Which type of change occurs when substances move across a membrane along a concentration gradient?

The correct choice relates to the movement of substances across a membrane in a manner that follows the concentration gradient, which is the process known as diffusion. This phenomenon occurs when molecules move from an area of higher concentration to an area of lower concentration until equilibrium is reached.

Focusing specifically on the nature of concentration gradients, when substances passively transport across membranes due to differences in concentration, it signifies that a concentration change is happening. This type of transport does not require energy input, as it relies on the natural tendency of substances to balance their distribution in a given space.

Understanding concentration changes is vital as it supports various biological functions, including nutrient uptake and waste elimination in cells. This principle also lays the groundwork for more complex processes in cellular biology, such as osmosis and facilitated diffusion. Recognizing this concept emphasizes the importance of concentration gradients in biological systems.