Which of the following best describes the concept of passive transport?

The concept of passive transport refers specifically to the movement of molecules from an area of higher concentration to an area of lower concentration, which occurs naturally without the need for energy input. This process relies on the concentration gradient, allowing substances to move freely across cell membranes until equilibrium is achieved.

This phenomenon is crucial in biological systems, as it allows for the efficient transport of essential molecules, such as nutrients and gases, into and out of cells. By utilizing the natural kinetic energy of molecules, passive transport plays a significant role in maintaining cellular homeostasis.

In contrast, other options describe different transport mechanisms. Movement against a concentration gradient involves active transport, which requires energy. Energy input is also needed for processes requiring assistance by proteins, while the mention of movement across a semi-permeable membrane indicates a specific condition, rather than defining passive transport itself. Thus, the description of passive transport as the movement of molecules down a concentration gradient accurately captures its essence.