Final answer:
Passive transport is a naturally occurring phenomenon that allows small molecules or ions to move across the cell membrane without the input of energy. It includes diffusion, osmosis, and facilitated diffusion.
Step-by-step explanation:
Passive transport is a naturally occurring phenomenon in which small molecules or ions move across the cell membrane without the input of energy by the cell. It includes three main types: diffusion, osmosis, and facilitated diffusion. In diffusion, substances move from an area of higher concentration to an area of lower concentration down a concentration gradient. Osmosis is the movement of water across a semipermeable membrane from an area of lower solute concentration to an area of higher solute concentration. Facilitated diffusion involves the use of carrier proteins or channel proteins to move substances across the membrane.