Particles tend to spread out evenly in an available space. Passive Transport is when molecules spread out over an availabe space by moving through a permeable membrane.
When molecules move down a concentration gradient through a membrane, no energy is used.
Oxygen enters our blood stream and carbon dioxide escapes by using passive transport.
Image: Osmosis is a form of passive transport. There are 9 water molecules on the right side of the membrane (high concentration) and 6 water molecules on the left side of the membrane (low concentration). In Passive Transport, molecules move from high concentration to low concentration across a membrane.