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. Author manuscript; available in PMC: 2014 Sep 1.
Published in final edited form as: Clin Obstet Gynecol. 2013 Sep;56(3):591–601. doi: 10.1097/GRF.0b013e3182993a2e

Fig 2. A model for placental transport of neutral amino acids.

Fig 2

The uptake of neutral amino acids from the maternal circulation across the microvillous plasma membrane (MVM) into the syncytiotrophoblast represents the active step of amino acid transport and is largely mediated by the System A and System L amino acid transporters. System A is predominantly expressed in the MVM and transport non-essential neutral amino acids (NEAA) against a concentration gradient energized by the inwardly directed Na+-gradient. System L is an exchanger, which uses the steep outwardly directed concentration gradient of some NEAAs to drive uptake of essential amino acids (EAA) against their concentration gradients. In all these cases, the energy for the uphill transport is ultimately generated by the Na+K+-ATPase. Amino acids are transferred across the basal plasma membrane (BM) by facilitated diffusion driven by the outwardly directed concentration gradient mediated by System L and efflux transporters. ST, syncytiotrophoblast; N, nucleus; ET, endothelial cells of the fetal capillary.