Diffusion is defined as the movement of molecules from an area of high concentration to an area of low concentration.

• A. From low to high concentration
• B. From area of high solute outside to inside
• C. Using energy to move against gradient
• D. From high concentration to low concentration

Answer: (C)

Diffusion is driven by the random motion of molecules and results in substances moving from regions of higher concentration to regions of lower concentration. It is a passive process, meaning no cellular energy is needed. That makes moving from high concentration to low concentration the correct description. Moving from low to high would require energy and is active transport, not diffusion. Saying diffusion involves energy or specifies movement from outside to inside describes a scenario or a different process, not the general definition of diffusion. For example, oxygen moving into cells and carbon dioxide leaving cells happens by diffusion down its gradient, without the cell expending energy.