Detailed Instructions: Given a scientific passage and an answer, generate a question for the given answer.
Problem:Passage: Facilitated Diffusion Across a Cell Membrane. Channel proteins and carrier proteins help substances diffuse across a cell membrane. In this diagram, the channel and carrier proteins are helping substances move into the cell (from the extracellular space to the intracellular space). Answer: cell membrane
Solution:
Channel proteins and carrier proteins help substances diffuse across what?