Placental Diffusion from Fetus to Mother

Steps:

• Identify diffusion process: In the placenta, substances move passively down concentration gradients between fetal and maternal blood.
• Determine direction: From embryo (fetus) to mother means higher concentration in fetal blood.
• Recall fetal metabolism: Fetus produces CO2 as a waste product, creating a higher partial pressure in fetal blood.
• Confirm exchange: CO2 diffuses to maternal blood for elimination via maternal lungs.

Why B is correct:

• Carbon dioxide follows Fick's law of diffusion, moving from high partial pressure in fetal blood (due to respiration) to lower in maternal blood.

Why the others are wrong:

• A. Amino acids are nutrients transported actively or via facilitated diffusion from mother to fetus.
• C. Glucose diffuses from higher maternal concentration to lower fetal concentration for fetal energy needs.
• D. Oxygen diffuses from higher maternal partial pressure to lower fetal partial pressure for fetal respiration.

Final answer: B