Catalytically perfect enzymes have the maximum catalytic efficiency with high turnover numbers, kcat, and strong affinities for their substrates, KM. The kcat to KM ratio measures catalytic efficiency. For catalytically perfect enzymes, this value ranges from 108 to 109 per molar substrate per second. Substrate diffusion is the constant movement of substrate molecules caused by molecular motion. Substrates must diffuse within the cell to interact with their enzymes. The reaction rates of catalytically perfect enzymes are limited only by the speed of substrate diffusion into or product release from the active site. Some enzymes have achieved catalytic perfection through electrostatic forces that attract and guide substrates into their active site. In catalytically perfect enzymes, every enzyme-substrate collision yields an intermediate that quickly transforms into products.