Diffusion Defined as Passive Molecular Movement

Steps:

• Recall the biological definition: diffusion is the net movement of particles from high to low concentration areas.
• Evaluate choices: identify which describes movement down a concentration gradient without external energy.
• Eliminate options involving air, heat, or upward movement as they mismatch the standard definition.
• Confirm B matches the core process in cells and solutions.

Why B is correct:

• Diffusion is defined as the passive movement of molecules from a region of higher concentration to lower concentration, following Fick's law of diffusion.

Why the others are wrong:

• A limits it to air, but diffusion occurs in liquids and gases generally.
• C incorrectly ties it to heated liquids, ignoring passive nature and broad applicability.
• D reverses the direction; diffusion moves down, not up, the gradient.

Final answer: B