“What is life?”
Though it seems like a philosophical question, this question is being studied by many scientists because they haven’t found an answer to it yet. It is a very common truth that all living things have life. However, it is not a simple matter to define life in one word. Many scholars have tried to define life, but they haven’t established one perfect theory. The current definition of life is the generalization of the common characteristics of life.
Living beings have the following characteristics: homeostasis, organic system, growth and development, reaction to stimulation, reproduction, and metabolism. All these characteristics are important features that determine living creatures, but one of the bases is metabolism.
Metabolism means all material changes that occur inside living organisms, including catabolism and anabolism. The most important purpose of this activity is to obtain energy needed to sustain lives. No living organism can live without energy. In other words, no life can be independent from energy. Energy is the essential element that makes the living organisms live, and it is the root of life. Then, where do living organisms obtain energy from?
The organ that is considered as the center of a human life is the heart. Medically, when one’s heart stops beating, he or she is considered dead and lifeless. How can the heart beat without resting even for a moment during the whole lifetime? What is it that gives life to the heart?
This is closely related to the mitochondria, the energy factory of our body. Mitochondrion is a small organelle in a cell, but it is in charge of producing energy through the cellular respiration1. Mitochondria are in most eukaryotes2, and humans have a large amount of mitochondria that take up to 10% of body weight. Mitochondria are dispatched in various parts of cells to efficiently deliver energy the cells need. That is why there are more mitochondria in the organs that require furious activities and breathing such as the brain, heart, liver, muscles, etc.
- Cellular respiration is a process that a cell gains oxygen, and breaks large molecules into smaller ones, releasing energy in the process. It is also called internal respiration.
- Eukaryota is any organism whose cells contain a nucleus. The cells contain organelles such as mitochondria and endoplasmic reticulum.
Mitochondria produce adenosine triphosphate [ATP] by using the oxygen delivered through breathing. ATP is like an energy currency. Each ATP has three phosphate groups. When each phosphate group falls off, a certain amount of energy is produced, and the cells use this energy. In other words, our body receives energy for maintenance of life, using the ATP produced in the mitochondria. Almost all the organs depend on the energy made in mitochondria. So, various types of diseases can be caused by mitochondrial deterioration or mitochondrial dysfunction. That is why the biological field and medical field have been active recently in researching the root cause of aging and diseases from mitochondria, and how to keep healthy mitochondria for a long period of time.
We see how mitochondria are essential for life. What is interesting is that all mitochondria in our cells are from our mothers; it is maternal inheritance. It is contrasted with the paternal inheritance, which is to receive Y chromosome from fathers.
The genes of mitochondria exist independently unlike most other cellular genes. Most genes in cells are condensed in a space called cell nucleus, but the genes of mitochondria are in independent spaces inside mitochondria, not in cell nuclei. As mentioned earlier, mitochondria exist in most cells, so when cells are divided, cell nuclei are not the only elements that are divided, but mitochondria are also divided to enter each cell.
There is no exception when sperm and egg, which are germ cells, are divided. Both the sperm which is from the father and the egg which is from the mother have mitochondria. However, when the sperm fertilizes the egg, the sperm’s mitochondria cannot enter the egg as its tail is cut off. It is because most of the sperm mitochondria are in the sperm’s tail. Even if some of them enter the egg, they are destroyed by the egg. As a result, the DNA in the nucleus of the embryo is half from the father and half from the mother, but mitochondrial DNA is fully from the egg, meaning the mother.
This maternal inheritance of mitochondria is useful in the field of biotechnology. There is only one nucleus in each cell, but as for mitochondrial DNA, the same genes are as many as the number of mitochondria. In other words, in one cell, there are thousands of the same mitochondrial DNAs, and those DNAs are most likely to remain without getting damaged for a long time because they are so small. So, when it is hard to analyze a nuclear DNA like the case of old bones, an identification test is done through the mitochondrial DNA. Since mitochondria are inherited through mothers only, it is mostly used to check the maternal line. In fact, a mitochondrial DNA test was used to identify the bodies of the people who died in the September 11 attacks in the U.S.
The source of power for a fertilized egg, which is the starting point of life, to bloom and make the hands and feet of a baby is the mitochondria left by the mother. The mother’s love is implanted on every part of our body and produces the energy of life.
Still, the exact origin and reason have not yet been identified why mitochondria are inherited only through mothers. Even today, we are breathing and living through mitochondria, the inheritance of the life energy which we can only receive from the mother.
- Reference
- Newton Editorial Department, Mitochondria, the Most Important Organ in Cells, Are the Main Current Interest, Newton, December 2012
- Masato Hiki, Mitochondria Perennial Youth Method (in Japanese, ｫﾟｫﾈｫｳｫﾉｫ・｢ﾜﾕ糶), Gentosha Media Consulting, 2009
