Optical measurement of ATP diffusion in 8-cm-long pipettes, monitored via firefly luciferase luminescence ( Rasmussen and Nielsen, 1968 ). All solutions contained luciferase (150 ng/ml) and luciferin (15 μM). (A) One group of pipettes was loaded one-half with gelatin-free solution containing 0.5 mM ATP and one-half with ATP-free gelatin. (B) A second group was loaded entirely without gelatin and with ATP in one-half of the pipette. Chemiluminescence was then evaluated after 1 and 5 h. The ATP gradient moved on average threefold further in the water phase (B) than in the gelatin phase (A), indicative of a ninefold smaller diffusion coefficient in 10 g% gelatin. **P < 0.01. (C) Simulation of results with the diffusion coefficients indicated and with an ATP dissociation coefficient of 2 μM.