Among all kinds of organelles, mitochondria is special because it contains ribosomes and carries its own genetic material. Mitochondrial DNA is circular, and there are some changes different from the genetic code of standard eukaryotes.
These characteristics led to the endogenous theory that mitochondria originated from endogenesis. This widely accepted theory holds that bacteria that once lived independently propagated in the same ancestor of eukaryotes and formed today's mitochondria.
Mitochondria (from the Greek mitos "thread" +Khondry "granule") are organelles existing in most eukaryotic cells (including plants, animals, fungi and protists) in cell biology. Some cells, such as trypanosoma, have only one large mitochondria, but usually there are hundreds of mitochondria in a cell. The specific number of mitochondria in cells depends on the metabolic level of cells. The more intense the metabolic activity, the more mitochondria there are. Mitochondria can account for 25% of cytoplasm volume.
It can be regarded as a "cell energy factory" because its main function is to convert the energy generated by organic oxidation into ATP, which is the main place for aerobic respiration to generate energy.
Mitochondria have various shapes, generally linear, granular or short. The diameter of mitochondria is generally 0.5 ~ 1.0 μ m, and the length varies greatly, generally, it is 1.5 ~ 3 μ m, and the length can reach 10μm, while the mitochondria of human fibroblasts are longer, reaching 40 μ m. Different tissues sometimes have abnormally enlarged mitochondria under different conditions, which are called giant mitochondria.
In most cells, mitochondria are evenly distributed in the whole cytoplasm, but in some cells, mitochondria are unevenly distributed, and sometimes mitochondria gather at the edge of cytoplasm. In cytoplasm, mitochondria tend to be concentrated in areas with active metabolism, because these areas need more ATP. For example, there are many mitochondria in muscle fibers of muscle cells. In addition, sperm cells, flagella, cilia and the base of renal tubular cells have many mitochondria. Mitochondria are not only distributed in areas where ATP is needed, but also concentrated in areas where there are many oxidation substrates, such as fat droplets, because there are a lot of fat in fat droplets to be oxidized.
Generally speaking, cells must have energy supply to be active. Mitochondria are organs that generate energy in cells, and the scientific community has also named mitochondria "power room", which is the power factory of cells. The number of mitochondria in a cell ranges from a dozen to hundreds. The more active a cell is, the more mitochondria it contains. For example, beating heart cells and brain cells that often think about problems contain the most mitochondria, while skin cells contain fewer mitochondria. Scientists have found that the mitochondria of farmers' skin cells are damaged much more than those of other indoor professionals because they work outdoors all the year round. When mitochondria are damaged, cells will die due to lack of energy. Our face is exposed all the year round, and it is always attacked by wind, rain and various pollution particles, so facial cells often die prematurely because of excessive pain.