Symbiosis: 2 Billions Years in the Making

Mitochondria

I woke up this morning feeling tired; as if I had no energy. Of course, by the time my feet hit the floor, I realized this was quite impossible. Thanks to a 2 billion-year-old symbiotic relationship that lives within every one of our cells, thanks to our mothers. I am referring of course to mitochondria, often referred to as the power station of the cell, but in reality, this is the result of a symbiotic relationship that has power not only our cells but the evolutionary process that has taken us from a simple single-celled creature to the multitude of higher order life forms that inhabit our planet.

Approximately 2 billion years ago, our ancient ancestors engulfed another life form. When this occurs, it is generally referred to as consuming, however in this case both life forms survived and thrived in this unique partnership. Because one life form, the mitochondria were able to produce so much energy, the host cell was able to utilize it to create evermore-complex forms, from multicellular organisms to functioning eyes, neurological systems, and organs. Of course, this did not happen overnight, but over the first billion years or so, these two life forms became intertwined, eventually becoming dependent upon each other for survival at the cellular level. What was to separate and diverse life forms had become one.

If you could examine any of your cells, you will see that there are many hundreds of mitochondria within every cell (except your red blood cells). These mitochondria still resemble their ancient original form. However, they depend on our cells for survival as our cells rely on them. Almost all complex living organisms, from humans and fish, fungi to trees that have evolved over the past 2 billion years are dependent upon mitochondria and the symbiosis of the cell. Moreover, yet these ancient life forms still have their agenda. They have their DNA and split and divide on their time scale (although some of their DNA has been shared with host cells). Compared to our human genomes, mitochondria DNA are foreign. While it is understood that you share 99.8% of your genes with chimpanzees, and 60% with a banana plant, mitochondria DNA are entirely unlike any other despite residing within our own (and virtually all) cells.

The renowned biologist and prolific writer Dr. Lynn Margulis laid out this symbiotic relationship in her groundbreaking work, Origin of Eukaryotic Cells (1970), which has become the standard for modern biological science and understanding the process of evolution of higher-order organisms. Mitochondria are passed down almost entirely through female inheritance exclusively from the mother in sexual reproduction. This occurs because (almost) all of the mitochondria contained in the mammalian sperm cells are destroyed by the egg upon fertilization. This is why mitochondrial DNA is used to trace matrilineal lines.

The Powerhouse of the cell.

The calories that we eat, the oxygen we breathe, the water we drink are all processed by the mitochondria and converted into energy to power our biological processes, from moving, to thinking, to sleeping. Mitochondria use oxygen to convert chemical energy from calories into a form of energy that can be used by our cells through the process of oxidative phosphorylation. Part of this process includes the generation of adenosine triphosphate (ATP). The Krebs cycle is the name given to the sequence of chemical reactions that take place in the mitochondria where oxygen is consumed, carbon dioxide and water are waste products through the continual conversion of adenosine diphosphate (ADP) to ATP. Because our cells require constant energy input, ADP is continually converted to ATP by adding phosphate during the process of cellular respiration. That energy is stored in the form of chemical bonds, which can be broken apart, and from which energy can be gained. In return for all this work, the mitochondria receive not only a constant supply of food and oxygen but reside in the relative safety and protection of the host cell.

The system seems to be, at least from the outside, self-regulating. In host cells that require more energy, for example muscle cells or neurons, we find larger numbers of mitochondria. In heart muscle cells, for example, nearly 40% of the space inside individual cells are filled with mitochondria. Still, there is a cost for all this energy production. Mitochondria produce many waste products, some in the form of free radicals, highly reactive oxygen particles that have been associated with aging and certain health conditions. While our cells could not function without mitochondria, there is a price to pay. Because mitochondria DNA mutate 50 times faster than host cells DNA, these mutations, if detrimental, can be multiplied quickly. In 1988, the first mitochondrial mutation disease was identified, not surprisingly directly related to the enormous amount of free radicals produced in energy captured. Moreover, because of the matrilineal nature of mitochondria, these conditions can be passed down to maternal offspring.

These mitochondrial conditions can become manifest at any age, and result in a wide range of conditions and symptoms including blindness, muscle wasting disorders, seizures, organ failure, or hearing loss. Because of the nature of mitochondrial DNA, the conditions are at least at present incurable, although there are investigations into how to prevent transmission of conditions from mother to child. Fruit flies, those tiny denizens of laboratories that seem to enjoy drowning in our soft drinks, share over 44% of our genes, and reproduce so quickly that they are excellent specimens for genetic testing. Recently, scientists have been able to genetically engineer the mitochondrial mutations in these fruit flies that allow them to live 40% longer than average. This has obvious repercussions for human life spans.

Despite having existed in a symbiotic relationship for over 2 billion years, mitochondrial DNA is just now becoming understood along with its complicated  relationship to health and disease.

 

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