Key Role of Concentration Gradient in Gas Diffusion

Steps:

• Identify factors for efficient diffusion: short distance, large surface area, and steep concentration gradient.
• Recall Fick's law: diffusion rate depends on surface area, diffusion distance, and concentration difference.
• Evaluate options: assume 1=large surface area, 2=short distance, 3=steep gradient based on standard biology.
• Determine only the gradient is strictly required for diffusion itself, while others enhance but aren't core to the process.

Why D is correct:

• Per Fick's law, diffusion requires a concentration gradient to drive net movement of O2 and CO2 across membranes.

Why the others are wrong:

• A includes all, but surface area and distance optimize, not require, basic diffusion.
• B omits gradient, which is essential for any net gas exchange.
• C omits surface area or distance, but still includes non-essential factors over the core gradient.

Not enough information on exact statements 1-3 for full certainty.

Final answer: D