Given a table of absorption behaviors: (A) Small hydrophobic molecules equilibrate across membranes, (B) Glucose transport saturates at high luminal glucose, (C) Large intact proteins taken up in neonates, (D) Certain ions moved into cells despite higher intracellular concentrations. Match each behavior to the correct transport mechanism and justify which pairing indicates transporter-mediated saturation.

• A. A = Endocytosis, B = Active transport, C = Passive diffusion, D = Facilitated diffusion. Saturation indicates vesicle formation limits rate.
• B. A = Facilitated diffusion, B = Endocytosis, C = Active transport, D = Passive diffusion. Saturation is only relevant to passive diffusion processes.
• C. A = Active transport, B = Passive diffusion, C = Facilitated diffusion, D = Endocytosis. Saturation indicates the process is passive.
• D. A = Passive diffusion, B = Facilitated diffusion (transporter-mediated saturation), C = Endocytosis, D = Active transport. Facilitated diffusion shows saturation because carrier availability limits rate.

Answer: (D)