Mitochondrion

Mitochondria are the power houses of the cell. They contain the "molecular machinery" for the conversion of energy from the breakdown of glucose into adenosine triphosphate (ATP), the energy currency of the cell. The energy stored in the high energy phosphate bonds of ATP is then equipped to power functions in the cell. Mitochondria are mostly protein, but some lipid, DNA and RNA are present. The unique makeup of these organelles can be seen with the use of an electron microscope. These usually spherical organelles have an outer membrane that surrounds an inner membrane that folds into a scaffolding for oxidative phosphorylation, the synthesis of ATP by phosphorylation of ADP for which energy is obtained by electron transport and which takes place in the mitochondria during aerobic respiration, and electron transport enzymes. Most mitochondria have flat shelf-like cristae, any of the inwardly projecting folds of the inner membrane of a mitochondrion, but those in steroid secreting cells may have tubular cristae. The matrix of the mitochondrion holds the enzymes of the citric acid cycle, fatty acid oxidation and mitochondrial nucleic acids, any of various acids (as DNA or RNA) that are composed of nucleotide chains .

Mitochondiral DNA is double stranded and circular. Mitochndrial RNA comes in the three standard varieties. These are ribosomal, messenger and transfer, but each is specific to the mitochondria. Some protein synthesis occurs in the mitochondria on the ribosomes of mitochondria that are unlike the cytoplasmic ribosomes. Other mitochondrial proteins are made on cytoplasmic ribosomes with a signal peptide that directs them to the mitochondria. The metabolic activity of the cell is closely related to the number of cristae and the number of mitochondria within a cell. Cells with a lot of metabolic activity, such as a heart muscle, have many well developed mitochondria. New mitochondria are formed from preexisting mitochondria when they grow and divide, the process of mitosis.