Final answer:
Passive transport does not require energy and involves movement of molecules with the gradient. Active transport requires energy and moves molecules against the gradient.
Step-by-step explanation:
Passive transport does not require energy input. Passive transport involves the movement of molecules across a membrane from an area of higher concentration to an area of lower concentration. It relies on the presence of a concentration gradient and the permeability of the membrane to allow molecules to diffuse.
On the other hand, active transport is an energy-requiring process that moves molecules against their concentration gradient, from an area of low concentration to an area of high concentration. It uses ATP as a source of energy and relies on specific membrane proteins to pump molecules across the membrane.
While active transport requires energy, it cannot concentrate a molecule against its gradient because it is limited by the laws of thermodynamics. Concentrating a molecule against its gradient would require continuously inputting energy, which is not possible in biological systems.