The type of transport that requires no energy to move substances across cell membranes is known as passive transport. This process relies on the natural movement of molecules from areas of higher concentration to areas of lower concentration until equilibrium is achieved. Examples include diffusion, facilitated diffusion, and osmosis. Passive transport is essential for cellular function, allowing the movement of vital substances such as oxygen, carbon dioxide, and nutrients without the expenditure of energy, thereby maintaining cellular homeostasis.