Which of the following substances crosses the placenta via active transport?

Calcium is indeed an example of a substance that crosses the placenta via active transport. This is important for fetal development, particularly for the formation of bones and teeth, as adequate levels of calcium are crucial for these processes. Active transport allows calcium to move against its concentration gradient from the maternal bloodstream into the fetal circulation, ensuring that the fetus receives sufficient amounts despite the lower levels typically found in fetal plasma compared to maternal plasma.

While sodium, chloride, and glucose also cross the placenta, they do so through different mechanisms. Sodium and chloride primarily utilize passive transport mechanisms, such as diffusion or facilitated diffusion, rather than active transport. Glucose crosses the placenta via facilitated diffusion but also relies on specific transporters (GLUT) rather than active transport, which requires energy and is typically used for substances that need to be concentrated on one side of the membrane. Thus, the unique requirement for calcium transport in pregnancy is what distinguishes it as a substance that utilizes active transport mechanisms effectively.