Fig. 1. A schematic of a muscle fiber in cross section shows the influence of mitochondrial distribution on diffusion distance. While mitochondrial distribution can be complex, shown here are two idealized patterns where mitochondria may be distributed throughout the fiber, leading to short diffusion distances (A; intermyofibrillar mitochondria), or clustered exclusively at the sarcolemmal membrane, leading to longer diffusion distances (B; subsarcolemmal mitochondria). More commonly, a combination of both subsarcolemmal and intermyofibrillar mitochondria are observed. ATP must be transported via diffusion (black arrows) from sites of production in the mitochondria (colored green) to cellular ATPases throughout the cytoplasm. Likewise, ADP must be transported back to the mitochondria (not shown). In some fibers the mitochondrial distribution changes during fiber growth from primarily intermyofibrillar to primarily subsarcolemmal, meaning the diffusion distance between mitochondria may change dramatically as muscle fibers increase in size (see text).