What role do concentration gradients play in passive transport?

Concentration gradients are fundamental to the process of passive transport within biological systems. In passive transport, molecules naturally move from an area of higher concentration to an area of lower concentration. This movement occurs because molecules are in constant random motion due to the thermal energy they possess.

When a concentration gradient exists, there is an imbalance in the concentration of molecules across a membrane or within a space. Molecules tend to move down this gradient until equilibrium is reached, where the concentration is uniform throughout. This movement does not require energy input from the cell, as it is driven solely by the difference in concentration. The spontaneous tendency of molecules to spread out promotes this process.

In contrast, movement from lower to higher concentration would require energy input to counteract the natural tendency of molecules to move down a gradient, making this option inconsistent with the definition of passive transport. Additionally, if energy is required for movement or if molecules are transported actively against a gradient, these processes are classified as active transport, which is not representative of passive transport principles. Thus, the correct understanding of passive transport highlights that it revolves around the movement of molecules from a region of higher concentration to one of lower concentration.