Mitochondrion, membrane-bound organelle found in the cytoplasm of almost all eukaryotic cells (cells with clearly defined nuclei), the primary function of which is to generate large quantities of energy in the form of adenosine triphosphate (ATP). Mitochondria are typically round to oval in shape and range in size from 0.5 to 10 μm. In addition to producing energy, mitochondria store calcium for cell signaling activities, generate heat, and mediate cell growth and death.
The number of mitochondria per cell varies widely—for example, in humans, erythrocytes (red blood cells) do not contain any mitochondria, whereas liver cells and muscle cells may contain hundreds or even thousands. The only eukaryotic organism known to lack mitochondria is the oxymonad Monocercomonoides species. Mitochondria are unlike other cellular organelles in that they have two distinct membranes and a unique genome and reproduce by binary fission; these features indicate that mitochondria share an evolutionary past with prokaryotes (single-celled organisms).
Most of the proteins and other molecules that make up mitochondria originate in the cell nucleus. However, 37 genes are contained in the human mitochondrial genome, 13 of which produce various components of the electron transport chain (ETC). In many organisms, the mitochondrial genome is inherited maternally. This is because the mother’s egg cell donates the majority of cytoplasm to the embryo, and mitochondria inherited from the father’s sperm are usually destroyed.
Role in energy production
The three processes of ATP production include glycolysis, the tricarboxylic acid cycle, and oxidative phosphorylation. In eukaryotic cells the latter two processes occur within mitochondria. Electrons that are passed through the electron transport chain ultimately generate free energy capable of driving the phosphorylation of ADP.
The outer mitochondrial membrane is freely permeable to small molecules and contains special channels capable of transporting large molecules. In contrast, the inner membrane is far less permeable, allowing only very small molecules to cross into the gel-like matrix that makes up the organelle’s central mass. The matrix contains the deoxyribonucleic acid (DNA) of the mitochondrial genome and the enzymes of the tricarboxylic acid (TCA) cycle (also known as the citric acid cycle, or Krebs cycle), which metabolizes nutrients into by-products the mitochondrion can use for energy production.
The processes that convert these by-products into energy occur primarily on the inner membrane, which is bent into folds known as cristae that house the protein components of the main energy-generating system of cells, the ETC. The ETC uses a series of oxidation-reduction reactions to move electrons from one protein component to the next, ultimately producing free energy that is harnessed to drive the phosphorylation of ADP (adenosine diphosphate) to ATP. This process, known as chemiosmotic coupling of oxidative phosphorylation, powers nearly all cell
Some diseases and disorders associated with mitochondrial dysfunction are caused by mutations in mtDNA. Disorders resulting from mutations in mtDNA demonstrate an alternative form of non-Mendelian inheritance, known as maternal inheritance, in which the mutation and disorder are passed from mothers to all of their children. The mutations generally affect the function of the mitochondrion, compromising, among other processes, the production of cellular ATP. Severity can vary widely for disorders resulting from mutations in mtDNA, a phenomenon generally thought to reflect the combined effects of heteroplasmy (i.e., mixed populations of both normal and mutant mitochondrial DNA in a single cell) and other confounding genetic or environmental factors. Although mtDNA mutations play a role in some mitochondrial diseases, the majority of the conditions actually are the result of mutations in genes in the nuclear genome, which encodes a number of proteins that are exported and transported to mitochondria in the cell.
There are numerous inherited and acquired mitochondrial diseases, many of which can emerge at any age and are enormously diverse in their clinical and molecular features. They range in severity from relatively mild disease that affects just a single organ to debilitating and sometimes fatal illness that affects multiple organs. Both inherited and acquired mitochondrial dysfunction is implicated in several diseases, including Alzheimer disease and Parkinson disease. The accumulation of mtDNA mutations throughout an organism’s life span are suspected to play an important role in aging, as well as in the development of certain cancers and other diseases. Because mitochondria also are a central component of apoptosis (programmed cell death), which is routinely used to rid the body of cells that are no longer useful or functioning properly, mitochondrial dysfunction that inhibits cell death can contribute to the development of cancer.
source
Mitochondria || Structure & Function || Cell Biology || +3 1st Sem || DSC Paper – 2 || Lecture – 04
