Substance Direction of movement Oxygen mother to fetus Carbon dioxide ................... Glucose ................... Urea fetus to mother During pregnancy ...

This question is about exchange across the placenta, where the mother's and fetus's blood come close but never mix.

(a) Completing the table, each substance moves down its concentration gradient by diffusion:

Carbon dioxide is a waste product made by the fetus, so it passes to the mother; glucose is food the fetus needs, so it passes to the fetus.

(b) Oxygen crosses the placenta into the fetus's blood by diffusion [1], from the higher oxygen concentration in the mother's blood to the lower concentration in the fetal blood.

(c) The fetus cannot breathe because its lungs are not yet working and are full of fluid [1], and there is no air around it as it is surrounded by fluid in the uterus [1]. It therefore gets its oxygen from the mother across the placenta.

(d) Two functions of the placenta (1 each): it allows exchange of oxygen and food (glucose) to the fetus [1]; and it removes wastes such as carbon dioxide and urea from the fetus [1] (also accepted: passes some antibodies, anchors the fetus).

(e) Any two harmful substances that can cross from mother to fetus: alcohol, nicotine, other drugs, or viruses such as rubella or HIV [1 each, max 2].

(f) One advantage of the bloods not mixing: the mother's and fetus's blood may be different blood groups or at different pressures [1]; mixing could make them react, or the higher maternal pressure could burst the delicate fetal blood vessels [1].

(g) Two functions of the amniotic fluid: it protects and cushions the fetus against mechanical damage or knocks [1]; and it supports the fetus and helps keep its temperature stable [1].