What characterizes passive transport?

Passive transport is characterized by the movement of substances across a cell membrane without the expenditure of energy. This process relies on the concentration gradient, where substances move from areas of higher concentration to areas of lower concentration, seeking to reach equilibrium. Because it does not require energy input from the cell, this mechanism is efficient for maintaining homeostasis and allows cells to take in essential nutrients while expelling waste products.

In contrast, active transport is the mechanism that involves the movement of substances against their concentration gradient, which does require energy. This helps in situations where cells need to concentrate substances inside or outside of the cell, despite existing gradients that would otherwise make such movement energetically unfavorable. Additionally, while passive transport can occur with or without the aid of protein channels (such as in facilitated diffusion or simple diffusion), it remains fundamentally energy-efficient due to its reliance on kinetic energy from the natural movement of molecules rather than the cell providing energy.