Diffusive flux of aerobically-produced ATP-equivalents to cellular ATPases and of ADP-equivalents from the fiber core to mitochondria is mediated by the arginine kinase (AK) reaction. As cellular dimensions increase, diffusive flux would be expected to exert more control over aerobic metabolic flux.

Understanding the diffusive behavior of metabolites in cells is necessary to model intracellular energetics. Shown above are pulsed-field gradient NMR measurements of the time- and orientation-dependence of arginine phosphate diffusion in giant, fast-twitch fibers from the Spiny lobster. Radial diffusion (filled squares), which is important for ATP-equivalent flux from mitochondria to the fiber core, is more attenuated by intracellular structures than axial diffusion. Mathematical modeling has revealed that the sarcoplasmic reticulum is the principal intracellular barrier that induces the observed diffusive anisotropy (from Kinsey and Moerland, 2002).

Inside the cell metabolites have diffusion coefficients inversely proportional to their molecular weight, as would be expected if the aqueous phase of the cytoplasmic environment were similar to a bulk solution (from Kinsey and Moerland, 2002).