What are mitochondria and what do they do?
Mitochondria are a subcompartment of the cell bound by a double membrane. Although some mitochondria probably do look like the traditional cigar shaped structures that appear in standard textbooks, it is more accurate to think of them as a budding and fusing network similar to the endoplasmic reticulum (fig 1). Mitochondria are intimately involved in cellular homeostasis. Among other functions they play a part in intracellular signalling and apoptosis, intermediary metabolism, and in the metabolism of amino acids, lipids, cholesterol, steroids, and nucleotides. Apoptosis is discussed in other articles of this series and will not be considered here. Perhaps most importantly, mitochondria have a fundamental role in cellular energy metabolism. This includes fatty acid ß oxidation, the urea cycle, and the final common pathway for ATP production—the respiratory chain.
The mitochondrial respiratory chain is a group of five enzyme complexes situated on the inner mitochondrial membrane (fig 2). Each complex is composed of multiple subunits, the largest being complex I with over 40 polypeptide components. Reduced cofactors (NADH and FADH2) generated from the intermediary metabolism of carbohydrates, proteins, and fats donate electrons to complex I and complex II. These electrons flow between the complexes down an electrochemical gradient, shuttled by complexes III and IV and by two mobile electron carriers, ubiquinone (ubiquinol, coenzyme Q10) and cytochrome c. The electron transfer function of complexes I–IV is accomplished through subunits harbouring prosthetic groups (for example, iron–sulphur groups in complexes I, II, and III, and haem iron in cytochrome c and complex IV). The liberated energy is used by complexes I, III, and IV to pump protons (H+) out of the mitochondrial matrix into the intermembrane space. This proton gradient, which generates the bulk of the mitochondrial membrane potential (the asymmetrical distribution of ions, such as Na+, K+, and Ca2+, across the inner membrane makes up the "chemical" portion of the gradient), is harnessed by complex V to synthesise adenosine triphosphate (ATP) from adenosine diphosphate (ADP) and inorganic phosphate. The overall process is called oxidative phosphorylation (OXPHOS). ATP is the high energy source used for essentially all active metabolic processes within the cell, and it must be released from the mitochondrion in exchange for cytosolic ADP. This is carried out by the adenine nucleotide translocator (ANT), which has various tissue specific isoforms.
Thus the respiratory chain is an elaborate system that must respond to the energy requirements of the cell. While these requirements may be constant (for example, in hepatocytes), they may also change dramatically over short periods of time (as in skeletal muscle). We are only just beginning to understand the mechanisms that maintain and regulate a healthy respiratory chain, and it is likely that many additional unknown genetic and environmental factors will be involved.
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