Which statement correctly contrasts diffusion and facilitated diffusion?

• A. Diffusion rate depends on gradient; facilitated rate depends on carrier saturation
• B. Both depend solely on gradient
• C. Diffusion does not depend on gradient
• D. Facilitated rate increases indefinitely with concentration

Answer: (A)

Diffusion is a simple, gradient-driven process: molecules move down their concentration gradient across a membrane without help from specific transporters, so the rate of transfer increases as the gradient increases. Facilitated diffusion, on the other hand, relies on carrier or channel proteins. These proteins can become occupied, so the rate of transport cannot keep rising with increasing substrate concentration; it saturates once all carriers are filled. That makes the rate depend on carrier availability (saturation) rather than purely on the gradient.

This is why the statement that diffusion rate depends on the gradient and facilitated diffusion rate depends on carrier saturation is the best description. The other ideas don’t fit: diffusion does depend on the gradient, so claiming it does not is incorrect; and facilitated diffusion does not increase indefinitely with concentration due to transporter saturation, so saying it does is incorrect.