Glucose cannot cross the lipid bilayer of the syncytiotrophoblast on its own — it is hydrophilic. Instead it is moved by GLUT1, a carrier protein that binds glucose on one face of the membrane, flips conformation, and releases it on the other. Because GLUT1 works in both directions (a symmetric facilitated-diffusion carrier), the net maternal→fetal flux follows a difference of two Michaelis-Menten terms:
J = Vmax · [ Cm /(Km+Cm) − Cf /(Km+Cf) ]
Cm, Cf : maternal / fetal glucose concentration (mM)
Km : ≈ 6 mM, apparent affinity of placental GLUT1 for glucose
Vmax : transport capacity, ∝ transporter density ÷ diffusion distance
dCf/dt : J − k·Cf (k = fetal glucose utilisation rate)
- Maternal glucose — sets Cm. Raising it (as in gestational diabetes) pushes the carrier toward saturation but does not raise flux proportionally once Km is exceeded — this is why maternal hyperglycemia does not translate into unlimited fetal glucose delivery.
- GLUT1 density — the placenta upregulates transporter expression as pregnancy progresses (and further in diabetic pregnancies); more carriers raise Vmax and therefore the ceiling on flux.
- Membrane thickness — the syncytiotrophoblast genuinely thins over gestation, from tens of micrometres early on to roughly 1–4 µm at term. A shorter diffusion path raises effective transport capacity (Vmax ∝ 1/thickness here), independent of transporter number — this is one reason near-term placentas transfer nutrients far more efficiently than early ones.
- Saturation % — the fraction of maternal-side carriers occupied by glucose, Cm/(Km+Cm). Near 100% saturation, adding more maternal glucose barely changes flux — the classic signature of carrier-mediated (not simple) diffusion.
Gold spheres are glucose molecules; density on each side visualises Cm and Cf. Particles crossing the pink membrane band do so only where a GLUT1 carrier (small pink cylinder) is drawn, at a rate matched to the computed flux J — the visual traffic is not decorative, it tracks the equation above.