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

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

Single-celled organisms and multicellular organisms have this in common: both have a way to get rid of waste materials.
Which characteristic do single-celled organisms and multicellular organisms have in common?

Epithelial cells cells continue the digestion and absorption of nutrients and transport these nutrients to the lymphatic and circulatory systems.

Which cells continue the digestion and absorption of nutrients and transport these nutrients to the lymphatic and circulatory systems?