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. Energy of the cell is got from the enzymatic oxidation of chemical compounds in the mitochondria. Hence, the mitochondria re referred to as the 'power houses' of the cell. Almost all the eukaryotic cell have mitochondria, though they are lost in the later stages of development of cell like in the red blood cells or in elements of phloem sieve tube.
In 1890, mitochondria was first described by Richard Altmann and he called them as bioblasts. Benda in the year 1897 coined the term mitochondrion. In the 1920s, a biochemist Warburg found that oxidative reactions takes place in most tissues in small parts of the cell.
Mitochondria is a membrane bound cellular structure and is found in most of the eukaryotic cells. The mitochondria ranges from 0.5 to 1.0 micrometer in diameter. The mitochondria are sometimes described as power plants of the cells. These organelles generate most of the energy of the cell in the form of adenosine triphosphate (ATP) and it is used a source of chemical energy. The mitochondria also involved in other cellular activities like signaling, cellular differentiation, cell senescence and also control of cell cycle and cell growth. Mitochondria also affect human health, like mitochondrial disorder and cardiac dysfunction and they also play important role in the aging process. The term 'mitochondrion' is derived from a Greek word 'mitos' which means 'thread' and 'chondrion' which means 'granule'.
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:
- 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.
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.
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.
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 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 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.
Mitochondrial DNA or mtDNA or mDNA is the DNA in the mitochondria, rest of the DNA present in the eukaryotic cells is in the nucleus, in plants DNA is also found in chloroplasts.
The mitochondria have a small amount of DNA of their own. Human mitochondrial DNA spans about 16,500 DNA base pairs, it represents a small fraction of the total DNA in cells. The mtDNA contains 37 genes. All these genes are essential for normal function of the mitochondria.
These DNA help the mitochondria divide independently from the cell. mtDNA is maternally inherited. The fact that mt DNA is maternally inherited enables to trace the maternal lineage far back in time.
The mt DNA in most multicellualr organisms is circular, covalently closed, double-stranded DNA. mt.DNA is susceptible to free oxygen radicals. Mutations in the mitochondrial DNA leads to a number of illness like exercise intolerance.
Disease of mitochondria results due to the failure of mitochondria. Dysfunction in the mitochondria fails to produce energy that is needed for the sustainment of life and growth of an organism. Injury in the cell or even cell death results in the production of less energy. If the process happens throughout the body, the whole system begins to fail. The disease primarily affects young. The mitochondrial disease causes most of the damage to the cells of brain, heart, liver, muscles, kidney, respiratory and the endocrine systems.
The symptoms may be as follows depending upon the cells that are affected:
- Loss of motor control,
- Muscle weakness and pain,
- Gastro-intestinal disorders,
- Swallowing difficulties,
- Poor growth,
- Cardiac disease,
- Liver disease,
- Respiratory illness,
- Visual/hearing problems,
- Lactic acidosis,
- Developmental delays and
- Susceptibility to infection.
Like in other eukaryotic cells, the mitochondria in plants play an important role in the production of ATP via the process of oxidative phosphorylation. Mitochondria also play essential roles in other aspects of plant development and performance. It also has various properties which allows the mitochondria to interact with special features of metabolism in plant cell.
Mitochondria are known as "power houses" of the cells, they are unusual organelles and are surrounded by a double membrane. These organelles have their own small genome. They divide independently by simple fission. The division of the mitochondria is a result of the energy demand, so the cells with high need of energy have greater number of mitochondria.
The process creating energy for the cell is known as cellular respiration. Most of the chemical reactions of this process happen in the mitochondria.
A typical animal cell may have about 1000 to 2000 mitochondria.
Structure of Mitochondria
Mitochondria are bound by a double membrane - the outer membrane and the inner membrane. The outer membrane is smooth and covers the organelle. The inner membrane has many folds known as cristae. The cristae increases the available surface are to enhance the productivity of cellular respiration.
The double membrane of the mitochondria divides the organelle into two distinct parts - the intermembrane space and the mitochondrial matrix. The intermembrane space is the narrow space between the outer and the inner membrane. The mitochondrial matrix is the content enclosed by the inner membrane.
The fluid inside the mitochondria is called the matrix. Most of the chemical reactions of the cellular respiration process happens in the inner membrane of the mitochondria due to the high concentration of enzymes.
Mitochondria are independent organelles, they have their own DNA and ribosomes. They can replicate and multiply on their own and make their own proteins. They have circular DNA similar to bacteria and replicate by fission.
Functions of Mitochondria
The most prominent function of mitochondria is to produce energy to the cell in the form of ATP through oxidative respiration process. It also regulates the metabolic activity of the cell. The set reactions for ATP production is known as the TCA cycle, or the citric acid cycle or the Kreb's cycle.