Diffusion as passive movement down a concentration gradient

Steps:

• Define diffusion: passive process where molecules move from high to low concentration without energy input.
• Evaluate each option: identify if the process requires active transport, mechanical force, or facilitated mechanisms.
• Eliminate options involving energy or bulk movement.
• Confirm the remaining option matches simple diffusion criteria.

Why C is correct:

• Carbon dioxide, a small nonpolar gas, diffuses directly across cell membranes down its concentration gradient, per Fick's law of diffusion (rate proportional to concentration difference).

Why the others are wrong:

• A: Magnesium ion uptake requires active transport against the concentration gradient using ATP.
• B: Food movement relies on peristalsis, a mechanical muscle contraction process.
• D: Glucose transfer uses secondary active transport via sodium-glucose cotransporters.

Final answer: C