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Given a scientific passage and an answer, generate a question for the given answer.

Passage: Diffusion can occur across a semipermeable membrane, such as the cell membrane, as long as a concentration gradient exists. Molecules will continue to flow in this manner until equilibrium is reached. At equilibrium, there is no longer an area of high concentration or low concentration, and molecules flow equally in both directions across the semipermeable membrane. At equilibrium, equal amounts of a molecule are entering and leaving a cell. Answer: concentration gradient
Output:
Diffusion can occur across a semipermeable membrane, such as the cell membrane, as long as a what exists?