PASSIVE TRANSPORT: DIFFUSION AND OSMOSIS
Passive transport does not require an expenditure of metabolic energy, and materials flow down the concentration gradient.

Examples of passive transport are diffusion, osmosis, and facilitated diffusion.

Diffusion is the movement of substances with the concentration gradient. Osmosis and facilitated diffusion are kinds of diffusion. When someone wears cologne or perfume, the aromatic molecules are initially concentrated on that person's skin. As the constant, random motion of the perfume molecules spreads them throughout the room, the aroma disperses. Even when the molecules reach equilibrium, spread evenly in the room, they will continue to move randomly.

Osmosis is the diffusion of water through a semipermeable membrane. Water moves from an area of higher concentration to an area of lower concentration, that is, toward the area where there is more solute, and thus less water. The area of less solute is called the hypotonic solution, and the area of more solute is called the hypertonic solution. If a semipermeable membrane separates the hypotonic solution from the hypertonic solution, water will move across the membrane from the hypotonic to the hypertonic solution. No metabolic energy is involved.