Which term describes the movement of solutes along a concentration gradient without cellular energy input?

• A. Capillary action
• B. Bulk flow
• C. Diffusion
• D. Osmosis

Answer: (C)

Diffusion is the passive movement of solutes from areas of higher concentration to lower concentration, driven by the random motion of molecules and requiring no input of energy. This means the solute spreads downhill along its concentration gradient until equilibrium is reached, with the rate influenced by the gradient size, temperature, and how easily the solute can move through the medium. In soils, diffusion helps nutrients reach plant roots when there’s a higher concentration of ions in the soil solution away from the root, working without cellular energy.

Capillary action describes liquids rising in narrow spaces due to cohesive and adhesive forces, not the downhill movement of solutes along a concentration gradient. Bulk flow involves the movement of an entire volume of fluid driven by pressure differences, not the selective diffusion of individual solutes. Osmosis is the diffusion of water across a semipermeable membrane toward higher solute concentration, affecting water potential rather than the movement of solute particles themselves.