebenkern (middle piece) of sperm during spermiogenesis.

(8) Mitochondria release energy during respiration.

(9) Mitochondria contain electron transport system.