Describe the process of fertilization in humans.

Explain how the structure and function of the placenta helps to maintain pregnancy.

Outline the hormonal control of the process of birth.

sperm breaks through follicle cells/cells surrounding the ovum;
triggers acrosome reaction;
proteases/hydrolytic enzymes (of acrosome) released;
digestion of zona pellucida;
plasma membranes of sperm and egg fuse;
sperm nucleus enters egg;
cortical reaction;
hardening/cross linking of glycoproteins in zona pellucida;
preventing sperm from entering;

disc-shape organ that attaches to (inside of) uterus;
connected to the fetus by the umbilical cord;
produces hormones/HCG that maintain pregnancy;
estrogen and progesterone maintain uterine lining;
nutrients/oxygen from mother’s blood transferred to fetal blood;
antibodies from mother’s blood transferred to fetal blood (through umbilical vein);
waste products transferred from fetal blood to maternal blood (through umbilical artery);
embryonic tissue invades/grows into the uterine wall;
placental/chorionic villi increase the surface area (for exchange);
fetal capillaries in placenta/placental villi;
inter-villous spaces/sinuses through which mother’s blood flows;
small distance between fetal and mother’s blood / narrow placental barrier / mother and fetal bloods do not mix;
Allow reference to embryo instead of fetus throughout.

level of progesterone decreases (drastically) just before birth;
removing inhibition of oxytocin secretion;
oxytocin produced by pituitary gland;
oxytocin causes contractions of uterus;
uterine contractions cause impulses to be sent leading to more oxytocin secretion;
positive feedback;

Part (a) was another question where it rapidly became clear to examiners how well the candidate understood the biology involved. Weaker answers often included many details of how sperm travel from the male reproductive system to the oviduct of the female. This was not needed as the events of fertilization were considered to start with the arrival of sperm at the surface of the oocyte. There were many full accounts, including the acrosome reaction and the cortical reaction. 

A full range of answers to (b) was seen, from muddled brief accounts confusing the placenta with the amniotic sac, to very full and detailed explanations of structure and function. The best answers included accurate descriptions of blood flow, with a clear distinction between fetal blood flow through capillaries in the placenta and maternal blood flow through the sinuses.

Part (c) of the question also elicited answers of very variable quality. Many candidates remembered that a positive feedback mechanism was involved but not all could describe it correctly. Some discussed cervical dilation but did not mention uterine contraction. In questions of this type it is always worth mentioning the source of the hormones involved, in this case the pituitary gland as the source of oxytocin.