The Origin of Mitochondria

We have all been taught that mitochondria are the powerhouse of the eukaryotic cell (cells containing membrane bound organelles such as the nucleus, golgi apparatus, mitochondria …). It is within this membrane bound organelle where most of the energy needed for our cells to survive is produced. What exactly is this energy, and how did mitochondria come about?

The energy, quite simply, is ATP (adenosine triphosphate). ATP is produced by a series of pathways involving many enzymes, cofactors, and of course glucose. With every glucose molecule that enters a cell, roughly 36-38 molecules of ATP are produced. Glucose begins its breakdown in the cytoplasm of the cell through the anaerobic pathway of glycolysis. The products of glycolysis are then transferred into the mitochondria where they are shuttled into the Krebs cycle and then on to oxidative phosphorylation. In respect to cellular respiration, the difference between the cytoplasm and the mitochondria besides the enzymes that are present, is the use of oxygen. Aerobic respiration, which consumes oxygen in the reaction, only occurs in the mitochondria. It is the addition of this extraordinary organelle that aided in the evolution of the eukaryotic cell and therefore the higher species.

mitochondria

It is believed that mitochondria arose through a process called endosymbiosis nearly 2.1 billion years ago. This theory suggests that a bacterium entered a cell, lived in a mutually beneficial relationship with its host and hence evolved into the mitochondria that we are familiar with today. The diagram at the left illustrates how this could have happened. In A, the cell membrane surrounds a bacterial cell and engulfs it. In B, we see that the bacteria now contains an outer membrane, compliments of the host cell. The original bacterial plasma membrane becomes highly folded and becomes the inner mitochondrial membrane that we see in C. This folded structure increases the surface area where oxidative phosphorylation takes place, therefore many of these reactions can occur simultaneously, optimizing the amount of ATP produced. Quite an efficient system for something that had once been a lowly bacterium.

The evidence for this theory is overwhelming. First, mitochondria are similar in size and shape to some strains of bacteria. They are between 0.5-1 μm in diameter and 1-10 μm long. The inner membrane of the mitochondria contain enzymes similar to those found in some bacterial cells. Mitochondria also contain their own genome. It is a circular molecule like that found in bacterial cells. mtDNA containing ~16 kilobases. Thirty-seven genes have been identified. Thirteen of these genes encode proteins used in cellular respiration that takes place within the mitochondria, and 24 code for RNA molecules involved in mitochondrial protein synthesis. There is a greater degree of homology between mitochondria RNA and the RNA found in bacteria than there is between mtRNA and the RNA found in the cytoplasm of eukaryotic cells. Mitochondria reproduce within the cell by way of fission, as do bacteria.

Mitochondria are inherited by the mother. They are found in the egg, and any mitochondria that might have been inside the sperm and entered the egg during conception are destroyed. Because of this, maternal linages can be easily traced.

The number of mitochondria within a cell type varies considerably depending on the energy needs of that cell. As you may imagine, muscle cells and brain cells contain thousands because their energy needs are greatest. Red blood cells do not contain any mitochondria, as their life is short and their energy requirements are produced in their progeny cells.

Without mitochondria, the organism would not be able to meet its energy needs and we might still be one cell organisms swimming in an oxygen deficient bog.

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ENDORPHINS. Not the happy drug you’re looking for.

The human body has many mechanisms that signal that something is not right. For instance, a fever indicates the presence of a harmful pathogen, thirst is a natural response to dehydration, and feeling pain brings about an awareness that damage is occurring within the body. In turn, the body may release hormones or other molecules that help overcome the crisis.

We have known for some time that endorphins act as a natural analgesic, that is, it is a natural pain reliever.  They are considered endogenous opioids because they bind to the same cell surface receptor as do opium and morphine, and hence have similar effects. There are many myths surrounding the release of endorphins into the blood stream. Many people believe that endorphins are released when one is happy, during orgasms, even when someone smiles. The truth is, that there is NO scientific evidence supporting these claims. In fact, the level of endorphins in the blood stream has been shown to decrease during sex8.

Endorphins are released in the body in response to pain, or extraneous exercise. Simply put, they prevent someone from feeling the pain associated with tissue damage. β-endorphin is a 31-amino-acid-peptide that functions as a neurotransmitter, that is, its target cell is a neuron. β-endorphin is released from the anterior pituitary gland in response to a pain signal that has been sent to the hypothalamus.  It then enters the blood stream.

Let’s first look at the pain signal. There are pain receptors throughout the body, called nociceptors. When tissues are damaged, they release a variety of substances, such as histamine, potassium and arachidonic acid. These substances stimulate the nerves and cause the release of Substance-P, which activates the pain pathway and transmits the pain signal to the central nervous system. In this way, the brain not only recognizes pain, but it also commits its source to memory. It is this recognition that creates the reflex to move away from the source of the pain.

endorphin

The opiate receptor is located on the neurons that release Substance-P. When β-endorphin is release in response to pain, it will bind to this receptor and as a result Substance-P will not be released. Therefore, the pain pathway will not be initiated, or will be reduced. β-endorphin is quickly degraded by an enzyme, and the receptor will then be free to bind to its substrate again. If this degradation process did not take place, then the feeling of pain will be nullified and the hand that inadvertently sat on the hot stove will remain there.

Addiction to opioids such as morphine and opium is the result of the drugs remaining on the receptors for long periods of time. A person becomes numb after a while because the receptors become “down regulated,” that is, there are less of them on the surface of the cell. Quite simply, there are fewer receptors and it takes more of the drug to find them. It then takes more of the drug to reach the same level of the “pain free” high.

The physiological role of endorphins in the human body has not yet been fully elucidated. However, one thing is clear. Its role is not to induce happiness, but to remove the pain.