Which transport method primarily relies on concentration gradients?

The correct answer emphasizes that passive transport primarily relies on concentration gradients to move substances across cell membranes. In passive transport, molecules move from areas of higher concentration to areas of lower concentration, a process that occurs without the need for energy input from the cell. This phenomenon is driven entirely by the natural tendency of molecules to spread out and achieve equilibrium.

In this transport method, substances such as oxygen, carbon dioxide, and certain small molecules can diffuse through the lipid bilayer of the cell membrane or through specific channels. The driving force for this movement is the difference in concentration on either side of the membrane, which fosters a net movement of molecules down their concentration gradient until equilibrium is reached. This mechanism is essential for functions like gas exchange in the lungs or nutrient absorption in the intestines.

Other transport methods, such as active transport, require energy to move substances against their concentration gradient, while facilitated diffusion also depends on concentration gradients but involves specific carrier proteins to assist movement. Endocytosis involves the engulfing of substances into the cell and does not rely directly on concentration gradients. Hence, passive transport is correctly identified as the method primarily based on concentration gradients.