Final answer:
Passive transport is a way that small molecules or ions move across the cell membrane without input of energy by the cell. Diffusion, one form of passive transport, is the movement of molecules from an area of higher concentration to an area of lower concentration. This occurs due to the random motion of molecules, known as Brownian motion.
Step-by-step explanation:
Passive transport is a way that small molecules or ions move across the cell membrane without input of energy by the cell. The three main kinds of passive transport are diffusion, osmosis, and facilitated diffusion.
Diffusion, one form of passive transport, is the movement of molecules from an area of higher concentration to an area of lower concentration. This occurs due to the random motion of molecules, known as Brownian motion. Diffusion does not require the expenditure of energy and is important for the exchange of gases such as O2 and CO2 in the lungs and in cellular respiration.