Question

reading tool cause and effect small molecules have the ability to move across the cell membrane by dissolving through the lipid bilayer. this is called diffusion and it happens without any energy input. why do some molecules need help moving across a cell membrane in the form of energy?

Explanation:

Some molecules need energy - input to move across a cell membrane because they are moving against their concentration gradient (from low - concentration to high - concentration areas). Passive diffusion (without energy) only occurs down the concentration gradient. Active transport, which requires energy (usually in the form of ATP), is needed for movement against the gradient.

Answer:

Some molecules move against their concentration gradient and require active transport which needs energy input in the form of ATP to cross the cell membrane.