What does passive transport refer to in cellular function?

Passive transport refers to the movement of substances across cell membranes without the need for energy input, which is what makes it distinct from active transport mechanisms. In passive transport, molecules move down their concentration gradient, meaning they naturally flow from areas of higher concentration to areas of lower concentration. This process relies on the inherent kinetic energy of molecules rather than any external energy source like ATP.

This concept is fundamental in cellular functions, as it enables essential nutrients and ions to enter and exit the cell while maintaining homeostasis. Examples of passive transport include diffusion, osmosis, and facilitated diffusion, where specific proteins assist molecules in crossing the membrane.

The other processes mentioned, such as energy-dependent mechanisms or transport requiring ATP, are characteristics of active transport. Active transport specifically moves substances against their concentration gradient, which necessitates energy expenditure, distinguishing it clearly from passive transport.