Mechanisms Involved in the Control of Mitochondrial Volume and their Role in the Regulation of Mitochondrial Function

Abstract

Mitochondria are organelles within the cell which, in most tissues, are primarily concerned with the oxidation of respiratory fuels to produce useful energy in the form of ATP. This is then used to drive energy requiring processes such as biosynthesis, mechanical work and solute transport. Mitochondria vary in size and shape depending on their source, but in general terms they are about 1–2 µM in diameter. They are surrounded by two phospholipid membranes, the inner one of which is elaborately folded into cristae. The outer membrane is permeable to most small molecular weight solutes, whilst the inner membrane is impermeable to the majority of solutes except those for which specific permeability pathways exist. Such transport is catalysed by proteins which span the membrane and are called carriers or transporters. The impermeability of the inner membrane to small solutes such as sucrose allow mitochondria to act as perfect osmometers; increasing the extramitochondrial osmolality causes the mitochondria to shrink in order to maintain the osmotic balance, whilst decreasing the osmolality causes swelling and unfolding of the cristae. These responses can be followed by measuring the light scattering of a mitochondrial suspension. Light scattering decreases as the mitochondrial volume increases (see Halestrap, 1989).