Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: The electrochemical potential across a membrane determines the tendency of an ion to cross the membrane. The membrane may be that of a cell or organelle or other sub cellular compartment. The electrochemical potential arises from three factors:. Answer: cross the membrane
The electrochemical potential across a membrane determines the tendency of an ion to do what?