Placental exchanges that take place occur according to different mechanisms. Simple diffusion is the movement of molecules of gas and water from a high concentration to a low concentration. Iodine readily crosses the placental barrier. Active transpo is another method. This method transpos specific molecules across a membrane or against a concentration gradient which requires energy (ATP) (i. e. Ca++, Na +, and K+). Endocytosis is the method by which the macromolecules are captured by cell microvilli. Leakage is difficult to understand, but it has to do with the connections or like connections between fetal and maternal blood. Ref: Molina P.E. (2013). Chapter 9. Female Reproductive System. In P.E. Molina (Ed), Endocrine Physiology, 4e.