Sexual reproduction in humans

Acrosome = a cap of digestive enzymes that digests a path through the egg's jelly coat. Mitochondria = release energy by respiration. Flagellum = movement only. "The flagellum has enzymes that break down the egg coat" earns nothing — the flagellum is a whip-like structure built for swimming, never a source of enzymes. Keep the three strictly separate.

Motility means the ability to move independently (spontaneously) — for a sperm, to swim under its own power using its flagellum. Answers such as "how fast it swims", or descriptions of the sperm's energy stores instead of a definition, gained no credit. Define the word first, then add the mechanism if the question asks for it.

Asked what the umbilical cord and placenta carry, name the dissolved substances. To the fetus: oxygen, glucose, amino acids, vitamins, mineral ions and antibodies. To the mother: carbon dioxide, urea and other excretory products. Bulk terms — "food", "nutrients", "waste", "blood" — are too vague for the mark scheme and are the single most-flagged error here.

Exchange happens by diffusion (and some active transport) across thin placental membranes separating two independent circulations. The vessels run close together, giving a short diffusion distance and a large surface area, but no blood crosses — whole blood cells cannot pass. "The mother's blood flows into the baby" is a zero-mark answer.

It lets antibodies cross from mother to fetus, giving temporary passive immunity — name antibodies, not "antigens" or "alleles". But it is not absolute: some pathogens (e.g. rubella virus) and toxins (e.g. nicotine, alcohol) also cross and can harm the fetus. Equally, it does block some pathogens, so never claim maternal antibodies cover every infection.

Fertilisation is the fusion of the sperm and egg nuclei and normally happens in the oviduct. Implantation happens days later, once the embryo has reached the uterus and embeds in its thickened lining. Two separate, sequential events. Name implantation explicitly — describing the attachment without the word, or writing "implementation", loses the mark.

The oviduct is the narrow tube leading from the ovary; the uterus is the larger pear-shaped organ below it. Same care with: scrotum (the sac) vs testes (the organs inside it); sperm duct vs urethra; the cervix (neck of the uterus, not part of the vagina); the placenta (a disc on the uterus wall) vs the umbilical cord; and the amniotic sac vs the fluid it holds.