Pure diffusion in biological transport

Steps:

• Diffusion is passive movement of molecules from high to low concentration without energy input.
• Evaluate A: Water movement in xylem relies on transpiration pull and cohesion, involving bulk flow.
• Evaluate B: Sugar transport in phloem uses pressure-flow mechanism driven by active loading.
• Evaluate C and D: CO2 enters blood passively across thin alveolar membranes; glucose requires energy for uptake.

Why C is correct:

• CO2 uptake follows Fick's law of diffusion, moving passively down its partial pressure gradient from alveoli (high) to blood (low) without cellular energy.

Why the others are wrong:

• A: Involves active transpiration and mass flow, not solely diffusion.
• B: Depends on ATP-driven loading creating pressure gradients for bulk flow.
• D: Uses active transport via sodium-glucose symporters against concentration gradients.

Final answer: C