Mitochondria are the cell's powerhouse whose internal structure has already adapted to produce high-energy compound ATP rapidly and massively. The ultrastructure can’t be distinguished under a light microscope, only an electron microscope can clearly see them.
Outer membrane
The smooth outer membrane is 6-7 nanometers thick. Proteins and lipids each account for half in the outer membrane. Mitochondria selectively exchange substances with cytosol through it. Porins on the surface selectively open, and molecules smaller than 5000KDa can pass through, such as amino acids, ATP, and acetyl-CoA, etc. There are also proteins on the outer membrane that inhibit or activate apoptosis.
Between the outer and inner membranes, there is a 6-8 nm wide gap called intermembrane space. Its composition is close to cytosol. It is filled with metabolic intermediates and some enzymes related to respiration.
Inner membrane
It is the most unique and important structure of mitochondria. Only about 20% of its composition is lipids whose main component is phospholipids, and there is almost no cholesterol. There is also a special lipid called cardiolipin on the inner membrane. They have two more hydrocarbon tails than phospholipids, so their arrangement is tighter to reduce ions permeability, especially protons. Only small molecules without charge can penetrate them. Hence, some larger molecules or ions entering the matrix need the help of carrier proteins.
Inner membrane folds toward the central cavity of mitochondria to form protruding structures called cristae. The surface area increases by 5-10 times for biochemical reactions. In the cells with active metabolism, there are not only more mitochondria, but also more cristae. The inner membrane is embedded with a large number of enzymes related to aerobic respiration, so that approximately 80% of its components are proteins. These proteins are responsible for energy metabolism (electron transport chain and ATP synthesis) and passage of substances across inner membrane. You will observe a lot of granules about 10 nanometers apart on inner membrane under an electron microscope. They are actually ATP synthase.
Matrix
Inner membrane encloses mitochondrial matrix into a gel-like compartment, where many enzymes and substrates necessary for cellular respiration are located. This is not only the site of citric acid cycle (also called Krebs cycle), β-oxidation of fatty acids, and degradation of amino acids, but also includes some steps of urea cycle, a process that converts ammonia into urea to remove cytotoxicity. In the matrix, there are also mtDNA, RNA, and mitochondrial ribosomes for somewhat self-sufficient protein synthesis. However, most proteins are still encoded by cell nucleus and synthesized by cytosolic ribosomes. Therefore, mitochondria are also called semi-autonomous organelles.
Mitochondrial function: how does it produce energy?
Burning glucose is their main function, so they have earned the nickname "cell powerhouse." Glucose is broken down into pyruvate by anaerobic glycolysis in cytosol and completely oxidized into water and carbon dioxide in mitochondria. In the matrix, pyruvate is converted into acetyl-CoA that undergoes tricarboxylic acid cycle to release high-energy compounds ATP and NADH. Electrons contributed by NADH gradually lower their energy along the electron transport chain (respiratory chain). Meanwhile, complexes on inner membrane pump protons from matrix into intermembrane space (cristae). The pH difference across the membrane is about 1. The reduced electrical energy is stored in proton concentration gradient and electric potential difference. Finally, protons flow through ATP synthase that produces 3 ATP for every rotation. If rapid heat generation is needed, protons directly penetrate uncoupling proteins instead of ATP synthase into matrix. Just like a short-circuited battery, and all the electrochemical energy dissipates as heat. Uncoupling proteins are abundant in brown fat hibernating animals use to maintain body temperature.