Passive transport is a biological process that allows substances to move across cell membranes without the need for energy. This movement occurs along the concentration gradient, meaning substances move from areas of higher concentration to areas of lower concentration. Common examples of passive transport include diffusion, osmosis, and facilitated diffusion. In diffusion, small molecules like oxygen and carbon dioxide pass directly through the cell membrane. Osmosis specifically refers to the movement of water across a membrane. Facilitated diffusion involves the use of specific proteins to help larger or charged molecules cross the membrane, still without energy expenditure.