Mitochondria are double-membraned organelles that are found in most eukaryotic organisms. They use aerobic respiration to produce ATP, a chemical energy source for cells. This energy is then used by cells to sustain their life. Read on to learn more about mitochondria and their functions.
Functions
The functions of mitochondria in a cell are highly complex. They must operate in a precise manner in order to generate energy and perform other functions, including ion buffering. These functions depend on finely tuned interactions between mitochondria and other organelles. When this relationship is disrupted, the mitochondria may fail to perform their essential roles.
The most important function of mitochondria is to produce energy within the cell. This occurs by means of a metabolic process known as the TCA Cycle. It is an important metabolic pathway that can be studied in detail in introductory biology courses. However, it is not possible to cover all of the functions of mitochondria in one course, as many of them are specific to certain types of cells.
The outer mitochondrial membrane contains a special type of integral protein called porin. It forms large nonselective channels through the lipid bilayer and allows molecules up to 10000 daltons to pass through. The inner mitochondrial membrane is a thin, tube-like structure with a large surface area. It is surrounded by a matrix space. It is filled with soluble enzymes that break down acetyl-CoA, a precursor to energy, to produce CO2. In addition, hydrogen ions are used to reduce NAD and FAD molecules, and pass them on to the respiratory chain.
Structure
The structure of mitochondria is a subject of ongoing research in biology. While mitochondria can be visualized using a light microscope, their detailed internal structure can only be revealed using electron microscopy. In the 1990s, electron tomography of mitochondrial thin plastic sections revealed striking three-dimensional images of the inner membrane system. However, these images were incomplete because chemical fixation, dehydration, and heavy metal staining lost molecular detail. However, recent developments in cryo-EM have enabled researchers to acquire increasingly detailed images of mitochondria. Now, single-particle cryo-EM of membrane protein complexes has been used to provide near-atomic resolution of the membrane.
The mitochondrial membrane consists of an outer membrane and an inner membrane. The outer membrane is porous and is composed of phospholipids. Its structure is similar to that of a wrinkled bag. It contains transport proteins and pore-forming membrane proteins. Porin channels allow molecules of more than ten thousand daltons to pass freely. Compared to the outer membrane, the inner membrane has a different structure and function. The outer membrane is made up of a lipid bilayer and contains several porins, which allow certain molecules to pass through. The inner membrane is a tight diffusion barrier, while the outer membrane is a phospholipid bilayer with pores. Various membrane transport proteins pass ions and other small molecules through it.
The inner membrane contains folds called cristae, which form compartments in mitochondria. These folds increase the surface area of the mitochondria and help them produce more ATP. There are approximately five times as many cristae in the inner membrane as there are in the outer membrane. The number of cristae depends on the energy demand of the cell.
Relationship to other organelles
The mitochondria are very small organelles that perform specific functions for the cells. They contain ribosomes and DNA. They also have structures called granules that help regulate ion concentrations. Cell biologists are still investigating how these structures function. The outer mitochondrial membrane is 60 to 75 angstroms thick.
The mitochondria represent the energy hub of eukaryotic cells. They are closely linked to other organelles within the cell, including the nucleus and endoplasmic reticulum. They regulate cellular energy metabolism and biosynthesis. Alterations in these interactions are associated with tumorigenesis.
The mitochondria and other organelles constantly communicate with each other. These interactions may occur through the release of mitochondria-derived vesicles (MDVs) to lysosomes and peroxisomes. These MDVs share ER components such as Fis1. In addition, they share several pathways that promote the biogenesis of both mitochondria and peroxisomes. Moreover, they are required for the degradation of damaged or excess peroxisomes.
The internal structure of mitochondria is similar to that of chloroplasts. The outer membrane is believed to have come from eucaryotic cells, while the inner membrane is derived from bacteria engulfed in host cells. The folds in the inner membrane provide a large surface area for reactions.
Number of mitochondria in eukaryotes
Almost all eukaryotes contain mitochondria, which are membrane-bound organelles that generate large amounts of energy in the form of ATP. They are typically round or oval in shape and range in size from 0.5 to 10 mm. They also store calcium for cell signaling activities, produce heat, and mediate cell growth and death.
The mitochondria, as well as the chloroplast, play important roles in eukaryotic cells. Both organelles are enclosed by double membranes. Mitochondria are thought to have evolved during the evolution of all life. Originally, eukaryotic cells ate single-celled organisms to obtain energy. In time, these “eaten” prokaryotes continued to produce essential cellular functions, and the host eukaryotes eventually came to depend on their contributions.
The mitochondria are found in complex cells, such as those found in humans. These cells are unable to function properly without them, and researchers have been trying to find a eukaryotic cell that lacked mitochondria. One example of such a cell was a human gut parasite called Giardia. Although researchers initially considered Giardia a living fossil, further research revealed that it contains mitochondria.
The number of mitochondria is highly variable between cells. For example, red blood cells don’t contain mitochondria, while liver cells and muscle cells have thousands of mitochondria. The only known eukaryotic organism without mitochondria is the Monocercomonoides species. Mitochondria have two distinct membranes and unique genomes, and they reproduce by binary fission. Though mitochondria share the same evolutionary past as prokaryotes, they differ in size and shape.
Relative importance of mitochondria to cellular metabolism
The number and size of mitochondria in a cell depend on the metabolic needs of that cell, and can range from a single large mitochondrion to thousands of smaller organelles. They are found in nearly all eukaryotes, and their size makes them easy to observe with a light microscope. Despite their size, mitochondria are highly flexible and can change shape rapidly, and are often arranged into traveling chains or stable groups. The exact configuration depends on the cell’s needs and the microtubular network that surrounds the cell.
The role of mitochondrial proteins is not fully understood, but scientists have identified several factors that regulate their activities. For example, the ubiquitin-like protein SUMO is a key regulator of mitochondrial fusion, which counterbalances the role of mitochondrial division. Other factors that control mitochondrial fusion include the presence of certain nutrients.
Mitochondria are critical to cellular metabolism. A dysfunctional mitochondrial network has been associated with a variety of diseases, including cancer, type 2 diabetes mellitus, and neurodegeneration. Progress on mitochondria has also led to new ideas for therapeutic approaches.
Importance of mitochondria to homeostasis of somatic stem cells
Recent studies have shown that mitochondria play a critical role in stem cell pluripotency, which is essential for regenerative medicine and cell therapy. In addition to their crucial role in energy production through oxidative phosphorylation, mitochondria also play an essential role in calcium homeostasis and ROS signaling.
Mitochondria integrate environmental cues to regulate homeostasis. They regulate bioenergetics, redox and calcium balance, epigenetics, and cell death pathways, all of which contribute to the maintenance of stem cell pluripotency and genome integrity. This important role makes it crucial to understand how mitochondria interact with cell fate.
Moreover, mitochondria play a crucial role during human endoderm differentiation, providing ATP and maintaining ROS levels. Understanding this role may provide a novel insight into metabolic regulation of cell fate determination. This new understanding may help in developing new strategies for treating various conditions, including diseases.
Mitochondria also play a key role in the response of stem cells to low oxygen levels. As mitochondria are crucial for maintaining the regenerative potential of tissues, they require tightly controlled energy metabolism. As a result, mitochondrial dysfunction may compromise homeostasis and contribute to aging and tissue degeneration.
