Final answer:
Passive transport is the energy-free movement of small molecules or ions across a cell membrane, down their concentration gradient, through diffusion, osmosis, or facilitated diffusion.
Step-by-step explanation:
Passive transport is a method by which small molecules or ions move across a cell membrane without the need for energy from the cell. This process relies on the inherent kinetic energy that substances have, which leads them to diffuse from an area of higher concentration to one of lower concentration, naturally moving down their concentration gradient. There are several types of passive transport, with the three main kinds being diffusion, osmosis, and facilitated diffusion.
In diffusion, molecules move freely through the lipid bilayer of the cell membrane due to their small size and non-polarity. Osmosis is a special type of diffusion that involves the movement of water molecules through a selectively permeable membrane from an area of high water concentration to one of low water concentration. Lastly, facilitated diffusion occurs when specific substances are transported across cell membranes by means of protein channels or carriers without energy input.
Unlike active transport, which requires energy input from ATP to move substances against their concentration gradient, passive transport is a naturally occurring phenomenon that does not cost the cell energy. Consequently, passive transport plays a crucial role in regulating the internal environment of cells and maintaining homeostasis.