Given scientific fact as input, generate the question from this fact such that it can be answered from the input.
One example: Earth rotates on its axis once times in one day.
Solution is here: How many times does Earth rotate on its axis in one day?
Explanation: The given output correctly poses a question based on the scientific fact given.

Now, solve this: The active transport of ions across the membrane causes an electrical gradient to build up across the plasma membrane.
Solution:
The active transport of ions across the membrane causes an electrical gradient to build up across the what?