Facilitated diffusion is defined as:

Facilitated diffusion is defined as:

• A. Uses carrier proteins but does not require energy
• B. Requires energy
• C. Involves pumps
• D. Occurs only in mitochondria

Answer: (A)

Facilitated diffusion is a form of passive transport where solutes cross a membrane down their concentration gradient with the help of membrane proteins. It does not require cellular energy. Carrier proteins bind the solute and undergo conformational changes to move it across, while channel proteins provide a hydrophilic pore for diffusion. The key point is that movement is driven by the existing gradient, not by ATP hydrolysis. This differs from active transport, which uses energy to move substances against their gradient via pumps. It also isn’t limited to mitochondria; it occurs across many membranes, including the plasma membrane. Therefore, the definition that fits is: it uses carrier (or channel) proteins but does not require energy.

Facilitated diffusion is a form of passive transport where solutes cross a membrane down their concentration gradient with the help of membrane proteins. It does not require cellular energy. Carrier proteins bind the solute and undergo conformational changes to move it across, while channel proteins provide a hydrophilic pore for diffusion. The key point is that movement is driven by the existing gradient, not by ATP hydrolysis. This differs from active transport, which uses energy to move substances against their gradient via pumps. It also isn’t limited to mitochondria; it occurs across many membranes, including the plasma membrane. Therefore, the definition that fits is: it uses carrier (or channel) proteins but does not require energy.