This kind of double-membrane organelle exists in almost all eukaryotic cells. Mammals red blood cells are one of the few exceptions. About 95% of the energy in life activities is provided by aerobic respiration of mitochondria, thus they are called "powerhouses" of cell. Their own genome and ribosomes can only produce a little of protein, but most proteins are still encoded by cell nucleus. Therefore, mitochondria are a semi-autonomous organelle.
Discovery of mitochondria and their energy metabolism?
Research by scientists began in 1850. German biologist Albert von Kölliker first discovered regularly arranged granules in muscle cells, which swelled in fresh water and shrank in saline solution. He speculated that a semipermeable membrane enveloped these granules. Similar structures were also found in other tissues. After half century chaotic naming, Carl Benda created the term "mitochondria" in 1898, and it was quickly accepted by everyone. Its meaning is "thread-like granule."
In 1900, Leonor Michaelis invented Janus Green for staining liver cells. He found that the green mitochondria faded during active respiration. This indicated that certain redox reactions occurred, as this dye changes color during such reactions. Although the Krebs cycle (also called the citric acid cycle or tricarboxylic acid cycle) was discovered in liver cells in the 1930s, it wasn't until the mid-20th century that energy metabolism and mitochondria were really connected.
By using differential centrifugation techniques and a sucrose solution, Hogeboom, Schneider, and Palade were able to isolate mitochondria while maintaining their structure and function in 1948. Mitochondria were later proven by subsequent researchers to be the site of the Krebs cycle and electron transport by this technological breakthrough. The chemiosmotic hypothesis was proposed by Peter Mitchell in 1961. When electrons are passed from one carrier protein to another in the electron transport chain, protons are pumped from matrix into intermembrane space to maintain a transmembrane proton gradient and an electric potential difference. Finally, protons flow back into the matrix to produce ATP through ATP synthase located on the inner membrane. This process is much like a turbine that is driven by water flow in a dam.
The number, shape, and distribution of mitochondria reflect the metabolic rate.
In lower protists, like red algae, there is only one, but higher animal cells possess several hundred to thousands mitochondria whose number is far exceeding plants because plants do not need to consume large amounts of energy to move and respond quickly to the environment. Even in the same organism, their number varies significantly. Cells with active metabolism have more mitochondria, and they are very few in inactive cells, such as unactivated lymphocytes. The number in liver cells even reaches around 1,000.
Their distribution is uneven in cytoplasm. Generally speaking, they tend to cluster in energy-demanding areas, and form a net-like structure. Near Golgi apparatus, rough endoplasmic reticulum, and nucleus, there are many mitochondria providing energy for protein and nucleic acid synthesis. Neurons' axons and dendrites are also rich in mitochondria for providing energy to neurotransmitters and ion pumps. Muscle fibers are often surrounded by them in muscle cells.
Granules and short rods are their common shapes. Their diameter is between 0.5μm and 1μm, and their length ranges from 1μm to 3μm. A very long length is reached in some metabolically active cells, sometimes even span the entire cell. In neurons and fibroblasts, the length can exceed 10μm sometimes. When B lymphocytes are activated to produce antibodies to resist invasion, the number of mitochondria increases sharply.