Mitochondria are membrane-bound organelles found in all eukaryotic cells. They are oval in shape and have an outer and inner membrane with inward protrusions called cristae. They serve as the site of cellular respiration and help capture energy.
They break down sugars in the cell and produce ATP (energy molecules). All eukaryotic cells have mitochondria for this vital function.
Known as the powerhouses of the cell, mitochondria produce a constant supply of adenosine triphosphate (ATP) – the cell’s primary energy-carrying molecule. This is accomplished through a process called cellular respiration, in which fuel molecules are converted to energy and waste products are eliminated. Mitochondria perform this function both in darkness and light, but they are especially critical during photosynthesis.
In addition to making ATP, mitochondria are important for controlling the redox balance of the cell. They accomplish this by transferring electrons across their inner membrane, producing a proton gradient that generates the energy needed for many of the organelle’s functions.
Both mitochondria and chloroplasts have their own DNA and ribosomes, but they also depend on the cell nucleus to direct the production of proteins for the organelles’ membranes, which are synthesized by free ribosomes in the cytoplasm. The mitochondrial matrix has a pH of around 8 and contains enzymes that catalyze many of the same reactions as those in oxidative phosphorylation, including splitting water molecules to release oxygen.
Both mitochondria and chloroplasts are often described as “organelles within cells” because they both have a double membrane with folds called cristae that form a semi-permeable channel between the interior of the organelle and the cytosol. Both have a similar origin, with evidence that they evolved through endosymbiosis, a relationship in which two different organisms engulfed each other and began living in a symbiotic relationship. Despite this, they have many differences, including their size, shape, and protein content across tissue types and over development. They can also change their ATP-producing capacity on a moment-to-moment basis to meet the metabolic demands of their host cells. This bidirectional communication is crucial for promoting a healthy cellular environment and is the reason why dysfunction of mitochondria is linked to so many aging-related diseases.
Plants rely on mitochondria to break down the sugar that is created during photosynthesis into easy-to-use energy molecules, particularly for respiration during the night. This is a vital function, and it is performed by all eukaryotic cells. It is thought that the mitochondria are tools — or parts of a system of tools — to help the cell make use of the nutrients that are produced through photosynthesis.
As with other organelles, the number and types of mitochondria within a cell vary over time and in response to various stresses. Mitochondrial proteomic analysis has shown that the mitochondrial proteome varies widely among different cell, tissue, and species. Over the course of germination, for example, a dramatic change in protein abundance was observed. Proteins encoding components of the electron transport chain and the TCA cycle were induced early on in germination, while transcripts encoding proteins involved with mitochondrial import were induced later (Howell et al., 2006).
Mitochondria also function to communicate with the nucleus of the cell about cellular conditions and signaling. Studies have shown that the mitochondria can send messages that alter nuclear gene expression, and that these changes are triggered by the presence of certain stresses.
The bidirectional communication that takes place between mitochondria and the rest of the cell also helps to regulate redox conditions, which are important in maintaining the homeostasis of cells. For example, a high level of respiration can increase the production of reactive oxygen species, which can cause damage to the cell if they are not controlled. This type of imbalance is regulated by the antioxidant enzymes that are produced in mitochondria. For instance, the terminal oxidase (AOX) in plant mitochondria can scavenge oxidative stress by generating toxic metabolites that counteract ROS.
A major function of mitochondria is apoptosis, or cell death. Mitochondria help initiate apoptosis in order to get rid of old or damaged cells that are no longer needed. This is done by releasing a protein named cytochrome C which can activate another enzyme called caspase that begins to break apart the cell. Once the cytoplasm is free of its organelles, the cell will die and another cell can take over its functions.
Cells that require the most energy, such as immune cells or cardiac muscle cells, will have more mitochondria than other cells. This is because the shape of mitochondria can change to allow them to take up more space if necessary. This is a result of a conformational change that happens when the mitochondria are stimulated with ATP.
The structure of mitochondria consists of an outer membrane and an inner membrane that are both made from phospholipid layers. The inner membrane is a lot thinner than the outer, and it contains many folds called cristae that increase surface area and enhance the efficiency of the reactions that take place inside. The inner membrane is impermeable to most ions and small molecules, except for oxygen which passes across it with the help of porin proteins.
Mitochondria also have their own small genome that encodes about 50 proteins and a variety of tRNAs and rRNAs. Most of the other more than 1,000 proteins found in mitochondria are encoded by nuclear genes and imported into mitochondria from the cytosol as multisubunit complexes. These complexes are assembled by a set of enzymes known as mitochondrial import pathways. Larger proteins are only able to enter mitochondria if a signal sequence at the tip of the protein recognizes a receptor protein in the outer membrane, and sticks to it. The receptor then hands off the tethered protein to a translocator protein that channels it past both the inner and outer mitochondrial membranes.
A coenzyme is a specific group of molecules that facilitates a series of chemical reactions. These reactions are required for cellular respiration and other vital functions in the cell. They transfer energy from one molecule to another, allowing cells to perform many complex tasks. For instance, the coenzyme NADH transfers four hydrogen atoms from one mitochondrial compartment to the other during oxidative phosphorylation. This process replenishes the ATP supply of the cell and allows it to perform its normal functions.
The discovery of mitochondrial signaling pathways has revealed that these organelles are more than just a powerhouse for producing ATP. They also communicate bidirectionally with the rest of the cell on a moment to moment basis. This communication is primarily done through calcium and ATP monitoring and sophisticated internal regulatory processes. This communication is important for maintaining the correct function of mitochondria and the underlying cell. When this communication is disrupted, it can cause a variety of disease conditions.
Studies of mitochondrial biogenesis have shown that the size, shape, and protein content of mitochondria vary across different tissues and over the course of development. In plant seedlings, for example, mitochondria are found in small and limited numbers at the early stages of germination. However, they increase in number and morphology as the germination process progresses. The regulation of nuclear genes encoding mitochondrial proteins is also influenced by the varying cellular environment. For example, the alternative oxidase (AOX) is induced by various treatments and may help to coordinate mitochondrial biogenesis in response to changing environmental conditions.
Mitochondria are essential for plants to thrive. If they were not present, all of the glucose prepared by Chloroplast during photosynthesis would go to waste because the only way for a plant to use this glucose is by breaking it down in its own cells with the aid of mitochondria.
Mitochondria and chloroplasts are complex organelles that generate a high energy yield through cellular respiration. This is a multi-step process that uses oxidative phosphorylation to produce ATP. They are important for converting sunlight into energy inside the cell through photosynthesis. They are also involved in scavenging reactive oxygen species that are generated during this process. Dysregulation of these two organelles can lead to compromised complex bioenergetics and exacerbated oxidative stress. This exacerbates systemic pro-inflammatory responses and can contribute to major diseases including Type II diabetes, atherosclerosis and rheumatoid arthritis.
The electron transport chain of mitochondria ends with cytochrome (cyt) oxidase, also known as alternative oxidase. AOX is a cyt-specific enzyme with a lower affinity for oxygen, which reduces ATP production, but prevents the generation of superoxide radicals, thus preventing cell damage. During drought, the AOX gene in plants was found to be up-regulated. Cyt oxidase activity was increased in the knockdown and overexpression plants during reoxygenation, indicating that AOX may act to maintain cyt pathway function.
Another important function of mitochondria is regulating energy demand and production in response to different environmental conditions. During germination, the genes that encode for mitochondrial proteins were found to be activated in response to light, but not to darkness. This indicates that plants have a mechanism to switch between metabolically active mature mitochondria and dormant promitochondria in response to changing ambient light conditions. It is likely that this molecular switch is facilitated by dynamin-like GTPases, called DRPs, which can form oligomeric rings around membranes to facilitate their fission. Interestingly, many DRPs are dual-targeted to both mitochondria and peroxisomes. Loss-of-function mutants of these proteins showed altered morphologies of both organelles during germination.