1. PHY 301A
Nakul Surana Submitted to –
13418 Prof. K P Rajeev
SHORT STUDY POJECT
Mitochondria – Its’ Structure, history and Function
2. Mitochondria
Mitochondria are well-defined cytoplasmic organelles of the cell which take part in a variety
of cellular metabolic functions. Survival of the cells requires energy to perform different
functions. The mitochondria are important as the fact that these organelles supply all the
necessary biological energy of the cell, and they obtain this energy by oxidizing the substrates
of the Krebs cycle.
When and How Often Did Mitochondria Arise?
The oldest undisputedly eukaryotic microfossils go back 1.45 billion years in the fossil record.
Given the coincidence of mitochondria with the eukaryotic state, this can also be seen as a
minimum age for mitochondria and a rough best-guess starting date for eukaryotic evolution
It is hardly surprising that many independent eukaryotic lineages have preserved anaerobic
energy-producing pathways in their mitochondria
History
There are currently two main, competing theories about the origin of mitochondria. They
differ with regard to their assumptions concerning the nature of the host and the
physiological capabilities of the mitochondrial endosymbiont.
The traditional view posits that the host that acquired the mitochondrion was an anaerobic
nucleus-bearing cell, a full-fledged eukaryote that was able to engulf the mitochondrion
actively via phagocytosis.
Initial benefit of the symbiosis might have been the endosymbiont's ability to detoxify oxygen
for the anaerobe host.
Because this theory presumes the host to have been a eukaryote already, it does not directly
account for the ubiquity of mitochondria.
The oxygen detoxification aspect is problematic, because the forms of oxygen that are toxic
to anaerobes are reactive oxygen species (ROS) like the superoxide radical, O2
-.
In that sense, mitochondria do not solve the ROS problem but rather create it; hence,
protection from O2 is an unlikely symbiotic benefit. This traditional view also does not directly
account for anaerobic mitochondria.
An alternative theory posits that the host that acquired the mitochondrion was aprokaryote,
an archaebacterial outright. This view is linked to the idea that the ancestral mitochondrion
was a metabolically versatile, facultative anaerobe (able to live with or without oxygen)
The initial benefit of the symbiosis could have been the production of H2 by the
endosymbiont as a source of energy and electrons for the archaebacterial host, which is
posited to have been H2 dependent. This kind of physiological interaction (H2 transfer or
anaerobic syntrophic) is commonly observed in modern microbial communities.
3. Structure of Mitochondria
Mitochondria are rod shaped structure found in both animal and plant cells. It is a double
membrane bound organelle. It has the outer membrane and the inner membrane. The
membranes are made up of phospholipids and proteins.
The components of mitochondria are as follows:
Outer membrane
It is smooth and is composed of equal amounts of phospholipids and proteins.
It has a large number of special proteins known as the porins.
The porins are integral membrane proteins and they allow the movement of molecules
that are of 5000 daltons or less in weight to pass through it.
The outer membrane is freely permeable to nutrient molecules,ions, energy molecules
like the ATP and ADP molecules.
Inner membrane
The inner membrane of mitochondria is more complex in structure.
It is folded into a number of folds many times and is known as the cristae.
This folding help to increase the surface ares inside the organelle.
The cristae and the proteins of the inner membrane aids in the production of ATP
molecules.
Various chemical reactions takes place in the inner membrane of the mitochondria.
Unlike the outer membrane, the inner membrane is strictly permeable, it is permeable
only to oxygen, ATP and it also helps in regulating transfer of metabolites across the
membrane.
Intermembrane space
It is the space between the outer and inner membrane of the mitochondria, it has the
same composition as that of the cell's cytoplasm.
There is a difference in the protein content in the intermembrane space.
Matrix
The matrix of the mitochondria is a complex mixture of proteins and enzymes. These
enzymes are important for the synthesis of ATP molecules, mitochondrial ribosomes,
tRNAs and mitochondrial DNA.
Functions
Functions of mitochondria depends on the cell type in which they are present.
The most important function of the mitochondria is to produce energy. The simpler
molecules of nutrition are sent to the mitochondria to be processed and to produce
4. charged molecules. These charged molecules combine with oxygen and produce ATP
molecules. This process is known as oxidative phosphorylation.
Mitochondria help the cells to maintain proper concentration of calcium ions within
the compartments of the cell.
The mitochondria also help in building certain parts of blood and hormones like
testosterone and estrogen.
The liver cells mitochondria have enzymes that detoxify ammonia.
The mitochondria also play important role in the process of apoptosis or programmed
cell death. Abnormal death of cells due to the dysfunction of mitochondria can affect
the function of organ.
Energy Production
This occurs by a process of cellular respiration, also known as aerobic respiration, which is
dependent on the presence of oxygen. (When oxygen is limited, the chemicals that would
otherwise be oxidized are, instead, metabolized by anaerobic respiration, via a process that is
independent of the mitochondria.)
The 3 main stages in the overall process of
aerobic cellular respiration are:
1. Glycolysis- splitting sugar molecules
2. TCA Cycle
3. Electron Transport
mtRNA
Mitochondria contain their own genetic material - which is independent of the cell in which
they are located.
Mitochondrial DNA (mtDNA) is maternally inherited. At fertilization only nuclear DNA enters
from the sperm because although the egg contains mitochondria, sperm cells do not. Sperm
are so tiny that mitochondria would hamper their passage toward the egg. (Therefore exercise
capacity e.g. for endurance sports tends to be maternally inherited. Maternal ancestral history
can also be traced via mtDNA.)
mtDNA accounts for about 1% of the total cellular DNA
5. Mitochondrial Eve
In 1987, A worldwide survey of human mitochondrial DNA (mtDNA) was published by Cann,
Stoneking, and Wilson in Nature magazine. Its main point was that "all mitochondrial DNAs
stem from one woman" and that she probably lived around 200,000 years ago in Africa.
This mitochondrial DNA, however, does not seem to come from both parents. Instead, it
comes only from the mother and not from the Father (There seems to be some rare
exceptions to the rule that only the mother contributes the mitochondrial DNA
Initially, it was thought that for humans, most of the sperm remained outside of the egg. Only
the head with the nuclear DNA and the centrosome, were thought to enter the egg. But that
view has changed. Now it has been determined that the whole sperm enters the egg.
However, virtually all of the sperm is broken down by enzymes. Only the chromosomes found
in the head of the sperm in crystalline form are preserved and used in the recombination
process to produce the final version of the new egg cell DNA. The sperm mitochondria and its
DNA are broken down by enzymes made for that purpose. However, the end result is still the
same. The mitochondria and its DNA from the sperm are not used. Only the mitochondria
from the egg are used for the newly developing person
So, our mitochondrial DNA is essentially identical to that of our mother. Mitochondrial DNA is
transfered from mother to daughter, generation after generation. The mitochondrial DNA in
the son, which he got from his mother, is a dead end street, since his mitochondrial DNA will
not be used in his children.
Nuclear DNA changes a lot since it undergoes recombination in every generation. However,
the mitochondrial DNA gets transfered from generation to generation without any
recombination. Only the normal mutation rate that occurs when DNA is replicated allows the
mitochondrial DNA to change. This is why the world wide survey was able to determine that
all people are related via some original mother which they called the "mitochondrial Eve".
They produced ancestral trees that depended on the slow mutation rate of mitochondrial DNA
to estimate how the whole human population came from a single woman.
Methodology
- The mitochondrial data was determined using restriction analysis
- They used African Americans rather than Africans from Africa to represent native Africans in
their study.
- They used a program called PAUP to build a phylogenetic tree.
