What is the main difference between passive transport and active transport?

The primary distinction between passive transport and active transport lies in the energy requirement for the movement of substances across cell membranes. Active transport requires energy, typically in the form of ATP, to move molecules against their concentration gradient, which means it can transport substances from areas of lower concentration to areas of higher concentration. This process is essential for maintaining the cell's internal environment and allowing it to function optimally.

In contrast, passive transport does not require energy; instead, it relies on the natural tendency of molecules to move from regions of higher concentration to areas of lower concentration until equilibrium is reached. This means that passive transport processes, such as diffusion and osmosis, occur spontaneously without the input of energy.

The other options do not accurately capture the fundamental difference between these two transport mechanisms. For instance, the speed of transport can vary regardless of whether it is passive or active. Moreover, passive transport can occur via various mechanisms and is not limited to water, while active transport is a critical mechanism for moving ions and molecules necessary for cellular functions, making it more common than any hypothesis might suggest.