What type of transport occurs due to variations in concentration across membranes?

The correct response involves passive transport, which is a fundamental biological process where substances move across cell membranes without the expenditure of energy. This transport occurs in response to concentration gradients, meaning that molecules will naturally diffuse from areas of higher concentration to areas of lower concentration until equilibrium is reached.

In passive transport, the driving force is the concentration difference, allowing substances such as small molecules and ions to move freely across lipid membranes. This process can include simple diffusion, where small molecules pass directly through the lipid bilayer, or facilitated diffusion, where specific carrier proteins help larger or polar molecules cross the membrane.

This understanding is crucial in the context of cellular physiology and the maintenance of homeostasis, as it allows cells to regulate their internal environments efficiently. In contrast, active transport would require energy input from ATP to move substances against their concentration gradients, and endocytosis involves the engulfing of large molecules or particles into the cell, which is a different mechanism altogether.