Amino acids and iron, for which the fetus has high demand, cross the placenta primarily by which mechanism?

1. The placenta moves different substances across using different transport mechanisms depending on the substance. 2. Lipid-soluble substances like oxygen and carbon dioxide cross by simple diffusion. 3. Water-soluble glucose crosses by facilitated diffusion. 4. Amino acids and iron, however, are needed in high, tightly regulated amounts by the fetus. 5. So simple diffusion and facilitated diffusion (options already used for other substances) are ruled out. 6. 'Bulk blood cell exchange' is ruled out because maternal and fetal blood cells never actually cross the placenta at all. 7. These are moved by active transport, distinguishing them from the diffusion-based routes used for oxygen and glucose. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 28 "Development and Inheritance", p.1333 §Development of the Placenta_