The Mighty Mitochondrion: Cellular Powerstation

What are mitochondria?

Mitochondria are tiny organelles found in our cells that are surrounded by two membranes whose primary role is to create adenosine triphosphate (ATP), the cell's primary energy source. Mitochondria are found in practically every type of human cell and are essential for our existence. Mitochondria are sometimes referred to as the cell's powerhouses. They play a role in the conversion of the energy we obtain from food into energy that our cells can utilize. In the process they produce the majority of the chemical energy required to fuel the metabolic activities of the cell.

Mitochondria are found in every cell in the body with the exception of our red blood cells. The number of mitochondria in a cell is determined by the quantity of energy it needs. Muscle cells, for example, have many mitochondria because they must generate energy to move the body. Red blood cells, which transport oxygen to other cells, have no requirement for energy production because they deliver oxygen around the body passively. Mitochondria multiply when a cell's energy demands grow and are very dynamic as they continually divide, merge, and morph into new forms depending on what the cell needs.

What do they do?

Producing energy

Mitochondria's primary function is to metabolize (break down) the carbohydrates and fatty acids in our diet to produce energy.

Adenosine triphosphate (ATP) is an energy-carrying molecule present in the cells of all living beings. ATP is a molecule that absorbs chemical energy from food molecules' degradation and can easily move around the cell to power other cellular processes. Most ATP is produced in mitochondria as a result of a chain of reactions, which are collectively known as the Krebs cycle or citric acid cycle. In this cycle mitochondria metabolise nutrients into by-products that can be used for energy production. This all takes place in the folds or cristae of the inner membrane; oxidation-reduction reactions take place resulting in phosphorylation (the addition of a phosphate molecule) of ADP (adenosine diphosphate) to ATP (adenosine triphosphate). This process is known as oxidative phosphorylation and in other words is the use of oxygen to power a process that adds a phosphate molecule to ADP to create a super energy carrier ATP.

Although energy production is mitochondria's most well-known function, they also perform other important functions. In addition to creating energy mitochondria also provide energy, store calcium for cell signaling, create heat, and regulate cell development and death.

Calcium regulation

Calcium is required for a variety of biological functions such as neurotransmission, muscle contraction, fertilization, blood clotting, cell migration and cell growth, etc.

Mitochondria serve as essential regulators of cellular Ca2+ by storing and releasing it as needed. Mitochondria contribute to calcium regulation by rapidly collecting calcium ions and storing them until they are required. The uptake of small physiological amounts of calcium into the mitochondria is considered to control mitochondrial metabolism and ATP production. Mitochondria are capable of absorbing huge quantities of calcium under pathological conditions of increased cytosolic calcium (calcium overload in the cell).

The discovery of the molecular machinery that controls mitochondrial Ca2+ accumulation, storage and release has increased the number of (patho)physiological situations that are dependent on mitochondrial Ca2+ homeostasis in recent years.

Cell Death

Mitochondria also play a role in signaling between cells and cell death. Cell death sounds a little serious but it’s a natural process that occurs when a cell is past its best by date or when something goes wrong and it’s in the body’s best interest that the cell is removed. Mitochondria have been recognized as playing a central role in cell death process. It’s a complex process but in short, the mitochondria activate enzymes involved in apoptotic cell death. This permits mitochondria to evaluate not just if a cell should die, but also the manner of that death, based on the degree of the injury to the cell.

Mitochondria are assumed to have a role in disease because some disorders, such as cancer, include a breakdown in normal apoptosis which, in some cases, mean that the cell becomes immortal and loses control of the replication process.

Other aspects of mitochondrial physiology, such as bioenergetics and dynamics, play a role in cell death processes that pass through the mitochondria. The proper management of these mitochondrial processes is critical for the cell's survival and death, as well as the organism's overall health.

Mitochondrial DNA

Although the majority of DNA (our cellular instruction set) is stored in chromosomes within the nucleus, mitochondria have a little quantity of DNA of their own. Each human cell includes several hundred to 1,000 mitochondria, with each mitochondrion having two to ten copies of mtDNA.

Unlike nuclear DNA, which comes from both the mother and the father, mitochondrial DNA is only handed down from your mother. During the fertilization event in sexual reproduction, only nuclear DNA is transmitted to the egg cell, with the remainder being destroyed. And it is for this reason that Mitochondrial DNA is only inherited from the mother.

Compared to nuclear DNA, the mitochondrial genome is hyper-mutable, which is owing to damage induced by high quantities of reactive oxygen species (ROS) to which it is exposed to in the mitochondria. mtDNA lacks the strong DNA-repair mechanisms found in nuclear DNA and so is much more likely to have mutated DNA. Once levels get too large the mitochondria can become inefficient and in some cases is broken down and its components recycled to make new fresh mitochondria.