Mitochondria

Abstract

One of the functions of science is to bring order out of chaos. At the end of the nineteenth century cyto-chaos reigned, at least in the mind of the observer. For fifty years almost every cytologist of note had reported granular thread-like particles in the cytoplasm of aerobic cells, and assigned a name and function to them—fila, chondriokonts, fädenkörner, blepharoblasts, vermicules and others. As the twentieth century progressed and a basic uniformity of function became apparent, the name mitochondrion (Benda, 1898) became generally accepted. It is now clear that, although mitochondria in different cell types may vary in biochemical details and in size, structure and frequency, they all contain the enzymes of the tricarboxylic acid cycle (TCA cycle) and carry out oxidative phosphorylation—ATP synthesis coupled to substrate oxidation. They also all conform to a basic structural pattern, namely ah outer membrane or envelope enclosing an inner membrane which has tube-like invaginations (cristae) into an inner compartment (the matrix). The cristae membrane of liver mitochondria (figure 4.1) is 3–4 times greater than the outer membrane area. Some mitochondria, particularly those from kidney, heart and skeletal muscle (figures 4.2 and 4.3), have more extensive cristae arrangements than liver mitochondria, while others (e.g. from fibroblasts, nerve axons and most plant tissues) have relatively few cristae. Mitochondria in epithelial cells of carotid bodies have only four or five cristae, and mitochondria from non-myelinated axons of rabbit brain have only a single crista. The cristae membranes are the sites of certain TCA cycle enzymes, respiratory chain components, and the enzyme, ATP synthetase; therefore, not surprisingly there is a rough correlation between the respiratory rates of mitochondria and the extent of their cristae. Typically tissues with high respiration rates have mitochondria with many cristae.