Given scientific fact as input, generate the question from this fact such that it can be answered from the input.

Let me give you an example: Earth rotates on its axis once times in one day.
The answer to this example can be: How many times does Earth rotate on its axis in one day?
Here is why: The given output correctly poses a question based on the scientific fact given.

OK. solve this:
In living systems, diffusion of substances into and out of cells is mediated by the plasma membrane.
Answer:
In living systems, diffusion of substances into and out of cells is mediated by the what?