a pH imbalance a pressure difference a temperature increase a concentration gradient
I gave my students this multiple choice question: In the human kidney, urea from the blood is filtered through the glomerular membrane into a nephron. The movement of urea across this membrane occurs without an input of energy. Which factor is the most likely reason urea absorption does not require energy?

Only one answer is correct among these 4 choices:
- a pH imbalance
- a pressure difference
- a temperature increase
- a concentration gradient

Could you tell me which one is correct?

a concentration gradient