Diffusion across semi-permeable membrane in dialysis

Steps:

• Recall dialysis uses a semi-permeable membrane allowing small waste molecules like urea to diffuse from blood (high concentration) to dialysis fluid (low concentration).
• Identify diagram showing unidirectional arrows for wastes from blood to fluid, with no net movement for balanced solutes like glucose.
• Eliminate diagrams with bidirectional arrows, active transport symbols, or incorrect directions.
• Select the diagram matching passive diffusion principles.

Why D is correct:

• It depicts passive diffusion of wastes from higher blood concentration to lower dialysis fluid concentration, per Fick's law of diffusion.

Why the others are wrong:

• A shows osmosis (water movement), not solute diffusion.
• B illustrates active transport with energy input, absent in dialysis.
• C reverses arrows, showing influx of wastes into blood.

Final answer: D