Placental exchange of nutrients, gases, and waste Steps:

• Recall placenta facilitates diffusion of oxygen and glucose from maternal to fetal blood for fetal respiration and energy.
• Note antibodies transfer from mother to fetus via pinocytosis for passive immunity.
• Identify waste products like carbon dioxide and urea diffusing from fetal to maternal blood for maternal elimination.
• Match options to correct bidirectional flow, eliminating those with reversed directions or toxins.

Why D is correct:

• Describes accurate exchange: oxygen and glucose diffuse down concentration gradients from mother to fetus, antibodies via receptor-mediated transport, while carbon dioxide and urea diffuse back per Fick's law of diffusion.

Why the others are wrong:

• A: Reverses glucose flow (mother to fetus) and includes alcohol, a harmful teratogen not beneficially exchanged.
• B: Includes nicotine, a toxin passing from mother to fetus, with glucose direction reversed.
• C: Reverses urea (waste to mother) and glucose (nutrient to fetus) directions.

Final answer: D