Answer the following question given this paragraph: 

Recall that the cell membrane is semipermeable. It does not allow everything to pass through. Some molecules can pass easily through your cell membranes, while others have more difficulty. Sometimes molecules need the help of special transport proteins to move across the cell membrane. Some molecules even need an input of energy to help get them across the cell membrane. The movement of molecules across a membrane without the input of energy is known as passive transport . When energy (ATP) is needed, the movement is known as active transport . Active transport moves molecules against their concentration gradient, from an area of low concentration to an area of high concentration.


Q: What do you call the movement of molecules across a membrane without the input of energy?


A:
passive transport