Reproduction and Inheritance · 7 question types
Past paper frequency (2018 to 2024)
This topic accounts for approximately 11% of your exam marks.
Sexual vs asexual reproduction comparisons appear frequently; IVF and cloning as application questions.
About a week after fertilisation, the zygote (now a ball of cells called a blastocyst) implants into the soft endometrium of the uterus. The blastocyst is now called an embryo, and the embryo begins to grow.
By around eight weeks of pregnancy, a special organ called the placenta has formed at the site where the embryo implanted. The placenta is the interface between the mother and the developing fetus. It is built from the tissue of the embryo and the wall of the uterus.
The placenta connects to the developing fetus by the umbilical cord, which contains:
Important: the mother's blood and the fetus's blood never mix directly. They are separated by a very thin barrier in the placenta. Substances cross this barrier by diffusion.
The placenta is highly adapted as an exchange surface, with a large surface area and a very thin wall between the two blood supplies.
From mother to fetus (down concentration gradients):
From fetus to mother (down concentration gradients):
The placenta is not a perfect barrier. Some harmful substances can cross from mother to fetus, with serious consequences:
This is why pregnant women are advised to avoid smoking, alcohol, certain medicines, and certain infections during pregnancy.
The developing fetus is surrounded by amniotic fluid, a clear liquid contained in the amniotic sac (also called the amniotic membrane). The amniotic fluid: