Human bodies are composed of millions upon millions of cells, and each cell is a complex unit composed of organelles, a nucleus, cytoplasm and a cell membrane that holds it all together. The only exception to this rule is red blood cells, which have no nuclei. One of the most important organelles in the cell is the mitochondrion (plural is mitochondria). Here, the energy required for cellular function, in the form of adenosine triphosphate (ATP), is produced.
The average cell has 200-2000 mitochondria, although individual cells may have up to 10,000 mitochondria. Fat cells, which store energy, tend to have higher concentrations but not as high as those of muscle cells. It is the large number of mitochondria in muscle cells that allow the muscle to react so quickly. Mitochondria are extremely tiny, and in-depth studies of them were not possible until the invention of the electron microscope in 1931 by Max Knoll and Ernst Ruska. At that time cellular biologists Professor George Palade and Professor Fritiof Sjostrand undertook major studies of this organelle.
The mitochondrion is a double-membrane organelle. It is usually oval or cigar-shaped but is in constant motion and can move within the cell and change its shape. The outer membrane is smooth and permeable to oxygen, pyruvate, ATP and other molecules. It is separated from the inner membrane by the intermembrane space. The inner membrane is folded in on itself many times, forming convoluted folds called cristae (singular is crista). It is on these cristae that oxidative phosphorylation occurs when the charged molecules derived from fats and carbohydrates react with oxygen to produce ATP. Many of the chemical reactions that occur within the mitochondrion happen on the cristae.
The interior of a mitochondrion is called the mitochondrial matrix. It is a highly complex mixture of enzymes, ribosomes and mitochondrial deoxynucleic acid (mtDNA). The enzymes are involved in the production of ATP and other chemical reactions within the mitochondrion. The ribosomes are of the 70s type and participate in the production of various proteins. The mtDNA is perhaps the most fascinating aspect of the mitochondrion.
Mitochondria have their own deoxynucleic acid (DNA) which is separate from that of the cell nucleus. They also produce their own ribonucleic acids (RNA) and proteins. They carry the genes for almost all of the mitochondrial proteins. Because of this separate mtDNA and the fact that mitochondria reproduce by fission, like bacteria, it has been hypothesized that mitochondria were originally bacteria that entered into an endosymbiotic relationship with eukaryotic cells.
All mtDNA is inherited maternally, which means that all of an individual’s mtDNA is inherited from its mother. This discovery has had a profound effect on evolutionary and genetic studies and earned mtDNA its popular nickname, the “Eve gene.”
The functions of mitochondria are extremely important to cellular life. Although their biggest contribution is in the production of energy in the form of ATP, they also contribute to the maintenance of calcium ionic balance by storing and releasing calcium ions as needed within the cell. Mitochondria also assist in the production of some components of blood and hormones such as testosterone and estrogen. Within liver cells, mitochondria detoxify ammonia. They also assist in programmed cell death. This occurs when unnecessary and excess cells are culled during the development of an organism.
Mitochondrial dysfunction can result in cell death and organ failure. It is associated with degenerative diseases such as Parkinson’s disease, Alzheimer’s disease, Huntington's disease, heart disease, hypertension, diabetes, osteoporosis, cancer and autoimmune diseases such as lupus, rheumatoid arthritis and multiple sclerosis. It has also been linked with mental disorders such as autism and dementia.
The importance of these tiny organelles within the cell cannot be underestimated. Without them there is no energy produced to fuel cell function, resulting in cell death and ultimately the death of the organism itself. They are an essential aspect of how eukaryotic cells work in all organisms.