Mitochondria
Mitochondria
is found in all eukaryotic species i.e. plants, animals, and fungi. It is also
known as 'The Power House Of The Cell'.These microscopic organelles are
double-membrane bound structures that are critical in the generation of energy
(in the form of ATP), that helps in sustaining the life alive at the cellular
level.
Mitochondria
are dynamic entities with a rich history connected with the emergence of
complex life forms, not just energy generators. These organelles are
hypothesized to have evolved from ancient symbiotic connections between
encapsulated prokaryotic bacteria and early eukaryotic cells. The endosymbiotic
theory tells about the origins of mitochondria in eukaryotic cells and the
evolution of multicellular creatures.
The structure of mitochondria
Mitochondria
are double-membraned bound structures. It consists of various complex
structures which we will go through one by one: -
1. The Outer Membrane:
The outer membrane covers the mitochondrion and is smooth and semi-permeable.
It act as a barrier between the cytoplasm and the mitochondrion's inner
components blocking the entry and exit of unwanted substances. Porins are
channel proteins that allow specific chemicals to flow through them.
2. Intermembrane Space:
The intermembranous space is the thin region between the mitochondrion's outer
and inner membranes. It contains a number of enzymes that are involved in
various processes taking place in it (eg: Adenylate cyclase).
3. Inner Membrane: The inner membrane is
strongly folded and has multiple cristae invaginations. These folds boost the
surface area available for chemical reactions and ATP generation substantially.
Many chemicals cannot pass through the inner membrane, which contains numerous
protein complexes involved in the electron transport chain and ATP production.
4. Cristae: Cristae are finger-like
projections created by the folding of the inner membrane. It helps in oxidative
phosphorylation machinery, which consists of a number of electron transport
chain complexes that generate ATP, which is the cell's primary energy currency.
5. Matrix: It is a semi-fluid substance
found within the inner membrane and surrounded by the cristae. It contains
various enzymes required for the Krebs cycle (also known as the citric acid
cycle) and other metabolic pathways such as the breakdown of fatty acids, and
carbohydrates which ultimately leads to the release of energy in the form of
ATP.
6. DNA: Mitochondria have their own DNA,
which is circular and looks similar to bacterial DNA. This mitochondrial DNA
(mt-DNA) contains the code for protein that are required for oxidative
phosphorylation and energy production. It is only inherited from the mother
because mitochondria in sperm are not normally passed down to the offspring.
7. Ribosome: Mitochondria have their own
ribosomes, which is similar to that found in prokaryotes. It is 70-S in nature
which is smaller than those found in the cell's cytoplasm. These ribosomes
produce their own proteins that are crucial for mitochondrial function.
8. Mitochondrial Matrix Space:
The region within the inner membrane containing the matrix. It contains enzymes
that help in the citric acid cycle and other metabolic processes (eg: citrate
synthetase, malate dehydrogenase etc).
Functions of Mitochondria
Energy
production in the form of ATP is the major function of mitochondria but it is
also involved in some other important cellular functions. Here are some of the
most important mitochondrial functions:
1) Energy Production (ATP Synthesis):
The most well-known mitochondrial task is the production of ATP(adenosine
triphosphate), which serves as the primary energy currency of cells. This
energy is produced via a chain of reactions which takes place within the
inner membrane of the mitochondria.
2) Cellular Respiration:
Mitochondria play an important part in cellular respiration. It is a multi-step
process that involves the breakdown of nutrients like glucose and fatty acids
to produce ATP. The three major steps of cellular respiration are glycolysis
(which occurs in the cytoplasm), the citric acid cycle/Krebs cycle (which
occurs in the mitochondrial matrix), and oxidative phosphorylation (which
occurs in the mitochondria and involves the electron transport chain and ATP
generation).
3) Oxidative Phosphorylation:
The electron transport chain (ETC) performs oxidative phosphorylation within
the inner mitochondrial membrane. The passage of electrons through a sequence
of protein complexes creates a proton gradient across the inner membrane. The
transport of protons back into the matrix via ATP synthase causes the synthesis
of ATP from ADP and inorganic phosphate.
5) Calcium Regulation:
Mitochondria assist in the regulation of cellular calcium levels. They have the
ability to accumulate, store and release calcium ions when needed, which are
necessary for many physiological activities such as muscle contraction,
neurotransmission, and cell signalling.
6) Apoptosis (planned Cell Death):
Mitochondria play an important role in starting apoptosis, a planned cell death
process required for tissue integrity, eliminating damaged cells, and managing
cell population. The cascade effect of apoptosis is triggered by the release of
certain chemicals from mitochondria.
7) ROS Control: Mitochondria are an important
source of reactive oxygen species (ROS), which are produced as a consequence of
electron transport during oxidative phosphorylation. ROS act as signalling
molecules/ligands in a variety of biological functions.
8) Cell Signalling: Mitochondria is
involved in various intracellular signalling pathways. They produce chemicals
that act as signals/ligands that have the ability to alter cellular functioning
such as gene expression and immunological responses.
9) Thermogenesis: In certain
specialised cells, mitochondria produce heat rather than ATP. This process, of
heat generation is known as thermogenesis, which helps in homeostasis i.e
maintaining internal body temperature which becomes crucial for hibernating
animals.
10) Inheritance and Evolution:
Mitochondria have their own genetic material (mitochondrial DNA, or mtDNA) and
can replicate independently of the nucleus of the cell. Although mt-DNA
is maternal in nature as it is transmitted from mother to child.
11) Cell Differentiation:
Mitochondria help with cell differentiation and development. They provide the
energy in the form of ATP which is required by cell for its various processes,
during embryonic development and tissue expansion.
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