Given scientific fact as input, generate the question from this fact such that it can be answered from the input.
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Question: The active transport of ions across the membrane causes an electrical gradient to build up across the plasma membrane.

Answer: The active transport of ions across the membrane causes an electrical gradient to build up across the what?


Question: On a global scale, plants and other photosynthetic organisms have generated all of the oxygen in the air.

Answer: On a global scale, plants and other photosynthetic organisms have generated all of what element in the air?


Question: Source is the name of the location in which a stream or river starts.

Answer:
What is the name of the location in which a stream or river starts?