Which of the following defines passive transport?

The concept of passive transport is characterized by the movement of molecules across a cell membrane without the use of energy. This process relies on the natural tendency of substances to move from areas of higher concentration to areas of lower concentration, a phenomenon termed the concentration gradient. When molecules move down this gradient, they are essentially following a path of least resistance, which does not require the expenditure of energy from the cell.

In passive transport, substances can diffuse across membranes by several mechanisms, including simple diffusion, facilitated diffusion, and osmosis, all of which occur spontaneously as a result of the concentration difference. This fundamental aspect of cellular processes is crucial for maintaining homeostasis within cells and allowing for the exchange of vital substances, such as nutrients and wastes, with the environment. Understanding this mechanism is essential in cell biology as it highlights how cells regulate their internal environment efficiently without needing to invest energy actively.