TPR Practice Test 3: BS Question number 12

[nindra]

The question states...

Which of the following processes do NOT occur at the placenta?

I. Diffusion of amino acids from fetal blood to maternal blood
II. Mixing of maternal and fetal blood and gas exchange
III. Exchange of CO2 and O2 between fetal and maternal blood

The answer is I and II.

I understand II, since the bloods do not mix. I also know that III does occur because CO2 and O2 are obviously non polar and are also because gas exchange are required for fetal survival.

But I don't get I. Are not nutrients exchanged between the mother and the fetus? Plus, amino acid are small molecules and thus could easily transfer through capillaries.
Their explanation states that since amino acids are charged, they cannot cross the placental and fetal capillary membranes.

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[Plane]

The question states...

Which of the following processes do NOT occur at the placenta?

I. Diffusion of amino acids from fetal blood to maternal blood
II. Mixing of maternal and fetal blood and gas exchange
III. Exchange of CO2 and O2 between fetal and maternal blood

The answer is I and II.

I understand II, since the bloods do not mix. I also know that III does occur because CO2 and O2 are obviously non polar and are also because gas exchange are required for fetal survival.

But I don't get I. Are not nutrients exchanged between the mother and the fetus? Plus, amino acid are small molecules and thus could easily transfer through capillaries.
Their explanation states that since amino acids are charged, they cannot cross the placental and fetal capillary membranes.

I'll make an attempt to answer your question:

http://www.vivo.colostate.edu/hbooks/pathphys/reprod/placenta/transport.html
"Amino acid concentrations in fetal blood are higher than in maternal blood. Amino acids are therefore transported to the fetus by active transport."

Diffusion implies movement from high to low concentrations, when in fact amino acids requires active transport pumps to move against this gradient. Hope this makes sense.