What is considered a mode of carrier-mediated transport across the placenta?

Carrier-mediated transport across the placenta is a specific mechanism that involves transport proteins facilitating the movement of substances from the maternal circulation to the fetal circulation. One notable example of this process is the transport of vitamin C.

Vitamin C, or ascorbic acid, is a hydrophilic nutrient that requires a specific transporter, as it cannot freely diffuse through the lipid bilayer of cell membranes, including the placental barrier. The human placenta utilizes a sodium-dependent vitamin C transporter (SVCT2) to facilitate the uptake of vitamin C from maternal blood into the placental cells, where it can then be delivered to the fetus. This active transport mechanism ensures that the fetus receives adequate levels of vitamin C, which is essential for collagen synthesis, antioxidant activity, and overall fetal development.

Other choices, while important in fetal development, are not primarily transported through carrier-mediated mechanisms. Fat-soluble vitamins, oxygen, and carbon dioxide do not require specific transport proteins in the same manner that vitamin C does. Fat-soluble vitamins can diffuse through cell membranes, while oxygen and carbon dioxide are exchanged through simple diffusion based on concentration gradients. Thus, vitamin C stands out as the correct answer due to its reliance on a carrier-mediated transport system at the placenta.