Structure and functions of Mitochondria

STRUCTURE AND FUNCTIONS OF
MITOCHONDRIA
By
Prof (Dr.) Ichha Purak
Department of Botany
Ranchi Women’s College, Ranchi
11/21/2020 Structure and function of Mitochondria 1

11/21/2020 2
CONTENTS
INTRODUCTION
HISTORY
ORIGIN AND EVOLUTION OF MITOCHONDRIA
SYNTHESIS OF MITOCHONDRIA
ISOLATION OF MITOCHNDRIA
SHAPE , SIZE AND NUMBER OF MITOCHONDRIA
STRUCTURE OF MITOCHONDRIA
 OUTER MEMBRANE
 INNER MEMBRANE
 INTERMEMBRANOUS SPACE
 MATRIX
CHEMICAL COMPOSITION OF MITOCHONDRIA
FUNCTIONS OF MITOCHONDRIA
MITOCHONDRIA –POWER HOUSE OF CELL
MITOCHONDRIAL DNA/ GENOME
TRANSPORT OF PROTEINS INTO MITOCHONDRIA
MITOCHONDRIAL INHERITANCE
MITOCHONDRIAL DISEASES IN HUMAN
SUMMARY
QUESTIONS
BOOKS CONSULTED
REFERENCES
Structure and function of Mitochondria

Mitochondrion , a membrane bound organelle found in the cytoplasm of almost all eukaryotic cells
(cells with clearly defined nuclei), the primary function of which is to generate large quantities of
energy in the form of adenosine triphosphate (ATP).
Mitochondria are oxygen consuming rod shaped cellular organelles of immense importance floating
free throughout the cell.
They are known as the “powerhouse of the cell” since these organelles supply all the necessary
biological energy to the cell by oxidizing the food substrates available.
Mitochondria are abundantly found on those sites where more energy is required such as sperm
tail, muscle cell, liver cell (up to 1600 mitochondria per cell ), microvilli etc.
Human oocyte contains 100,000 to 300,000 mitochondria per cell but each mitochondrion contains
only one copy of mtDNA.
Typically, there are about 2000 mitochondria per cell, representing around 25% of the cell volume.
INTRODUCTION

Rudolf Albert von Kölliker
(1817-1905) C Benda (1897)
Peter Mitchell (1961)
HISTORY OF MITOCHONDRIA

HISTORY
Rudolf Albert von Kolliker (1857), first observed cytoplasmic granules in striped muscles of insects. He
termed them as sarcosomes, now known as mitochondrion. He was studying human muscle cells when
he noted strange granules in them.
Richard Altman (1890) employed a dye technique to identify granules, terming them as “bioblasts”.He
hypothesized these organelles as basic units of cell activity in his book "Die Elementarorganismen"
("The Elementary Organism")
Benda (1897) coined the term mitochondria (mitos-thread,chondrion-grannules) for these cell
inclusions.
Michaelis (1898) demonstrated that mitochondria play a significant role in respiration. All these earlier
informations about mitochondria were obtained from animal cells.
The first evidence for the presence of mitochondria in plant cells (Nymphaea) was given by Meves
(1904). Since then mitochondria have been shown in all kinds of plant and animal cells.
Bensley and Hoerr (1934) for the first time isolated mitochondria from liver cells.
Eugene Kennedy and Albert Lehninger (1950) were able to prove that mitochondria contain the
respiratory assembly, the enzymes of the citric acid cycle, and of fatty acid oxidation.

All mitochondria present in a cell are collectively called chondriosome
In 1957, Philip Siekevitz termed mitochondria as the “powerhouse of the cell.”
Efraim Racker and co-workers (1960) isolated, from mitochondria, the enzyme "F1- F0 ATPase" now
called ATP synthase
Peter D. Mitchell (1978) was awarded the Nobel Prize in Chemistry "for his contribution in
formulation of the chemiosmotic theory (1961) for synthesis of ATP in Mitochondria.
Wallace (1992) identified degenerative diseases caused by mtDNA mutations.
Professor Paul Boyer, John E Walker and Skou J (1997) won the Nobel Prize for discovering the
role of mitochondria in the combination of adenosine diphosphate and inorganic phosphate to
produce ATP (a high energy compound)

ORIGIN AND EVOLUTION OF MITOCHONDRIA
About 1.45 billion years ago , some independent prokaryotic bacteria like organisms which accidently
entered a host (Eukaryotic) cell and established a successful symbiotic union with it as cell organelle
mitochondria
Mitochondria are similar to bacteria in certain features, which support endosymbiotic theory of origin of
mitochondria from bacteria.(Table :- 1)
Similarities are as follows :
1) Mitochondrial DNA is circular (like bacteria)
2) Mitochondrial ribosomes are like bacteria (55S & 70S)
3) Protein synthesis of bacteria and mitochondria are sensitive to different antibiotics
(e g Chloramphenicol and streptomycin) while cytoplasmic protein synthesis is inhibited by antibiotics
like cycloheximide.
4) New mitochondria arise by growth and fisson (as in bacteria) , not de novo.
5) Special adhesion sites are present in between outer and inner membrane of mitochondria. These
sites facilitate the transport of many proteins (synthesized in the cytoplasm) from cytoplasm to matrix .
In bacteria (E coli) similar sites are present in the membrane which play important role in transport of
proteins from cytoplasm to outside of cell.

Character Mitochondria Bacteria
1. size-width
length
0.2μm -1.0μm
1.0-4 μm-10 μm
0.2μm -2.0 μm
0.3- 5 μm -10 μm
2. Lipoprotein membrane 6nm -7 nm 7nm -8 nm
3.Invagination of membrane Cristae Mesosome
4. Respiratory control Marked Low or absent
5.Inhibitors of phosphorylation
like CN-CO azide , antimycin,
DNP and oligomycin
Effective Less effective or ineffective
6. DNA shape Closed circle Closed circle
7. Inhibitor of protein synthesis Chloramphenicol Chloramphenicol
8. Ribosomes 55S/ 70S (variable) 70S
9.Growth & Reproduction By fission as bacteria By Binary fission
10. Adhesion sites Between outer and Inner membrane
help in transport of proteins from
cytoplasm to matrix
In membrane,help in transport of
proteins from cytoplasm to outside
cell
Table: - Comparision between mammalian mitochondria and bacteria

Ivan Wallin(1926) an American Biologist made first experimental work on endosymbiotic theory.
Lynn Margulis (1967) later on proposed her Serial Endosymbiotic theory stating that
eukaryotic cells (cells with true nuclei) evolved from the symbiotic merger of non-nucleated bacteria
that had previously existed independently
Lynn Margulis (1938-2011)
Figure :- 1 Serial Endosymbiosis Theory (SET) proposed by Lynn Margulis (1967) for Evolutionary
origin of Mitochondria

Monophyletic origin of mitochondrion from a free living proteobacterium
(rather than polyphyletic origin)
Woese et al (1985) analyzed sequence of rRNA of small subunit (16S) of bacterial and mitochondrial
ribosome mentioning similar structure. Complete sequences of numerous mitochondrial, many
prokaryotic, and several nuclear genomes are now available.
These data confirm that the mitochondrial genome originated from some bacteria , Rickettsia l
subdivision of the α –proteobacteria.
During 1997-99, complete DNA sequence of bacteria like mitochondrial genome
(eg Reclinomonas americana) and mitochondria like eubacterial genome sequence
(eg Rickettsia prowazekii) have been worked out, suggesting strongly that all extant mitochondrial
DNAs had their origin in a single ancestral promitochondrial genome, which in turn must have
originated from a eubacterial genome.

A serial endosymbiosis model has also been proposed for the origin of nuclear genome and
mitochondria
In this model, it is assumed that in the first step, the nuclear genome of the host itself resulted from
fusion of eubacterial and archaebacterial partners and that only in the second step, the mitochondrion
was acquired as a symbiont
A group of eukaryotes known as Archezoa, do lack mitochondria, suggesting that perhaps eukaryotes
without mitochondria originated first as a primitive form and that the mitochondria entered the cells later.
The above view, however, has been challenged (1997-99) and it is suggested that the eukaryotes
acquired the nucleus and mitochondria simultaneously, and that Archezoa (lacking mitochondria) must
have resulted due to loss of mitochondria.
This view was supported by the observation that genes for several mitochondrial proteins of bacterial
origin are found in the genome of a mitochondriate Archezoa and also in the hydrogenome, recently
discovered in protists lacking mitochondria.

This view is described as ‘ hydrogen hypothesis’ since it assumes chimeric origin of
eukaryotic nucleus, due to symbiotic association between a eubacterium ( a
protobacterium, the symbiont), producing H2 as the end product of anaerobic
metabolism and a hydrogen requiring autotrophic archaebacterium (the host) .
This hypothesis thus allows the possibility of simultaneous origin of the ancestor of
eukaryotic cell and its mitochondrion. (Figure:- 2)

Figure : 2 Alternative hypotheses describing the origin
of eukaryotic cell and its mitochondria
Simultaneous creation of the
eukaryotic nucleus (gray) and
mitochondrion (orange) by fusion of
a hydrogen-requiring, methanogenic
Archaebacterium (host) with a
hydrogen-producing α-
Proteobacterium (symbiont).
Initially involving formation of an
amitochondriate eukaryote by fusion
of an Archaebacterium and a
Proteobacterium followed by
acquisition of the mitochondrion
through endosymbiosis with an
α- Proteobacterium

In concern with origin of mitochondria, it can be summarized that all mitochondria are descendants
from a single, alpha proteobacterial ancestor.
The acquisition of the mitochondrial endosymbiont triggered eukaryogenesis.
The host of the mitochondrial, alpha proteobacterial endosymbiont was a prokaryote.
Figure :-3 Endosymbiotic origin of Mitochondria

SYNTHESIS OF MITOCHONDRIA
The DNA in the cell nucleus does not code for the construction of mitochondria. Mitochondria are not
formed de novo, existing mitochondria divide by binary fission as that of bacteria.
Mitochondria are generated by existing mitochondria by inward furrowing like bacterial binary fission.
Mitochondrial Fission and Mitochondrial Fusion (when two separate mitochondria join) are almost
balanced.
Figure : -4 Mitochondrial fission and fusion
These processes involve both outer and inner
mitochondrial membranes. (A) An electron
micrograph of a dividing mitochondrion in a liver cell.
(B) Fusion (courtesy of Daniel S. Friend.)
From : Molecular Biology of the Cell. 4th edition.
Alberts et al. (2002). The Genetic Systems of
Mitochondria and Plastids
As mitochondria are enclosed by a double membrane, fisson and fusion are complex processes.
Mitochondria require dynamin on the cytosolic face for fission and for outer membrane fusion GTPase
and Mitofusin 1 & 2 and OPA1 for inner membrane fusion.

ISOLATION OF MITOCHONDRIA.
Mitochondria can be isolated from plant tissue for example potato tuber or roots of pea seedling or
animal tissue as mouse liver or skeletal tissue.
Isolation of mitochondria involves cell disruption (breaking open of cell to spill out contents within cell)
and differential centrifugation.
The cell disruption step should be gentle enough not as to mutilate structure of the organelles
The fresh tissue is gently homogenized to disrupt cells and release contents.
Mitochondria are pelleted by differential centrifugation, which separates cell components based on
differences in rate at which they may sediment by using specific solvents and buffers.
Further purification is carried out by sucrose gradient centrifugation.

SHAPE , SIZE AND NUMBER OF MITOCHONDRIA
Mitochondria can be seen under high power of light microscope by using specific vital stain Janus
Green ,which was used by Michaelis (1900) .
Commonly mitochondria appear rod shaped (2 μm -3 μm ) in higher plant cell.
Shape of mitochondria under goes considerable change, as they move around by cytoplasmic
streaming .
They are some time globular, cylindrical or branched and sometimes split into portions or fuse with
one another ,when globular have diameter (0.5 μm -1.5μm) and when cylindrical reach 6 μm-8 μm
in length .
In some cases oval mitochondria range in size from 0.5μm to 10 μm.
The number of mitochondria in a cell depends on the type and functional state of the cell. It varies
from cell to cell and from species to species.
Cells with higher cellular activity and high energy requirements, contain large number of mitochondria.
Less active cells possess fewer mitochondria.
The number of mitochondria for a given cell is fixed per unit volume of cytoplasm (1/5th ) .

Mitochondria are often concentrated in more active regions of the cell where energy requiring
processes take place ( e g at base of cilia) .
Number of mitochondria in Amoeba (Chaos chaos) are about 50,000 mitochondria.
In rat liver cells, about 1000 to 1600. Sea urchin eggs contain about 14,000 to 15,000
mitochondria per cell .
Some oocytes contain as many as 300,000 or even more mitochondria.
The green plant cells possess less number of mitochondria in comparison to animal cells, since
chloroplast in mesophyll cells can also synthesize ATP by photophosphorylation. Some algal cells
and Trypanosoma contain only one mitochondrion.
The number of mitochondria per cell varies widely, in humans, erythrocytes (red blood cells) do not
contain any mitochondria, whereas liver and muscle cells may contain hundreds or even
thousands.
The only eukaryotic organism known to lack mitochondria is Oxymonad Monocercomonoides
species. (Karnkowska et al., 2016) (Encyclopaedia Britannica)

STRUCTURE OF MITOCHONDRIA
The fine structure of a mitochondrion can change in different cells of a tissue at different stages of
development or in different physiological conditions.
Mitochondria are surrounded by double membranous envelope of phospholipid bilayer and proteins.
(Figure:-5)
If outer membrane of mitochondria is removed then structure is called mitoplast.
Two mitochondrial membranes have different permeability properties. The outer membrane is smooth,
freely permeable to low molecular weight compounds and number of proteins, whereas inner
membrane is impermeable to many ions, low molecular weight compounds and proteins and
possesses specific trans-membranous transport system.
Both mitochondrial membranes are very rich in proteins
Outer membrane has more phospholipids (phosphatidylcholine and cholesterol) as compared to inner
membrane.
Phospholipid in inner membrane is mainly diphosphatidyl glycerol (Cardiolipin)
The outer membrane has 3 times higher lipid content than inner membrane and has a totally different
components of enzymes, some of which are identical to Endoplasmic Reticulum.

Each membrane is 60-75 A° thick and separated by 80-100 A° thick peri - mitochondrial space
having enzymes required for oxidation of fats and pyruvic acid.
Outer surface of inner membrane is called C-face while inner surface called M-face.
Figure: - 5 Showing components of mitochondria (From Encyclopaedia Britanica)

Mitochondrion has an outer membrane and an inner membrane. The inner membrane contains
folds, called cristae, which increase its surface area.
The space between the two membranes is called the intermembrane space, and the space inside
the inner membrane is called the mitochondrial matrix.( Figure:-6)
ATP synthesis takes place on the inner membrane.
Outer mitochondrion membrane is quite smooth (6nm -7.5 nm thick) having some pores.
It has protein: phospholipid ratio 1:1 as that of plasma membrane.
It contains large numbers of integral membrane proteins called porins.
Porins allow small molecules (less than 5000 dalton ) to be exchanged between the cytoplasm and
the intermembrane space
ULTRASTRUCTURE OF MITOCHONDRION

ULTRASTRUCTURE OF MITOCHONDRION
Figure :6 This electron micrograph shows a mitochondrion as viewed with a transmission
electron microscope.
(credit: modification of work by Matthew Britton; scale-bar data from Matt Russell)
Intermembrane space

.Some sessile particles , attached to outer membrane are known as “subunits of parson”
Outer membrane of mitochondria is similar in structure to gram-negative bacterial membrane.
Larger proteins can enter the mitochondria if a signaling sequence at their N-terminus binds to
translocase (a large multisubunit protein ) present in outer membrane, which then actively moves
them across the membrane.
Outer membrane contains certain important enzymes as monoamine oxidase, rotenone-insensitive
NADH-cytochrome-C-reductase, kynurenine hydroxyalase, and fatty acid CoA ligase. (Novikoff and
Holtzman,1970).
The mitochondrial outer membrane can associate with the endoplasmic reticulum (ER) membrane, in
a structure called MAM (mitochondria-associated ER-membrane).
This is important in the ER-mitochondria calcium signaling and is involved in the transfer of lipids
between the ER and mitochondria.

Intermembrane space
The outer mitochondrial membrane is separated from Inner mitochondrial membrane by Inter
membranous (perimitochondrial) space which is 6nm-10 nm wide.
It has same composition as that of cell’s cytoplasm.
Intermembranous space continues with the intracristae space.
It contains Cytochrome c , adenylate kinase and nucleoside diphosphokinase enzymes .
Mitochondrial Inner membrane
It contains proteins with three types of functions
 Specific transport proteins that regulate passage of metabolites in and out of mitochondrial matrix
 ATP synthase enzyme complex which generates ATP in the matrix.
 Proteins of Electron transport chain for redox reactions.
Inner membrane is rich in phospholipid cardiolipin but does not have cholesterol.
The inner mitochondrial membrane is rich in many enzymes, coenzymes, and other components of
electron transport chain.

Inner membrane is completely impermeable to all molecules even small molecules
(With the exception of O2, CO2, and H2O) .
It also contains proton pumps and many permease proteins for the transport of various molecules such
as citrates, ADP, phosphate, and ATP.
Numerous transporters in the inner membrane ensure import and export of important metabolites.
Proteins are transported into the matrix via Translocase of Inner Membrane (TIM) complex.
Cristae
The inner mitochondrial membrane gives out finger or plate like outgrowths crista(S),cristae (P) or crests
towards the lumen (matrix) of the mitochondrion .
Palade (1952) coined the term cristae for these infoldings of inner membrane.
The cristae may be unbranched or branched forming a complex network which provides access to the
respiratory enzymes.
The cristae are tubular in shape, having a narrow neck extending into a globular or plate like expansion.

The inner membrane is folded differently.
These foldings are tubular in plants, are known as tubuli or microvilli, crest like in animals. In
some mitochondria intermediary types of foldings are seen.
In Fungi crista are plate like while in Euglena cristae are vesicle shaped.
Mitochondria from cells that have a greater demand for ATP, such as muscle cells, contain more
cristae.
Regarding the location of Electron transport chain and ATP synthase enzymes there are different
views and concepts.
According to old concept
The cristae bears stalked particles (respiratory particles) or oxysomes extending into matrix.
The particles are present at constant distance having a spherical head, stalk and base . These
particles carry out the electron transport chains as complexes (I,II,III & IV).
The spherical head of particles contain ATPase or coupling factor or F1 particle which again has a
stalk and head, transports protons for creating proton gradient which facilitates ATP synthesis.

According to Modern concept
The components of respiratory electron transport chain are directly built into inner membrane or
infoldings of inner membrane (cristae) as structural units and occupy about 10% area of inner
membrane in the form of complexes( I,II,II &IV).
It contains NADH dehydrogenase, Flavoproteins ,Succinic dehydrogenase , Ubiquinone, Cytochrome b
& c , Cytochrome a & a3 (cytochrome oxidase) as final electron acceptors.
Inner mitochondrial membrane as well cristae contain along with components of ETC ,
ATPase (coupling factor) or F1-F0 stalked particles ,involved in synthesizing ATP
(Figure:- 7& 8 )
The proteins of ETC are embedded in the inner mitochondrial membrane or cristae membrane, actually
traversing the lipid bilayer and protruding from the inner and outer surfaces

Figure : 8 Electron Transport Chain (ETC) and
ATPsynthase located on inner membrane of
Mitochondria
Figure : 7 Structure of inner membrane
of mitochondria

Intracristal Space is the space enclosed by cristae. It is continuous with intermembranous space.
Matrix
Inner mitochondrial membrane encloses a finally granular, dense, highly proteinaceous matrix.
The matrix is generally homogenous, but contains some fibrillar (Nucleoid) and filamentous structures.
Matrix contains two types of electron dense particles, ribosomes and calcium containing granules (
consisting of calcium phosphate )
The mitochondrial ribosomes (70S) are smaller than cytoplasmic ribosomes having 2 subunits 30S and
50S.
30S contains only one rRNA 16S and 50S contains 2 rRNAs 5S and 23S . All rRNAs are coded by
mitochondrial DNA.
Both subunits of mitoribosome contain considerable number of proteins, majority of which are coded by
nuclear genes.
Mitochondria of mammals have 55S ribosomes .
The matrix contains a highly concentrated mixture of hundreds of enzymes, mitoribosomes, tRNAs,
and several copies of the mitochondrial DNA genome.

The mitochondrial matrix contains all the enzymes for Citric Acid Cycle and for oxidation of fatty
acids, except flavoproteins and succinic dehydrogenase which are bound to inner surface of inner
membrane or cristae.
Colloidal or gel like matrix of mitochondria contains soluble enzymes of Krebs cycle which completely
oxidize acetyl-CoA to produce CO2, H2O and hydrogen ions, which reduce NAD and FAD, both of
which pass on hydrogen ions to respiratory or electron transport chain where oxidative
phosphorylation takes place to generate energy-rich ATP molecules.
Mitochondria contains several copies (2-6) of circular histone free DNA ,similar to bacterial DNA in
shape ,but smaller molecules having about 15000-75000 base pairs.
Mitochondria have their own genetic material, and machinery to manufacture their own RNAs and
proteins.
About 100 or more proteins are present in mitochondria, majority of which are coded by nuclear
genes.
Since mitochondria can synthesize 10 percent of their proteins by their own protein-synthetic
machinery, they are considered as semi-autonomous organelles

Enzyme DNA polymerase is found in matrix, it shows that mitochondrial DNA is independently
synthesized in mitochondria
Mitochondrial DNA carries information for synthesis of only 30 proteins required in mitochondria, but
this is not enough to build all the proteins required to make a new mitochondria, therefore, for rest of
the proteins, it has to depend upon nuclear DNA, cytoplasmic enzymes and other molecules supplied
by cell.
Kreb’s cycle enzymes, electron carriers and outer membrane elements are synthesized by the
cytoplasmic nuclear protein synthesizing system
The proteins of matrix and inner membrane are synthesized by both nuclear and mitochondrial
genomes.
Human mitochondrial DNA has been sequenced, which reveals 16569 base pairs encoding
37 genes, 22tRNA, 2rRNA and 13 peptide .
Mt-DNA is rich in G-C content.

CHEMICAL COMPOSITION OF MITOCHONDIA
Mitochondria contains 65-70% protein, 25-30% lipids, 0.5% RNA and small amount of DNA in matrix.
Proteins and lipids make about 80-99% of the organelle.
The important lipids present are lecithins, cephalins, cholesterol, and cardiolipins.
The inner membrane possess more of unsaturated fatty acids in the lipids.
Proteins are also abundant in inner membrane and act as electron carriers.
In matrix of Mitochondria, ribosomes are present which are diverse in size having 2 subunits (SSU
and LSU). Mitoribosomes consist of several specific proteins and less rRNAs
Mammalian mitochondrial ribosomes (mitoribosome ) are 55S particles having two subunits 28S and
39S (Greber et al,2015)
Mitochondria also has a number of enzymes in outer membrane, perimitochondrial space, Inner
membrane and matrix. Enzymes for DNA replication (DNA polymerase), RNA polymerase, Amino
acid activating enzymes are also present. (Table No- 2 )
The mitochondrial DNA differs from nuclear DNA and resembles bacterial DNA.
Mitochondrial DNA is extrachromosomal DNA .
Mitochondrial genes cause cytoplasmic or maternal inheritance .

Table :-2 Enzyme distribution in mitochondria
Enzymes of outer
membrane
Enzyme of peri -
mitochondrial space
Enzymes of inner membrane Enzymes of matrix
Monoamine oxidase
NADH-cytochrome
C reductase
Kynurenin
hydroxylase
Fatty acid CoA
ligase
Adenylate kinase
Nucleoside
diphosphokinase
NADH dehydrogenase
complex(NAD)
Flavin adenine dinucleotide (FAD)
Diphosphopyridine nucleotide
(DPN)
Cytochrome b-c complex
Cytochrome oxidase complex
Ubiquinone or Coenzyme Q
ATP synthase /F1F0ATPase
Succinate dehydrogenase
Β-hydroy butyrate dehydrogenase
Carnitine fatty acid transferase
Pyruvate dehydrogenase
complex
Malate dehydrogenase
Isocitrate dehydrogenase
Fumarase
Aconitase
Citrate synthethase
Keto glutaric acid
dehydrogenase

Although the best-known role of mitochondria is in aerobic respiration (ATP synthesis for production of
energy) , they carry out other important tasks as well.
1. ATP Synthesis/ Production of energy
The mitochondrion is the site of ATP synthesis for the cell. (Figure:--9)
ATP is produced in mitochondria through a series of reactions ( Krebs cycle) which takes place on the
folds or cristae of the inner membrane resulting in formation of chemicals as NADH2 and FADH2
which are oxidized through electron transport chain to produce ATP (oxidative phosphorylation ) .
In molecules of ATP, energy is stored in the form of chemical bonds, when these chemical bonds are
broken, the energy released can be used.
The most primary and important function of mitochondria is to produce energy through aerobic
cellular respiration which is dependent on presence of oxygen so Seikevitz (1957) proposed
mitochondria as “power house” of the cell .
FUNCTIONS OF MITOCHONDRIA

The 3 main stages in the overall process of aerobic respiration are
1. Glycolysis : spilting sugar ( Glucose ) molecules to pyruvic acid in cytoplasm
2. TCA cycle (in mitochondria) : Pyruvic acid to Acetyl CoA
3. Electron Transport (in mitochondria) and oxidative phosphorylation (ATP synthesis)
Figure: 9 Role of Mitochondria in Aerobic Respiration
Note : Visit Appendix Role of Mitochondria in Aerobic Respiration for details

2. Production of Heat: non-shivering thermogenesis
Under certain conditions, protons can re-enter the mitochondrial matrix without contributing to ATP
synthesis.
This process is known as proton leak or mitochondrial uncoupling and is due to the facilitated
diffusion of protons into the matrix.
The process results in the unharnessed potential energy of the proton electrochemical gradient
being released as heat.
The process is mediated by a proton channel found in brown adipose tissue or brown fat and is
responsible for non-shivering thermogenesis.
In humans, brown adipose tissue is present at birth and decreases with age.
In brown adipose tissue mitochondria have an alternative function of heat production using the
electron transport chain.

3. Mitochondria plays various roles as independent unit within eukaryotic cells by virtue of having its
own mitochondrial DNA (mt-DNA) and protein synthesis machinery (ribosomes and RNAs) , which
can replicate itself and synthesize some proteins and RNAs.
Mitochondria contain their own genetic material which is independent of the cell in which they are
located.
Mitochondrial genome can code for a number of proteins acting as enzymes for its metabolic
activities .
Each mitochondria has 2-6 circular DNAs .
The human mitochondrial genome contains 37 genes that encode 13 proteins, 22 tRNAs, and 2
rRNAs.
Mitochondrial DNA (mtDNA) is maternally inherited.
Mutations of mitochondrial genes can result in certain mitochondrial diseases.

4. Role in the process of apoptosis (Programmed cell death )
Cell death, also called apoptosis, is an essential part of life.
As cells become old or broken, they are cleared away and destroyed. Mitochondria help
decide which cells are to be destroyed.
Mitochondria release cytochrome C, which activates caspase, one of the chief enzymes
involved in destroying cells during apoptosis.
Cytochrome emerges from the mitochondrion into the cytosol or the intracellular fluid of
the eukaryotic cell which stimulates a cascade of biochemical changes that lead to
apoptotic death of the cell.
Because certain diseases, such as cancer, involve a breakdown in normal apoptosis,
mitochondria are thought to play a role in the disease.

5. Mitochondria can store calcium, which maintains homeostasis of calcium levels in the cell.
Mitochondria help the cells to maintain proper concentration of calcium ions within the compartments of
the cell.
Storage of Calcium (Ca2+ ) ions has many important functions in the physiology and biochemistry of
cells as
 Signal transduction pathway
 Neurotransmitter release from neurons
 Contraction of muscle cells
 Many enzymes need Ca2+ as a cofactor. e. g. in blood clotting
 Fertilization
Mitochondria play a part in this by quickly absorbing calcium ions and holding them until they are
needed.
In case of mammals, including humans, bone tissue is the main mineral storage site.
Release and re-absorption of Ca2+ from and into bone is regulated by hormones .
At a cellular level , Ca2+ is held within two types of organelles, the endoplasmic reticulum and
mitochondria.

6. Some mitochondrial functions are performed only in specific types of cells. For example,
mitochondria in liver cells contain enzymes that allow them to detoxify ammonia, a waste product of
protein metabolism.
7. An increase in Reactive Oxygen Species (ROS) generated by mitochondria is associated with a
decrease in regeneration potential of human bone marrow stem cells that are important for making
and repairing skeletal tissues, such as cartilage and bone.
8.Regulation of Innate Immunity- Mitochondrial antiviral signalling protein (MAVS) plays a key role in
the innate response to viral infections, helping to induce antiviral and anti-inflammatory pathways.
9.Enzymes required for the synthesis of lipids ,amino acids ,cholesterol and hormones as testosterone
and Oestrogen are present in the mitochondria .

10. In plants Mitochondria play additional role in a process known as Photorespiration which takes
place in mesophyll cells in leaves of C3 Plants
Photorespiration involves three cell organelles chloroplast, peroxisome and Mitochondria.
Photorespiration is enhanced respiration during day time under high oxygen levels Ribulose 1,5,di
phosphate reacts with oxygen instead of CO2 by the enzyme RUBISCO forming 3PGA and 2C
compound Phosphoglycolic acid ,which diffuses from chloroplast to Peroxisome where it is converted
to glycine which passes to mitochondria .
2 molecules of Glycine in mitochondria are converted to one molecule of serine ,releasing one
molecule of CO2 (respiration ) and NH3 , Serine passes back to peroxisome and then to
chloroplast as 3PGA.

MITOCHONDRIA –POWER HOUSE OF CELL /ROLE IN ATP PRODUCTION
Mitochondria are commonly known as power house of the cell as they are involved in synthesis of
ATP (energy rich compound ) by a process oxidative phosphorylation with the help of components of
Electron Transport Chain (ETC) embedded in cristae (in foldings of inner membrane of mitochondria)
along with the Enzyme ATPase (F1-F0) particle.
Mitochondria are tiny organelles inside cells that are involved in releasing energy from food.
This process is known as cellular respiration.
It is for this reason that mitochondria are often referred to as the powerhouses of the cell.
The popular term “powerhouse of the cell” was coined by Philip Siekevitz in 1957.
Philip Siekevitz (1957) in his paper “Powerhouse of the cell” mentioned that it is the mitochondrion,
a small body which appears to play a central role in the oxidation of foodstuff. Its structure, as
revealed by the electron microscope, mirrors its function.
The energy is released in the form of adenosine triphosphate (ATP) .
It is an energy currency of the cell.

MITOCHONDRIAL DNA/ GENOME
Mitochondria contain their own genetic system, which is separate and distinct from the nuclear
genome of the cell.
Mitochondrial genomes are usually circular DNA molecules, like those of bacteria but are much
smaller, which are present in multiple copies (2-6) per organelle.
The similarity supports the hypothesis that mitochondria arose from intracellular bacterial symbiosis
(endosymbiotic theory).
In some organisms mtDNA may be linear as for example ciliates, protozoa, algae, slime molds and
oomycetous fungi.
Entire nucleotide sequence of mitochondrial DNA of many plant and animal species has been
worked out , which throws light on origin of mitochondria from bacteria.
Mitochondrial RNA – is different from RNA of nuclear origin as it is resistant to ribonuclease enzyme.
Mitochondrial ribosomal proteins (MRPs), are encoded by the nuclear genome, translated in the
cytoplasm and then imported into the mitochondria.

Figure : -10 The human mitochondrial genome
Courtsey : The Cell: A Molecular Approach. 2nd edition. Cooper
GM.Sunderland (MA): Sinauer Associates; 2000
The genome contains 13 protein coding
sequences, which are designated as components
of respiratory complexes I, III, IV, or V.
In addition, the genome contains genes for
12SrRNA(for SSU of ribosome )and 16SrRNA (for
LSU of ribosome) and for 22 tRNAs, which are
designated by the one-letter code for the
corresponding amino acid.
The region of the genome designated “D loop”
contains an origin of DNA replication and
transcriptional promoter sequences.
The human mitochondrial genome (mt DNA) is a double stranded, circular molecule of 16,569 bp
(representing a small fraction of the total DNA in cells), contains 37 genes of which 13 encodes
proteins involved in electron transport and oxidative phosphorylation .
In addition, human mitochondrial DNA encodes 12S and 16S rRNAs and 22 tRNAs, which are
required for translation of the proteins encoded by the organelle genome. (Figure :-10)

Mitochondrial DNA molecules range in size from less than 6kb in Plasmodium to more than 300kb in
some land plants
In malaria parasites of human, Plasmodium falciparum mt genome is of particular relevance, due to its
small size (~6 kb), containing only 3 protein-coding genes viz. cytochrome c oxidase I (cox1),
cytochrome c oxidase III (coxIII) and cytochrome b (cytb ),these proteins are required for heme and
coenzyme Q biosynthesis, and oxidative phosphorylation.
Plants contain large mitochondrial genomes, which are several times as complex as those in animals,
fungi or algae. However, genome size is not correlated with information content. Mitochondrial genome
(mtDNA) shows large range of variation in size, from 15-19 kb in diptera, mammals, amphibians and
fishes; from 15-176 kb in protists and fungi.
The complete DNA sequences for more than 200 mitochondrial genomes have been determined. The
lengths of a few of these mitochondrial DNAs are shown to scale as circles for circular genomes and lines
for linear genomes. The largest circle represents the genome of Rickettsia prowazekii, a small pathogenic
bacterium whose genome most closely resembles that of mitochondria. (Figure:- 11)

The mitochondrial DNA(mtDNA) of Saccharomyces cerevisiae (yeast) is 85.8 kb in size and codes for
about 9 proteins , small (rns) and (rnl) rRNA subunits and approximately 24 tRNA genes. It also
encodes subunits I,II and III of cytochrome c oxidase (cox 1, cox 2 and cox 3), apocytochrome b (cob),
subunits 6, 8 and 9 of ATPase and a ribosome-associated protein (VAR1).
The size of mitochondrial genomes does not correlate well with the number of proteins encoded in
them: while human mitochondrial DNA encodes 13 proteins, the 22-fold larger mitochondrial DNA of
Arabidopsis encodes only 32 proteins.
The extra DNA that is found in Arabidopsis, Marchantia, and other plant mitochondria may be “junk
DNA”.
The mitochondrial genome (mtDNA) of Arabidopsis specifies only 57/58 genes in 367/372 kb,
whereas the 184 kb mtDNA in the liverwort Marchantia polymorpha codes for 66 genes, 58 kb
genome in the green alga Prototheca wickerhamii encodes 63 genes and 67kb genome of Chara
vulgaris encodes 68 genes( Unseld et al 1997.Turmel et al 2003, Wolff et. al 1994 ). (Figure:-12)

Figure:-11 Various sizes of mitochondrial
genomes From :- The Genetic Systems of
Mitochondria and Plastids
Molecular Biology of the Cell. 4th
edition.Alberts et al (2002) New York
Figure:-12 Scale comparison of mitochondrial genome size
between different organisms.
The two outermost circles represent the size of two plant
mitochondrial genomes, Nicotiana tabacum and Arabidopsis
thaliana.
Most animal mitochondrial genomes are 16.5 kbp in length.
Great size disparity can be seen between plants and
organisms from different phyla as yeast,insects and mammals

The largest number of mitochondrial genes has been found in mitochondrial DNA of the protozoan
Reclinomonas americana, which is 69 kb and contains 97 genes. (Gray et al ,1999).
The mitochondrial genome of Reclinomonas closely resemble the bacterial genome from which
mitochondria evolved than most present-day mitochondrial genomes, which encode only a small
number of proteins that are essential components of the oxidative phosphorylation system.
Mitochondrial genomes encode all of the ribosomal RNAs and most of the transfer RNAs needed for
translation of these protein-coding sequences within mitochondria.
Other mitochondrial proteins are encoded by nuclear genes, which are thought to have been
transferred to the nucleus from the ancestral mitochondrial genome.
The DNAs of many other animal mitochondrial genomes have also been completely sequenced. Most
of these animal mitochondrial DNAs encode precisely the same genes as humans, with the gene
order being identical for animals that range from mammals to fish.
The genomes of human and most other animal mitochondria are only about 16 kb, but substantially
larger mitochondrial genomes are found in yeasts (approximately 80 kb) and plants (more than 200
kb).

The number of genes in mtDNA varies from 5 to 94.
The four genes that are common in all mtDNAs are cob (cytochrome b) ,cox1(cytochrome c
oxidase 1) and two genes for ribosomal RNAs (rns and rnl)
The gene content in mtDNA is relatively low, when compared with that of chloroplast DNA in green
plants.
Number of genes in chloroplast DNA (cpDNA) is generally more than 100.
Generally Mitochondrial DNA is about less than 1% of cellular DNA ,but yeast mtDNA represents
as high as 18% of cellular DNA.
Mitochondria are not self-supporting entities but rely heavily for their functions on imported
nuclear gene products.
Mitochondria contain one or more molecules of DNA which are circular in shape , highly twisted
and measure about 5 μm in length having about 15000-75000 base pairs.
Mt- DNA behaves like a chromosome and duplicates in usual manner into several circles. Due to
DNA, mitochondria are capable to undergo self reproduction and thus may exhibit cytoplasmic
inheritance.

Mitochondrial DNA differs from nuclear DNA in several aspects such as
 Mitochondrial DNA contains more (GC) contents than nuclear DNA,having higher density.
 Denaturation or melting temperature of mitochondrial DNA is higher than nuclear DNA
 Mitochondrial DNA is generally circular in shape as in bacteria. But in some organisms
mtDNA may be linear
 Rate of renaturation of Mt-DNA is more
 The Mt-DNA is shorter than nuclear DNA.
 The genome contains little non-coding DNA ( Junk DNA or introns).
 Mt-DNA can undergo replication and duplication.
 Some mitochondrial codons resemble those of purple non-sulfur bacteria.
 Different stop codons are present in the mitochondrial DNA : AGA AAG but not UGA.
 DNA polymerase of Mitochondria is also different from nuclear DNA polymerase .
 In human mitochondrial genome is built of 16,569 DNA base pairs, whereas the nuclear
genome is made of 3.3 billion DNA base pairs
 Some bases are part of two different genes, as the last base of one gene and as the first
base of the next gene

Transport of proteins into Mitochondria
Mitochondria contains a number of proteins of which only few are encoded by mitochondrial genome.
Mitochondrial genes code and form some subunits of protein complexes meant for inner membrane of
mitochondria.
All other proteins are encoded by nuclear genes, so are synthesized by free ribosomes present in
cytoplasm and later transported into mitochondria.
New mitochondria are produced by the growth of preexisting mitochondria, it mainly depends on the
import of proteins from the cytosol .
Some important mitochondrial proteins imported from cytosol are alcohol dehydrogenase (yeast),Citrate
synthase and other citric acid cycle enzymes, DNA polymerase, Ribosomal proteins, RNA
polymerase,F1ATPase subunits, CoenzymeQ, Cytochrome c
reductase, Cytochrome c peroxidase and Mitochondrial porins.

These proteins may belong to either outer membrane or inner membrane or intermembranous
space or matrix, the mechanism of transport may differ in each case.
Transport of protein to outer membrane or matrix requires a single signal peptide whereas
transport of protein to inner membrane and inter membranous space requires second signal
peptide also.
Mitochondrial proteins are first fully synthesized as precursor proteins in the cytosol and then
translocated into mitochondria by a post translational mechanism.
Most of the mitochondrial precursor proteins have a signal sequence at their N terminus that is
rapidly removed after import by a protease (the signal peptidase) present in the mitochondrial
matrix

Figure:- 13 A signal sequence for
mitochondrial protein import in
cytochrome oxidase
Cytochrome oxidase is a large multiprotein complex
located in the inner mitochondrial membrane acts as
terminal enzyme in the ETC.
The first 18 amino acids of the precursor of subunit IV
of this enzyme serve as signal sequence for import of
the subunit into the mitochondrion.
The signal sequence becomes folded as an
amphipathic α helix by clustering of positively charged
residues on one side and negatively charged residues
on opposite face, which is recognized by specific
receptor proteins on the mitochondrial surface.
(Figure:-13 )
Courtsey: Alberts et al 1994.Molecular
Biology of the cell)

Figure:- 14 Three protein translocators
(TOM,TIM and OXA) in the
mitochondrial membranes (Courtsey:
Alberts et al 1994.Molecular Biology of
the cell)
Protein translocation across mitochondrial membranes is
mediated by multi-subunit protein complexes that function
as protein translocators as one TOM (translocase of
outer membrane) and two TIM (Translocase of Inner
membrane) TIM 23 and TIM 22 complexes and one OXA
complex
(Figure :- 14 )
TOM is required for import of all nucleus encoded
mitochondrial proteins .
TIM23 transports some of these proteins into matrix space
.
TIM22 complex mediates insertion of a subclass of inner
membrane proteins including carrier proteins.
A third protein translocator in the inner mitochondrial
membrane, the OXA complex, mediates the insertion of
inner membrane proteins that are synthesized within the
mitochondria.11/21/2020 Structure and function of Mitochondria 54

Mitochondrial precursor proteins are imported as unfolded polypeptide chains
Mitochondrial precursor proteins remain unfolded through interaction with other proteins (chaperones
belonging to hsp70 family ),bind directly to their signal sequence.
In the import process, as a first step, the mitochondrial precursor proteins bind to import receptor
proteins of the TOM complex, which recognize the mitochondrial signal sequence. The interacting
proteins are then stripped off, with the help of signal sequence , the unfolded polypeptide chain is fed
into the translocation channel.
Transport of proteins to mitochondrial matrix
Translocation into the mitochondrial Matrix depends on a signal sequence and protein translocators
Proteins imported into the matrix of mitochondria are usually taken up from cytosol within seconds or
minutes of their release from ribosomes.

Mitochondrial precursor proteins are imported into the matrix at contact sites that join the inner and
outer membranes
Electron microscopic studies of single mitochondrion reveal several contact sites at which the inner
and outer mitochondrial membranes are closely apposed, and it seems likely that translocation
occurs at or near these sites.
At these contact sites, there appear to be separate protein translocators in outer and inner
membranes, whose functions are coupled to allow translocation across both the membranes at the
same time.
It is thought that the TOM complex first transports the mitochondrial targeting signal across the outer
membrane. Once it reaches in the intermembrane space, the targeting signal binds to a TIM
complex, opening the channel in the complex through which the polypeptide chain either enters the
matrix or inserts into the inner membrane. (Figure:- 15)

Figure:- 15 Protein import by mitochondria utilizing
signal peptide in the protein and a receptor in the
mitochondrial membrane (redrawn from Alberts et
al 1994.Molecular Biology of the cell)
The N-terminal signal sequence of the
precursor protein is recognized by receptors of
the TOM complex.
The protein is thought to be translocated across
both mitochondrial membranes at or near
special contact sites.
The signal sequence is cleaved off after import
by a signal peptidase in the matrix to form the
mature protein.
The free signal sequence is then rapidly
degraded.
Although the functions of the TOM and TIM
complexes are usually coupled to allow protein
transport across both membranes at the same
time, both protein types of translocator can
work independently

Thus transport of a protein from the cytosol to mitochondrial matrix requires
i) a signal peptide attached to NH2 terminus of protein to be transported
ii) a receptor protein at the outer membrane
iii) ATP hydrolysis to supply energy and
iv) In addition, another energy source is required: an electrochemical H+ gradient across the
inner mitochondrial membrane. (Figure:-16)
It has also been shown that cytosolic chaperone proteins ( chaperonins) belonging to the hsp70
family also help in import of mitochondrial and ER proteins by binding this protein in its unfolded
state during translocation.
These chapronins also help in correct folding of cytosolic proteins.
Despite the independent functional roles of the TOM and TIM translocators, the two mitochondrial
membranes at contact sites may be permanently held together by the TIM23 complex, which spans
both membranes

Figure: 16 The role of energy in protein
import into the mitochondrial matrix
(Alberts et al 1994,Molecular Biology of
the cell)
1) Bound cytosolic hsp70 is released from the protein
in a step that depends on ATP hydrolysis. After initial
insertion of the signal sequence and of adjacent
portions of the polypeptide chain into the TOM
complex, the signal sequence interacts with a TIM
complex.
(2) The signal sequence is then translocated into the
matrix in a process that requires an electrochemical H+
gradient across the inner membrane, positioning the
unfolded polypeptide chain so that it transiently spans
both membranes.
(3) Mitochondrial hsp70 binds to regions of the
polypeptide chain as they become exposed in the
matrix, there by pulling the protein into the matrix. ATP
hydrolysis then removes the mitochondrial hsp70,
allowing the imported protein to fold.

Transport of proteins into inner membrane and intermembrane space
Protein transport into inner mitochondrial membrane and intermembrane space requires two signal
sequences
Proteins that are integrated into the inner mitochondrial membrane or the intermembrane space are
initially transported from the cytosol by the same mechanism that transports proteins into the matrix.
In some cases they are first transferred into the matrix .
A hydrophobic amino acid sequence, however, is strategically placed after the N-terminal signal
sequence that guides import into the matrix.
Once the N-terminal signal sequence has been removed by the matrix signal peptidase, the
hydrophobic sequence functions as a new N-terminal signal sequence to translocate the protein from
the matrix into or across the inner membrane, using the OXA complex as the translocator .
The OXA complex is also used to insert proteins encoded in the mitochondrion into the inner
membrane (Figure :-17 A).
11/21/202017A17 Structure and function of Mitochondria 60

These proteins may follow one of the two pathways
i) they are prevented from completing journey to the matrix and get localized to intermembrane
space/inner membrane
ii) they may reach the matrix and then transported back across inner membrane to reach their final
destination.
In the first direct pathway, a translocator in the inner membrane binds the hydrophobic sequence
following the amino-terminal end of the signal peptide.
The signal peptide is then cleaved off and disintegrates in the matrix.
The translocators in the outer and inner membrane become uncoupled , so that the protein is
pulled into inter membrane space and may remain attached to the inner membrane with the help
of stop-transfer peptide.
In case of proteins destined for the intermembranous space , the protein is cleaved off by a signal
peptidase to release the mature soluble protein in the space.
They may later become attached to the outer surface of inner membrane. (Figure:17 )

Figure:-17(A-D) Protein import from the cytosol into the inner mitochondrial membrane or
intermembrane space

A)Pathway that requires two signal sequences and two translocation events is thought to be used to move some
proteins from the cytosol to the inner membrane. The precursor protein is first imported into the matrix space .
Cleavage of the signal sequence (red) used for the initial translocation, however, unmasks an adjacent hydrophobic
signal sequence (orange) at the new N terminus.
This signal then directs the protein into the inner membrane, presumably by the same OXA-dependent pathway that
is used to insert proteins encoded by the mitochondrial genome.
B) In some cases, the hydrophobic sequence that follows the matrix-targeting signal binds to the TIM23 translocator
in the inner membrane and stops translocation. The remainder of the protein is then pulled into the intermembrane
space through the TOM translocator in the outer membrane, and the hydrophobic sequence is released into the inner
membrane.
(C) Some soluble proteins of the intermembrane space may also use the pathways shown in (A) and (B) before they
are released into the intermembrane space by a second signal peptidase, which has its active site in the
intermembrane space and removes the hydrophobic signal sequence.
D) The import pathway used to insert metabolite carrier proteins into the inner mitochondrial membrane utilizes the
TIM22 complex, which is specialized for the translocation of multipass membrane proteins.
Explanation of Figure : 17 - A,B,C & D

The genes controlling cytoplasmic inheritance are present outside nucleus, in the cytoplasm, they are
known as plasma genes, cytoplasmic genes, cytogenes, extra nuclear genes or extra chromosomal
genes.
Nass and co-workers (1963) proved the existance of DNA in mitochondria
The sum total of the genes present in cytoplasm of a cell are known as Plasmon. All genes present in a
mitochondria are known as chondrioms and all genes present in a plastid are known as plastoms.
During reproduction (Sexual) nuclear DNA is inherited in equal proportions from both parents, but
mtDNA has a unique inheritance pattern.
MtDNA is inherited from maternal parent (mother ) over generations .
It is passed down strictly along the direct maternal line from a mother to all of her children.
Males will carry mtDNA of their mother, but when they have children, their children will carry the mtDNA
of their own mother, not their father.
Thus, only daughters pass the mtDNA on to future generations.

During fertilization (oogamous) which takes place in higher plants, many animals and human, ovum
is very large and non motile ,contains cytoplasm along with cell organelles like mitochondria ,where
as the sperm is very small and motile, has very little cytoplasm, only nucleus of sperm enters the
ovum. (Figure: -18)
The male and female nuclei fuse to form diploid zygote nucleus.(Figure:-19)
The zygote contains cytoplasm contributed mostly by ovum (Figure :-20)
Further development of embryo takes place by mitotic division of zygote.
The mitochondria are transmitted into daughter cells during cell division
Since mtDNA is found only in the cytoplasm, all of our mtDNA comes from our mother, not our
father.
As the embryo continues to develop into a full grown human, all of the cells in the resulting human
contain strictly the cytoplasm and mtDNA of the mother, not the father.

Figure :-18 The steps in fertilization
and formation of zygote
Figure:-19 Haploid
sperm fuses with
haploid egg to form
diploid zygote
Figure:- 20 zygote/embryo receives most of
cytoplasm from egg, so mitochondria are
contributed by mother

Mitochondria originate from pre-existing mitochondria only.
Mitochondria contain DNA, RNA and ribosomes.
Many plasmagenes are located in the mitochondrial DNA (mt-DNA).
The available evidences show that, generally mitochondria are transmitted to the progeny from only
maternal parent.
Therefore characters governed by the genes located in mt-DNA show cytoplasmic inheritance
(Maternal inheritance) .
Examples of maternal inheritance:
 Petites mutant (very small yeast)
 Poky mutant of Neurospora
 Cytoplasmic male sterility in some crops

Extra-Nuclear Inheritance by Mitochondria of Yeast
In Yeast (Saccharomyces cerevisiae)- unicellular ascomycetes fungus, sexual reproduction takes
place by fusion of two somatic cells to form diploid zygote nucleus which divides by two successive
divisions (meiosis) forming four haploid daughter nuclei ,all of which take part in ascospore formation
in Zygote (ascus) cell .
Respiration in yeast cell takes place both aerobically and anaerobically (Fermentation).
Certain mutant yeast cells are comparatively small sized and are unable to utilize oxygen for aerobic
respiration .
These mutant strains are known as petites.
Petite strains lack cytochrome b and c and also enzyme cytochrome oxidase a and a3
for aerobic terminal respiration.
These components are present in the cell of normal strain where they are associated with the inner
membrane of mitochondria.

Petite strain can be maintained indefinitely in the vegetative state and can be mated with normal yeast
cells.
When such matings are carried out, three petite varieties can be classified
Figure: 21 Results of crosses between different kinds of petites and Normal strain of yeast

i)Nuclear (segregational) Petites:When a normal haploid strain of yeast is crossed with a haploid petite
strain, a normal diploid zygote is produced.
The haploid ascospores produced from zygote by sporulation are segregated in the ratio 1 : 1 (petite :
normal), following ordinary nuclear mendelian inheritance
ii) Neutral Petites: In this type, only normal wild ascospores are produced from mattings between petite
and normal strain of yeast.
The petite characteristic is absent in product of segregation. It shows the non-Mendelian
inheritance,caused by extra nuclear inheritance, not by nuclear genes
iii) Suppressive Petites: In this type, all ascospores produced from mating between normal and petite
strain are petite type.
Such petites seem to suppress normal respiratory behaviour and the suppressive petite factor acts as a
dominant.(Figure :-21)
It has becomes clear that there are two different genetic causes for respiratorty deficiency in yeast, one is
nuclear and other is extra nuclear.

Extra-Nuclear Inheritance by Mitochondria of Poky Strain of Neurospora crassa
Neurospora crassa (Ascomycetes fungi) has a slow growing mutant called poky.
This mutant strain exhibits poorly differentiated mitochondria which are deficient in Cytochrome b and
enzymes cytochrome oxidase a and a3 of respiratory ETC
When poky female parent is crossed with a normal strain as a male parent, the progeny are found
to be poky.
In reciprocal cross, the progeny are normal.
The only difference between reciprocal crosses is the contribution of cytoplasm.
Thus nuclear genotype has no effect on this particular phenotype.
The trait is inherited via female parent in non-mendelian fashion indicate its extra chromosomal
nature

Cytoplasmic male sterility in some crops is controlled by mitochondrial DNA
Male sterility is the failure of plants to produce functional anthers, pollen grains , or male gametes.
Male sterility in plants can be controlled by nuclear genes or cytoplasmic genes or by both.
Therefore at least three types of male sterility are seen in plants.
Genetic male sterility, Cytoplasmic male sterility and Cytoplasmic-genetic male sterility. Cytoplasmic
male sterility (CMS) is produced by plasmagenes located in mt- DNA
In maize, wheat, sugar-beets, onions etc. male sterility is controlled by cytoplasmic factors.
This type of male sterility is referred to as cytoplasmic male sterility.
Nuclear genes does not control this type of male sterility
It is transmitted over generations through egg cytoplasm (mitochondria ) .
In other plants however ,male sterility is controlled entirely by nuclear genes.

Rhodes (1933) discovered and analyzed cytoplasmic male sterility in maize
In this plant pollens are aborted in the anther.
He demonstrated that male sterility was contributed by female parent.
This was shown by crossing male sterile plants with wide range of fertile males and by observing
that in subsequent generations all progenies were male sterile .
The male sterile plant is produced when an egg cell containing cytoplasmic male sterility factor is
fertilized by pollen from normal male fertile plants.
In such cases if female parent is male sterile, F1 progeny would always be male sterile (Fig. -22),
because cytoplasm is mainly derived from egg obtained from male sterile female parent
In maize, three cytoplasmic male sterile sources (CMS) are known, which are T(Texas), C (Charnua)
and S(USDA) .
Some dominant nuclear genes, called restorer genes, nullify the effects of CMS cytoplasm on male
fertility.

Figure :-22 Maternal inheritance of cytoplasmic male sterility in plants.

Three types of cytoplasm, T, C and S show differential response to the restorer gene .
The normal male fertile cytoplasm is called N cytoplasm.
CMS is extensively used in hybrid seed production in crops like maize, jowar,bajara,etc.
Each of the three CMS cytoplasms exhibits strict maternal inheritance and even when all
chromosomes were replaced by a male fertile source through repeated backcrosses, male sterility
could not be overcome
Recent studies have shown that cytoplasmic male sterility is controlled by either some unique
polypeptide produced by mitochondria or by some plasmid like elements which are not found in the
mitochondria of normal cytoplasm
It indicates that male sterility is contributed by the cytoplasm of female parent. It is also observed
that when a male sterile female plant is crossed with wide range of fertile males, all progenies are
male sterile in the subsequent generations.
Therefore, it is confirmed that male sterility is not controlled by nuclear gene. In rare cases, male
sterile plants produce a few fertile pollen grains.

MITOCHONDRIAL DISEASES IN HUMAN
Human Mitochondrial DNA contains 37 genes which all are essential for normal function of the
mitochondria.
Mitochondrial DNA (mtDNA) is highly susceptible to mutations, largely because it does not possess
DNA repair mechanisms found in nuclear DNA.
Many genetic conditions are related to changes (mutation) in particular mitochondrial genes.
Mitochondria are energy factories in cells of human body.
Mitochondrial diseases occur when mitochondria fail to produce enough energy required for the body
to function properly.
Mitochondrial diseases can be present at birth, but can also occur at any age and are often inherited.
One in 5000 individuals has a genetic mitochondrial disease.
Symptoms might include poor growth, developmental delays and muscle weakness.

Mitochondria produce 90% energy our body needs for various metabolic activities and support organ
function. When mitochondria malfunction, organs start to fail, people get sick and even die.
Mitochondrial diseases can affect almost any part of the body, including the cells of the brain, nerves,
muscles, kidneys, heart, liver, eyes, ears, pancreas, skeletal muscles and digestive tract .
Mitochondrial diseases are not curable but treatment and physiotherapy can reduce symptoms and
slow decline in health.
Mutations in at least three mitochondrial genes can cause cytochrome c oxidase deficiency.
The mitochondrial genes associated with cytochrome c oxidase deficiency provide instructions for
making proteins that are part of a large enzyme group (complex) called cytochrome c oxidase
Cytochrome c oxidase is responsible for the last step in oxidative phosphorylation before the
generation of ATP.

A mitochondrial disease that causes prominent muscular problems is called a mitochondrial
myopathy (myo means muscle, and pathos means disease), while a mitochondrial disease that
causes both prominent muscular and neurological problems is called a mitochondrial
encephalomyopathy (encephalo refers to the brain).
The symptoms of mitochondrial myopathies include muscle weakness or exercise intolerance, heart
failure or rhythm disturbances, dementia, movement disorders, stroke-like episodes, deafness,
blindness, droopy eyelids, limited mobility of the eyes, seizures. diabetes, and stunted growth.

Some examples of Mitochondrial diseases :
Leigh Syndrome
MELAS : Mitochondrial Encephalomyopathy Lactic Acidosis and Stroke-like episodes
DAD- Diabetes mellitus and deafness
KSS- Kearns Sayre syndrome
Pearson syndrome
PD-Parkinson Disease
Leigh syndrome and MELAS are the most common mitochondrial myopathies
Leigh syndrome causes brain abnormalities that can result in ataxia (impaired coordination), dystonia
(involuntary muscle movement), external ophthalmoplegia (paralysis of eye muscles), progressive
neurodegeneration seizures, lactic acidosis (buildup of lactate in the body), vomiting, weakness,
hypotonia ,developmental delays, and altered control over breathing.
This condition is characterized by progressive loss of mental and movement abilities (psychomotor
regression) and typically results in death within two to three years, usually due to respiratory failure.

MELAS (Mitochondrial Encephalopathy, Lactic Acidosis, and Stroke-like episodes)
syndrome is a rare disorder that begins in childhood, usually between two and fifteen years of age,
and mostly affects the nervous system and muscles.
The most common early symptoms are seizures, recurrent headaches, loss of appetite and recurrent
vomiting.
Stroke-like episodes with temporary muscle weakness on one side of the body may also lead to
vision and hearing loss, loss of motor skills and intellectual disability.
DAD (Diabetes mellitus and deafness ) is a subtype of diabetes which is caused by point mutation in
human mitochondrial DNA which affects the gene encoding tRNAleu .
It is characterized by hearing loss
Maternal DD is characterized by both a defect in insulin secretion and hearing loss.
Other associated abnormalities are mascular dystrophy, myopathy, cardiac disorders, renal and
gastrointestinal disorders.

KSS (People with Kearns Sayre Syndrome ) develop paralysis of eye muscles that affects eye
movements and cause drooping eyelids.
The features of KSS usually appear before age of 20.
This disorder is defined by chronic progressive external ophthalmoplegia (CPEO), which consists in
slowly progressive weakness of the muscles that control the eye movement plus pigmentary
retinopathy, that can affect vision.
Other common symptoms are deafness, diabetes, anemia, and ataxia ( lack of coordination of
voluntary movements )
Pearson syndrome- This syndrome causes severe anemia, low level of immune cells (neutrophils)
,low blood platelet count and malfunction of pancreas.
PD (Parkinson's disease) is a progressive disorder that affects nerve cells in the brain responsible
for body movement.
Parkinson’s disease (PD) is a complex neurodegenerative disorder ,evidences indicate that
mitochondrial dysfunction is a central factor behind this disease

SUMMARY
Mitochondrion, a membrane-bound organelle found in the cytoplasm of almost all eukaryotic cells, the
primary function of which is to generate large quantities of energy in the form of adenosine triphosphate
(ATP).
Mitochondria are oxygen consuming rod shaped or spherical cellular organelles.
Mitochondria are known as the power house of the cell since these organelles supply all the necessary
biological energy to the cell by oxidizing the food substrates available.
Normally mitochondria vary in size from 0.5 µm to 2.0 µm to 10µm.
Mitochondria can be seen under high power of light microscope by using specific vital stain Janus
Green ,which was used by Michaelis (1900) .
Mitochondria are surrounded by double membranous envelope of phospholipid bilayer and proteins.
Mitochondrion has following distinct parts as Outer mitochondrial membrane, intermembrane space
,inner mitochondrial membrane, cristae (infoldings of inner membrane ) , matrix(space within the inner
membrane) and intracristal space (space inside cristae) .

Mitochondria contain 65-70% protein, 25-30% lipids, 0.5% RNA and small amount of DNA is present in
its matrix. The important lipids present are lecithins, cephalins, cholesterol, and cardiolipins. The inner
membrane possess more of unsaturated fatty acids in the lipids. Proteins are also abundant in inner
membrane and act as electron carriers.
The mitochondria are organelles that reproduce themselves semi-autonomously within eukaryotic cells.
Mitochondria contains a number of proteins of which only few are encoded by mitochondrial genome.
All other proteins are encoded by nuclear genes, so are synthesized by free ribosomes present in
cytoplasm and later transported into mitochondria.
Mitochondrial precursor proteins are imported into the matrix at contact sites that join the inner and outer
membranes.
Mitochondria have their own genome .Mitochondria contain multiple copies of circular DNAs.
The human mitochondrial genome (mt DNA) is a double stranded, circular molecule having 16569 bp,
consisting of 37 genes which encode 13 proteins,16S and 12S rRNAS and 22tRNAs

Maternal inheritance is expression of some characters which are governed by genes located in mt-
DNA.Examples of maternal inheritance are Petites mutant (very small yeast),Poky mutant of
Neurospora and cytoplasmic male sterility in some crops .
Mitochondrial DNA (mtDNA) is highly susceptible to mutations, largely because it does not possess
the DNA repair mechanisms found in nuclear DNA.
Mitochondrial diseases can affect almost any part of the body, including the cells of the brain,
nerves, muscles, kidneys, heart, liver, eyes, ears, pancreas, skeletal muscles and digestive tract .

MCQs (Multiple Choice Questions) on Mitochondria
1. Who introduced the name Mitochondria first?
a) Jams Long b) Benda c) L Pasture d) Harvard
2. Following in which Mitochondria is absent?
a) Muscle fibers b) RBC c) Renal tubular cells d) Liver cells
3. On which factor number of mitochondria depends?
a) Energy requirement of the cell b) Size of the plant or animal.
c) Condition of plasma membrane d) Requirement of secretion
4. Life span of mitochondria:-
a) 1 to 2 years b) 2 to 4 year c) 4 to 10 years d) 10 to 14 years.
5. Percentage of RNA in a mitochondria?
a) 1% b) 5% c) 1.5% d) 0.5%
6. Which of the following is Mitochondria’s job
a) To make energy b) To protect the cell c) To make proteins d) To hold genetic information
7. How many mitochondria are in a cell ?
a) 1 b) 2 c) 1000 d) Depends on the complexity of the cell
8. What molecule do mitochondria use to make energy?
a) DNA b) RNA c) ATP d) ADP
9. The mitochondria take food particles and combine them with
a) Carbon dioxide ) Nitrogen c) water d) Oxygen
10. Which of the following parts of the mitochondria is wrinkled with lots of folds?
a) Outer Membrane b) Inner Membrane c) Cristae d) Matrix
QUESTIONS

11. What is the purpose of the cristae?
a) To protect the mitochondria b) To increase the surface area of the inner membrane c) To hold
ribosomes and DNA d) To make proteins
12. Which of the following is the space inside the inner membrane?
a) Outer Membrane b) Inner Membrane c) Cristae d) Matrix
13) The inner membrane of the mitochondria is usually, highly convoluted forming a series of infoldings
known as
a) Thylakoids b) Lamellae c) Cristae d) grana
14) Oxysomes of F0-F1 particles occur on
a) Thylakoid b) Mitochondrial surface c) chloroplast surface d) Inner Mitochonfrial membrane
15) Select the wrong statement from the following .
a) Both chloroplast and mitochondria contain an inner and outer membrane.
b) Both chloroplast and mitochondria have an internal compartment, the thylakoid space bound by the
thylakoid membrane
c) The chloroplast are generally much larger than mitochondria
d) Both chloroplast and mitochondria contain DNA
16) Which of the following statements regarding mitochondrial membrane is not correct?
a) The outer membrane resembles a sieve
b) The enzymes of the Electron Transport Chain are embedded in the outer membrane.
c) The outer membrane is permeable to all kinds of molecules
d) The outer membrane is permeable to all kinds of molecules ?
17) In mitochondria, cristae act as sites for
a) Protein synthesis b) Phosphorylation of flavoproteins
c) breakdown of macromolecules d) Oxidation reduction reaction

18) Mitochondrial inner membrane is rich in phospholipid
a) Cardiolipin b) Phosphatidyl inositol c) Phosphatidyl serine d) Phosphatidyl choline
19) All the statements are true except
a) Mitochondria are called as the “power house “ of the cell b) mitochondrial DNA is called mt DNA c)
Mitochondria is the site of oxidative phosphorylation and Krebs cycle d) Mitochondria is the site of
Calvin cycle
20) Which of the following is not a function of mitochondria
a) Electron transport chain and associated ATP production
b) Glycolysis and associated ATP synthesis c) Fatty acid breakdown
d) Non shivering thermogenesis
21) mt DNA is
a) Simple,double stranded linear DNA molecule b) Simple single stranded DNA molecule c) Simple ,
single stranded circular DNA molecule d) Simple double stranded circular DNA molecule
22) The inner mitochondrial membrane has
a) NADH dehydrogenase complex b) Cytochrome b-c1 complex c) Cytochrome oxidase complex
d) All of these
23) Which of the following cell organelle can be viewed by a light microscope
a) Ribosome b) Endoplasmic Reticulum c) Golgi Apparatus d) Mitochondria
24) Which of the following statement about mitochondria is not true ?
a) Size and shape of mitochondria varies in a cell .
b) Mitochondria in the cell can fuse with one another .
c) large mitochondria in the cell can split into two
d) In all cells, one mitochondria will be exceptionally larger than others.

25) Nebenkern of Insect sperm cells is a modified ------------------------
a) Nucleus b) Golgi Apparatus c) Mitochondria d) Centrosome
26) Mitochondria in the human sperm cell are occupied at ---------------------
a) Sperm head b) Mid piece c) Sperm tail d) No mitochondria in the sperm
27) Which of the following statement is true regarding the membrane system of mitochondria ?
a) Chemical composition of both inner and outer membrane of mitochondria are same
b) Outer membrane contain more protein/lipid ratio (by weight) than inner membrane
c) Inner membrane contain more proteins/lipid ratio (by weight) than outer membrane
d) Phospholipids are completely absent on the outer membrane of mitochondria.
28) Which of the following membrane lipid constituent can be considered as the lipid marker of inner
mitochondrial membrane ?
a) Lecithin b) Cardiolipin c) Ceramide d) Sphongo-ceramide
29) The only other place where cardiolipin can be found naturally other than mitochondrial inner
membrane is -----------------------
a) Inner membrane of Golgi apparatus b) Apoptosome c) Bacterial plasma membrane d) Membrane
of glyoxysome
30) Which of the following statement regarding the distribution of cholesterol is true in the membrane
system of mitochondria ?
a) Cholesterol is completely absent in mitochondrial membrane.
b) Cholestrol is absent in the inner membrane c) Cholesterol is present only in the inner membrane d)
Cholesterol is present in both inner and outer membrane

31) Mitochondrial membrane system processes a variety of porin proteins which allow the passage
of selective molecules.Which of the following statement is true regarding the porin protein in the
mitochondrial membrane ?
a) Mitochondrial outer membrane possess porins but they lack the usual beta barrel structure of the
usual porins.
b) Both inner and outer mitochondrial membrane possess porin proteins.
c) Only the inner mitochondrial membrane possess porin proteins
d) Many porin proteins which are structurally similar to the porins of bacterial cell are present in the
outer membrane of mitochondria
32) ATP,NAD and CoA can be ----------------------.
a) Freely transported through both outer and inner membrane of mitochondria.
b) Freely transported through the outer membrane of mitochondrion.
c) Freely transported through the inner membrane of mitochondria.
d) Cannot be transported freely through outer and inner membrane of mitochondria.
33) Cellular organelles (s) involved in the regulation of Ca2+ level in the cell --------------
a) Mitochondria b) Endoplasmic Reticulum c) Endoplasmic Reticulum and Mitochondria d)
Mitochondria and vesicles
34) The only enzyme in citric acid cycle which is attached to the inner mitochondrial membrane is
a) Succinic dehydrogenase b) NADPH dehydrogenase c) Isocitric dehydrogenase d) Malate
dehydrogenase

35) Which of the following can be used as an enzyme marker for inner mitochondrial membrane ?
a) Succinic dehydrogenase b) ATP synthase c) Succinyl Co-A synthase
d) Cardiolipin
36) Which of the following is a mobile electron carrier in the mitochondrial electron transport system ?
a) NADH dehydrogenase b) FADH dehydrogenase c) Ubiquinone d) Succinic dehydrogenase
37) Which of the following cellular event is NOT directly involved with mitochondria ?
a) Apoptosis b) ATP synthesis c) Controlling cell cycle d) protein degradation
38) Which of the following statement is NOT true about mitochondria ?
a) Hepatocytes have more than 2000 mitochondria per cell.
b) RBC completely lack mitochondria.
c) Sperm cells of insects completely lack mitochondria.
d) Mitochondrial number may change in each cell of a species .
39) Examples of autonomous organelle (s)
a) Mitochondria b) Chloroplast c) Mitochondria and Chloroplast d) Chloroplast, Mitochondria and
Golgi Apparatus
40) Pick out the odd one :
a) Kearne-Sayre Syndrome b) Tay-sach Syndrome c) MELAS Syndrome d) Leber’s hereditary optic
neuropathy

41) Mitochondrial DNA (MtDNA) is considered as one of the best marker tool for population biologists
and evolutionary biologists . The reason for this is :
a) Mitochondrial DNA undergoes spontaneous mutation
b) Mitochondrial DNA can be easily isolated
c) Mitochondrial genes are specific to mtDNA
d) Absence of genetic recombination in mt DNA
42) Cyanide is a mitochondrial toxin. The mechanism of action of cyanide is by inhibiting :
a) NADH dehydrogenase b) Succinate dehydrogenase c) Cytochrome c oxidase
d) ATP synthase
43) Who observed the mitochondria first
a) Kolliker b) Robert Brown c) Robert Hook d) Altman
44) The mitochondrial DNA differes from the nuclear DNA because of
a) Being linear b) Having A=T and C-G c) Lacking binding histones d) Being highly twisted
45) ATP is formed in
a) Mitochondria b) Nucleus c) Nucleolous d) Ribosomes
46) F1 particles are also called
a) Electron transport particles b) Elementary particles c) Cytochrome d) Cristae
47) Prokaryotic origin of mitochondria was proposed by
a) Rabinowitch b) Altman and Schimper c) Salton d) Morrison

48) Mitochondria are related to
a) Prokaryotes b) Plasmids c) plastids d) Viruses
49) F1particles/oxysome/elementary particles are present in
a) Endoplasmic Reticulum b) Chloroplast c) Mitochondria d) Golgi complex
50) The number of mitochondria increases in cells of
a) Dormant seeds b) Germinating seeds c) Dry seeds d) Dead seeds
51) Who first introduced the term mitochondrion
a) Kolliker b) Robert Brown c) Benda d) Altman
52) Glycogen occurs in –
a) Mitochondria b) Kreb’s cycle c) Cytoplasm d) None of them
53) The proteins forming the membranes of mitochondria are called
a) Mitochondrial proteins b) Structural proteins c) Skeletal proteins d) All the above
54) Mitochondria are non-existent in
a) Red algae b) Some bacteria c) Green algae d) Brown algae
55) Mitochondria supply most of the necessary biological energy by
a) Breaking down of sugar b) Oxidizing substrates of TCA cycle c) Reducing NADP d) Breaking
down of protein
56) Mitochondria are usually found in
a) Reproductive cells b) Vegetative cells c) Both reproductive and vegetative cells d) Non of these
57) Which organelle has Electron Transport System
a) Ribosomes b) Sphaerosomes c) Mitochondria d) Lysosome

58) Respiratory enzymes are present in
a) Mitochondria b) Chloroplast c) Golgi bodies d) Lysosome
59) Autonomic genome system is present in
a) Ribosome and chloroplast b) Mitochondria and Ribosome
c) Mitochondria and Chloroplast d) Golgi bodies and Mitochondria
60) First plant cell in which mitochondria was observed
a) Lily b) Nymphea c) Nelumbium d) Nerium
61) In which part of mitochondria, ATP is generated
a) Matrix b) Cristae c) Outer membrane d) F1 particle
62) Function of mitochondria is
a) To provide CoA b) To synthesize PGA c) To release energy during respiration
d) All of the above
63) Single mitochondria is found in
a) Microsteria b) Rhizopus c) Nostoc d) Ulothrix
64) Oxidative enzymes occur mostly in
a) Lysosome b) Golgi bodies c) Mitochondria d) Ribosome
65) The presence of DNA in Mitochondria and Chloroplast supports the hypothesis that
a) Mitochondria and Chloroplast both originated as independent free living organisms
b) Glycolysis occurs in mitochondria and chloroplast
c) ATP is produced in Mitochondria and chloroplast
d) Mitochondria and chloroplast undergo meiosis and mitosis independent of nucleus
66) Percentage of mitochondrial DNA in the cell is
a) 10 % of the total cellular DNA b) 1 % of total cellular DNA
c) 2.5 % of the total cellular DNA d) None of the above
67) mitochondria are numerous and densely packed in
a) inactive tissue b) Less active tissue c) very active tissue d) Damaged tissue

68) In prokaryotes, the mitochondria are absent. Even then Krebs cycle takes place. What is the site of
krebs cycle in bacteria.
a) Ribosome b) Nucleoid c) Cytoplasm d) Mesosome
69) The site for cellular respiration is
a) Nucleus b) Ribosome c) Mitochondria d) Endoplasmic Reticulum
70) Which cell organelle participates in photo-respiration
a) Chloroplast b) Peroxisome c) mitochondria d) All of the above
71) Mitochondria arises
a) By growth and division of pre-existing mitochondria b) From non-mitochondrial membranes c) From
pre-cursors of the cytoplasm d) None of the above
72) Chondriospheres are formed due to
a) Fusion of mitochondria b) Division of mitochondria
c) DNA replication d) transcription
73) Mitochondria are store house of
a) Fats b) ATP c) Glucose d) Glycogen
74) Which of the following is correct pair
a) DNA synthesis-Ribosomes b) Protein synthesis-Smooth E R
c) Aerobic respiration-Cristae d) Suicidal sacs –Dictyosome
75) Mesosome were taken as
a) Golgi bodies b) Plastids c) Mitochondria d) Endoplasmic Reticulum
76) In which of the following parts of mitochondria succinic dehydrogenase enzyme is located
a) outer membrane b) Inner membrane c) Perimitochondrial space d) Matrix
77) Which of the following is present in mitochondria
a) Polysome b) Mesosome c) Quantasome d) Oxysome

78) Mitochondria are site for
a) phosphorylation b) Oxidative phosphorylation c) Transpiration d) Carboxylation
79) Which of the following organelle is considered to be rich in catalytic enzymes
a) E R b) Lysosome c) Golgi Body d) Mitochondria
80) The size of mitochondria is
a) 5-10u b) 50-100u c) 0.5-1.0u d) 150-300u
81) DNA is present in
a) Carboxysome b) Ribosome c) Lysosome d) Mitochondria and Chloroplast
82)Rackers particles are found in
a) chromosome b) Mitochondria c) Nucleus d) Golgi body
83) What is the typical size of a mitochondrion ?
a) 0.75-3 centimeters b) 0.75-3 micrometer c) 0.75-3 nanometers d)0.75-3 meters
84) What is the main function of the mitochondria ?
a) Site of glucose synthesis b) Site of ATP synthesis c) Site of protein synthesis
d) Site of lipid synthesis
85) Which of the following stored by the mitochondria, help in triggering apoptosis?
a) Caspases b) Lipases c) Endonucleases d) Peptidases
86) What alternate function can mitochondria carry out in brown adipose tissue ?
a) Heat production b) Glucose production c) Protein production d) lipid production
87) What is the lineage of mitochondria ?
a) Maternal b) Paternal c) Maternal and Paternal d)none

88) _______is called powerhouse of the cell
a) Mitochondria b) Endoplasmic reticulum c) Nucleus d) Golgi complex
89) The site of aerobic respiration in eukaryotic cells is___________
a) Peroxisome b) Plastid c) Mitochondria d) Cilia
90) How do the small molecules pass through the outer membrane of mitochondria?
a) ATP pump b) Carrier protein c) Channels d) Porins
91) Which of the following division technique is similar in mitochondria and bacteria?
a) Binary fission b) Budding c) Binary fusion d) Meiosis
92) Name the organelle which is used for aerobic respiration and ATP synthesis in Entamoeba
histolytica.
a) Hydrogenosome b) Mitochondria c) Mitosome d) Peroxisome
93) Which of the following is INCORRECT evidence to support the endosymbiotic theory?
a) Mitochondria are self-repeating bodies like microscopic organisms
b) Mitochondria have their own particular DNA
c) Mitochondrial ribosomes, and enzymes are similar to the bacteria
d) Mitochondria and bacteria differ in size
Answers : Note : Options in red colour are correct choice for MCQs

Short Answer Questions
Write short notes on the following
a) Outer membrane of mitochondria
b) Inner membrane of mitochondria
c) Cristae
d) Matrix of mitochondria
e) Mitochondrial DNA /genome
f) ATP Synthase in mitochondria
g) F1-F0 particle
h) Cytochrome c Oxidase
i) Electron Transport Chain (ETC) of Mitochondria
j) Mitochondrial fusion
k) Biogenesis of mitochondria
l) Intermembranous space of mitochondria
m) Chemical composition of mitochondria
n) Oxysome
o) Mitochondrial ribosome
p) Mitochondrial inheritance
q) D loop in mitochondrial DNA

Explain the following in brief
a) Symbiotic origin of mitochondria
b) Enzyme distribution in mitochondria
c) Succinate dehydrogenase
d) Oxidative phosphorylation
e) Chemiosmotic hypothesis as proposed by Mitschell (1961)
f) Role of Mitochondria in heat production
g) Mitochondrial inheritance
h) Cytoplasmic male sterility
i) Mitochondrial diseases
J) Role of mitochondria in Apoptosis
K) Protein transport into matrix of mitochondria
L) Protein transport into inner mitochondrial membrane
m) TOM complex
n) TIM complex

o) OXA complex
p) MITOCHONDRIAL DISEASES
q) MELAS
r) Leigh Syndrome
s) Mitochondrial myopathy
t) Chondrioms

Long Answer Questions
1.Why is mitochondria called powerhouse of cell ?
2. Describe briefly main functions of mitochondria.
3.How many mitochondria are in a cell ?
4. Why mitochondria is called endosymbiont ?
5. Mitochondria are called semi autonomous cell organelle –Discuss
6. How did mitochondria get into eukaryotic cells ?
7. Can eukaryotic cells survive without mitochondria?
8. Is mitochondria inherited from the mother?
9. Why mitochondrial DNA is passed from mothers only ?
10. Do all humans have the same mitochondrial DNA?
11. What is mitochondrial inheritance?

12.Describe the various components of mitochondria and their functions.
13. What Is the Purpose of Mitochondrial Membranes?
14. Briefly describe the structure of mitochondria
15. What are cristae?
16. What difference exist in structure and function between the inner and outer membranes of
mitochondria ?
17. Describe the biogenesis of the mitochondria
18. Give a short account of symbiotic origin of mitochondria.
19. What is the special function of brown fat mitochondria and how is it carried out ?
20. Describe with the help of suitable diagram mitochondrial genome.

Very short Answer Questions -2 marks each
1. Do mitochondria have circular DNA?
2. What kind of eukaryotic cells have mitochondria?
3. Do prokaryotic cells have mitochondria?
4. Where are mitochondria located in the cell ?
5. Do sperm have mitochondria?
6. Do eggs have mitochondria?
7. What Is the Origin of Mitochondria?
8. During evolution, When did mitochondria appear?
9. Why red blood cells do not have mitochondria?
10. Are mitochondrial diseases curable?
11. What is the most common mitochondrial disease?

Molecular Biology of the Cell. 4th edition. Alberts B, Johnson A, Lewis J, et al.New York: Garland
Science; 2002. Chapter 12 Transport of proteins into Mitochondria and Chloroplast and Chapter 14
The Mitochondrion and The Genetic Systems of Mitochondria and Plastids
The Cell: A Molecular Approach. 2nd edition. Cooper GM. Sunderland (MA): Sinauer Associates;
2000. ,Chapter 10: Mitochondria
Cell and molecular biology Concepts and Experiments Gerald Karp 5th Edition 2007 John Willey and
Sons, Inc Chapter 5 The Mitochondrion and Bioenergetics
Cell and Molecular biology By P K Gupta: Rastogi Publications
Cell Biology (Cytology,Biomolecules and Molecular Biology) By Verma P S and Agarwal V K S
Chand & Company Pvt Ltd.
Cell Biology by Dr S P Singh and Dr B S Tomar Rastogi Publication,Meerut,U P ,India
Cell Biology 1997 Dr SC Roy and Dr K K De New Central Book Agency (P ) Ltd. Kolkatta , India
BOOKS CONSULTED

Molecular Cell Biology. 4th edition.2000 H Lodish, A Berk and S L Zipursky W. H. Freeman; New York
A B Novikoff and E Holtzman ( 1970) Cells and Organelles Published by Holt, Rinchart and Winston
New York 337 pages
Lehninger Principles of Biochemistry 2001 Fourth Edition David L Nelson and Michael M Cox
Chapter 19: Oxidative Phosphorylation and Photophosphorylation
Biocyclopaedia Is online for students and researchers in Biology Biocyclopedia.com

REFERENCES
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vermehrte Auflage (The Elementary Organisms and Their Relationships to the Cells. Second
Extended Edition). Leipzig: Verlag Von Veit and Company, Oxford Univ. Press, New York
Benda C (1898). "Ueber die Spermatogenese der Vertebraten und höherer Evertebraten. II. Theil:
Die Histiogenese der Spermien". Arch. Anal. Physiol.: 393–398.
Bensley RR and Hoerr N.l (1934) Studies on cell structure by freezing -drying method VI ; The
preparation and properties of mitochondria. The Anatomical Records : 60:449–455
Boyer P., Walker B J and Skou J (1997) The Nobel Prize in Chemistry for their elucidation of the
enzymatic mechanism underlying the synthesis of adenosine triphosphate (ATP) and for the first
discovery of an ion-transporting enzyme, Na+, K+ -ATPase.
Gray M W (2017) Lynn Margulis and the endosymbiont hypothesis: 50 years later Molecular
Biology of the Cell 28:1285-1287
Gray M W,Burger G and Lang B F (1999) Mitochondrial Evolution . Science :05 (283): 1476-1481
Greber et al (2015) Ribosome. The complete structure of the 55S mammalian mitochondrial
ribosome.Science: 348 (6232) : 303-308

Karnkowska et al., 2016, A Eukaryote without a Mitochondrial Organelle oxymonad
Monocercomonoides species : Current Biology 26, 1274–1284
Kennedy, E. P., and Lehninger, A. L. (1949) Oxidation of Fatty Acids and Tricarboxylic Acid Cycle
Intermediates by Isolated Rat Liver Mitochondria J. Biol. Chem. 179(2): 957–972
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Margulis,L (1970) Origin of Eukaryotic Cells (Yale Univ. Press, New Haven, CT)
Meves F. (1904). "Über das Vorkommen von Mitochondrien bezw. Chondromiten in Pflanzenzellen". Ber.
Dtsch. Bot. Ges. 22: 284–286.
Michaelis, L. (1900). Die vitale Farbung, eine Darstellungsmethode der Zellgranula. Archiv für
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Mitchell, P. (1961). "Coupling of Phosphorylation to Electron and Hydrogen Transfer by a Chemi-
Osmotic type of Mechanism" Nature. 191 (4784): 144–148.
Mitchell,P (1978) was awarded the 1978 Nobel Prize for Chemistry for his discovery of the chemiosmotic
mechanism of ATP synthesis

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Sagan L (1967) :On the origin of mitosing cells; Journal of Theoretical Biology
Siekevitz P (1957) Powerhouse of the cell . Scientific American Vol 197(1) : 131-144
Turmel M,Otis C and Lemieux C(2003) The mitochondrial genome of Chara vulgaris: insights into the
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THE END

ROLE OF MITOCHONDRIA IN AEROBIC RESPIRATION
ATP Synthesis/ Production of energy
The mitochondrion is the site of ATP synthesis for the cell.(Figure:-1)
ATP is produced in mitochondria through a series of reactions ( Krebs cycle) which takes place on
the folds or cristae of the inner membrane resulting in formation of chemicals as NADH2 and FADH2
which are oxldized through electron transport chain to produce ATP (oxidative phosphorylation ) .
In molecules of ATP, energy is stored in the form of chemical bonds, when these chemical bonds are
broken, the energy released can be used.
The most primary and important function of mitochondria is to produce energy through aerobic
cellular respiration which is dependent on presence of oxygen so Seikevitz (1957) proposed
mitochondria as “power house” of the cell .
APPENDIX-1

The 3 main stages in the overall process of aerobic respiration are
1. Glycolysis : spilting sugar ( Glucose ) molecules to pyruvic acid in cytoplasm(Figure:-2)
2. TCA cycle (in mitochondria) : Pyruvic acid to Acetyl CoA
3. Electron Transport (in mitochondria) and oxidative phosphorylation (ATP synthesis)
Figure: 1 Role of Mitochondria in Aerobic Respiration
11/21/2020
Structure and function of Mitochondria 111

In Glycolysis which takes place in cytoplasm
Glucose through a series of reactions
changes into 2 molecules of Pyruvic acid.
2ATP molecules are synthesized by
Substrate level phosphorylation
2NAD+ is reduced to 2 NADH2
Figure: - 2 Flowchart diagram of
Glycolysis

Krebs Cycle : Pyruvic acid diffuses into mitochondria and by oxidative decarboxylation changes
into a 2C compound Acetyle CoA liberating a molecule of CO2 and reducing NAD+ to NADH2.
Acetyle CoA undergoes condensation reaction with a 4C compound Oxaloacetic acid (OAA) end
product of last Krebs cycle synthesizing a 6C compound citric acid which is further
decarboxylated to α Ketoglutaric acid (5C compound) liberating another CO2 molecules .
α KGA by oxidative decarboxylation loses CO2 and change to succinyl CoA (4C compound)
transferring protons to oxidants NAD+., ultimately forming a 4C compound Oxalo Acetic Acid
(Figure :- 3 Flowchart diagram of Krebs cycle)
The enzymes for TCA cycle are present in matrix of mitochondria .

Figure: - 3 Flowchart diagram
of Krebs Cycle
3 molecules of CO2 released
NAD and FAD are reduced
Which are oxidized by ETC
resulting in formation of ATP by
Oxidative phosphorylation

Electron Transport Chain and Oxidative Phosphorylation
The protons accepted by oxidants NAD+ and FAD+ are carried over by Electron Transport Chain
embedded in Inner Mitochondrial membrane or Cristae (Figure:-4 )
Electrons and protons are finally accepted by oxygen present in matrix of mitochondria with the
help of F1-F0 particle (ATP synthase –Figure -5 ) present in inner mitochondrial membrane
having stalk and head forming H2O and ATP in matrix side through a proton gradient from Inner
mitochondrial membrane to matrix (Chemiosmotic Hypothesis)
(Figure :-6) as proposed by Peter Mitschell (1961)

Figure :- 4 Electron Transport Chain of Mitochondria

Figure : 6 Pumping of protons from the matrix
to the cytosolic side of the inner mitochondrial
membrane
(From : Biochemistry Lubert Stryer)
Figure:5 Schematic diagram of ATP synthase
showing proton Conducting unit (F0) and
ATP synthesizing F1 unit (From :
Biochemistry Lubert Stryer

THE END

Structure and functions of Mitochondria

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