Mitochondrion

Mitochondria are known as the powerhouses of the cell. They have two membrane coverings, a porous outer membrane and a deeply-folded inner membrane. These folds increase the surface area for Adenosine triphosphate (ATP)-generating chemical reactions. Mitochondria release the energy needed for various chemical activities in daily life as ATP molecules. Energy stored in the ATP, the cell’s energy currency, is used by the body to create new chemical compounds and mechanical functions. Mitochondria are strange organelles given that they have their own DNA and ribosomes and are thus able to make some of their own proteins.

Mitochondria

Mitochondria, unless specifically stained, are not easily visible under the microscope. The number of mitochondria per cell is variable depending on the physiological activity of the cells. A considerable degree of variability is observed even in their shape and size. Mitochondria are generally sausage-shaped or cylindrical, with a diameter of 0.2-1.0µm (average 0.5µm) and a length of 1.0-4.1µm.

History

Kölliker (1880) was the first to discover mitochondria in the muscles cells of insects. He referred to them as sarcosomes. The mitochondria were termed ‘fila’ by Flemming (1882). Altmann discovered them in 1894 and called them Altmann’s granules or bioblasts. Benda coined the word “mitochondria” (1897–98). Hogeboom and his colleagues identified them as respiration sites in 1948.

Mitochondrial Morphology

Morphologically, mitochondria can have the appearance of filaments or tiny granules that can take the form of rod-like structures known as chondrocytes that can expand or combine to create gigantic spheroid forms known as chondrosphere.

Position – Mitochondria float freely in the cytoplasm, with the ability to move independently and take the shape of filaments. They can move freely in certain cells, bringing ATP where needed, while in others, they are constantly situated within the part of the cell where more energy is required.

Quantity – The number of mitochondria varies greatly across cells and between species. Only a few algae and protozoans contain a single mitochondrion. Their number is proportional to the cell’s activity, age, and kind. Cells that are growing, dividing, and actively producing have more mitochondria than other cells. There might be up to 50,000 mitochondria in an amoeba. These are small in number in rat liver cells, ranging from 1000 to 1600. Some oocytes have up to 300,000 mitochondria.

Size – On average, mitochondria are 0.5-1.0µ in diameter and 2–8µ in length. They are roughly 10µ long in mammalian pancreatic exocrine cells and 20–40µ long in frog Rana pipiens oocytes. The mitochondria in yeast cells are the smallest.

Mitochondria Ultrastructure

The electron microscope reveals the mitochondrion as vesicles bordered by an envelope of two unit membranes and filled with a fluid matrix.

Membranes – In molecular structure, the inner and outer mitochondrial membranes mimic the plasma membrane. Each is 60–70 trilamellar, with two layers of phospholipid molecules sandwiched between two layers of protein molecules. However, the two membranes differ in terms of the types of protein and lipids they contain and their characteristics. The outer and inner membranes have particular pumps or channels to transport chemicals across them. The membranes can be linked at adhesion sites, where proteins are transported to the inner membrane from the outside. The outer and inner membranes are separated by a small gap known as the intermembrane space, outer chamber, or peri-mitochondrial space.

• Outer membrane

The outer membrane is permeable to most small molecules and contains transmembrane channels produced by the protein ‘porin’. It is approximately 50% lipid, including a significant proportion of cholesterol. It includes certain enzymes but has a low protein content.

• Inner membrane

The inner membrane is selectively permeable and controls material passage into and out of the mitochondrion. It is high in enzymes and permease carrier proteins. It has a very high protein-to-lipid ratio (about 4:1 by weight). It is devoid of cholesterol. Cardiolipin is found in close proximity to some integral proteins and appears to be essential for their function.

The inner chamber is filled with a gel-like substance called the mitochondrial matrix, which fills the gap between the cristae. Proteins, lipids, some ribosomes, RNA, one or two DNA molecules, and various crystals, fibrils, and dense granules are found in it.

• Cristae

Plate-like infoldings on the inner mitochondrial membrane are known as cristae. They protrude inward to varying degrees and may merge with those from the other side, separating the mitochondrion into compartments. They are organised in distinct ways in various cells. They normally run perpendicular to the long axis of the rod-shaped mitochondria. Cristae are longitudinal folds parallel to the long axis of the mitochondria in cells of the proximal sections of the kidney tubules. They also differ in number. Active cells may have a large number of cristae, whereas inactive cells may have only a few. Cristae have a small intra-crista space that connects to the intramembranous space. Cristae significantly enhance the inner surface of the mitochondrion, allowing adequate space for enzyme assemblies to be housed. Cristae also allow mitochondria to expand or swell in response to changes in metabolic and environmental circumstances.

The inner mitochondrial membrane contains minute, regularly spaced particles known as inner membrane subunits, elementary particles (EP), or oxysomes. An oxysome comprises three parts: a spherical headpiece or an F1 subunit connected to a base piece, or an F0 subunit situated in the inner membrane by a short stalk. A single mitochondrion may contain 100,000 to 1,000,000 exosomes.

Functions of Mitochondria

• ATP production

The creation of ATP is possibly the most well-known function of mitochondria. This complicated, multistep process is required for the body to operate properly, and disruption can lead to a wide range of ailments, from diabetes to Parkinson’s disease to uncommon genetic abnormalities.

• Homeostasis of calcium

The movement of calcium into and out of a cell’s mitochondria is known as mitochondrial calcium exchange, and it is vital in metabolic control and cell death.

• Innate immunity regulation

Innate immunity refers to the inborn system that identifies and responds to pathogen infection by providing prompt, non-specific defence. MAVS (mitochondrial antiviral signalling protein) is important in the innate response to viral infections, assisting in the induction of antiviral and anti-inflammatory pathways.

• Programmed cell death

Apoptosis is the highly regulated process of planned cell death employed by multicellular organisms in various biological activities such as intrauterine development, sweeping up damaged cells, and cell population maintenance. Both an intrinsic and extrinsic mechanism can be used to induce the creation of apoptotic entities that are consumed by phagocytes.

Conclusion

Mitochondria, or “powerhouses”, are unique organelles enclosed by a double membrane and containing their own tiny genome. They also divide through simple fission, which occurs independently of the cell cycle. Mitochondrial division is driven by energy demand; therefore, cells with higher energy demand have more organelles than cells with lower energy needs. Mitochondria perform the majority of cellular oxidations and generate the majority of the ATP produced by the animal cell. The mitochondrial matrix is home to a wide range of enzymes, including converting pyruvate and fatty acids to acetyl CoA and those that oxidise this acetyl CoA to CO₂ via the citric acid cycle. These oxidation processes generate a large quantity of NADH (and FADH2).