Final answer:
The principal force driving movement in diffusion is concentration gradient, which continues until the gradient is eliminated without requiring energy from the cell.
Step-by-step explanation:
The principal force driving movement in diffusion is concentration gradient. Diffusion is a process where molecules move from an area of higher concentration to an area of lower concentration. This creates a concentration gradient, which means there's a difference in the amount of substance in two areas. This gradient is the driving force behind the movement of molecules during diffusion, and the process continues until there is no longer a concentration gradient remaining. Passive transport, including diffusion, does not require the cell to expend energy, which differentiates it from active transport that does require energy.