Final answer:
Passive molecular diffusion occurs when molecules move from regions of higher concentration to lower concentration without the input of energy.
Step-by-step explanation:
Passive molecular diffusion occurs when molecules move from regions of higher concentration to lower concentration without the input of energy. This process is driven by random collisions between molecules and does not require the use of ATP or the catalysis by enzymes. Variations in molecular sizes can affect the diffusion rate, as well as other factors such as concentration gradient, particle sizes, and temperature.