Home › Learning Hub › IGCSE Biology › 16b Human reproduction
Topic 16 · 16.4–16.6

Human reproduction

Core and Extended · Papers 1–6

🎯What you need to be able to do

  • Identify the parts of the male and female reproductive systems and state their functions.
  • Explain the adaptive features of sperm and egg cells and compare them; describe fertilisation and early development.
  • Identify the umbilical cord, placenta, amniotic sac and fluid; explain the placenta’s role EXTENDED.
  • Describe the roles of testosterone and oestrogen, and the menstrual cycle; explain the hormones FSH, LH, oestrogen and progesterone EXTENDED.
  • Describe STIs, HIV and AIDS, how HIV is transmitted and how the spread of STIs is controlled.

📚The biology

The reproductive systems

The male reproductive system from the front. Two testes inside the scrotum; a sperm duct from each testis loops up and joins the urethra below the bladder, where the prostate gland is; the urethra runs through the penis.
The male reproductive system (front view).
testes: produce sperm and testosterone
scrotum: holds the testes outside the body, where it is slightly cooler, the best temperature for making sperm
sperm ducts: carry sperm from the testes to the urethra
prostate gland: adds fluid to the sperm (to make semen)
urethra: carries urine or semen out of the body (not both at once)
penis: places semen in the vagina
The female reproductive system from the front. Two ovaries, each beside the funnel-shaped open end of an oviduct; the oviducts lead to the uterus; the cervix is the ring of muscle at the base of the uterus; the vagina leads to the outside.
The female reproductive system (front view).
ovaries: release egg cells and produce oestrogen
oviducts: carry the egg to the uterus; fertilisation usually happens here
uterus: where the embryo implants and develops into a fetus
cervix: a ring of muscle at the neck of the uterus; holds the fetus in and widens at birth
vagina: receives sperm during intercourse; the birth canal

Gametes and fertilisation

Fertilisation is the fusion of the nuclei from a male gamete (sperm) and a female gamete (egg cell).

A sperm cell, enlarged, with an acrosome at the tip of the head containing enzymes, a haploid nucleus, a midpiece packed with mitochondria, and a long flagellum. An egg cell, much larger, surrounded by a jelly coat, with cytoplasm containing an energy store and a haploid nucleus.
Not to scale: a human egg is about 20 times the diameter of a sperm head.
sperm: flagellum to swim; many mitochondria to release energy for swimming; enzymes in the acrosome to digest a way through the jelly coat
egg: energy stores in the cytoplasm for the early embryo; a jelly coat that changes after fertilisation so no more sperm can enter
compared: sperm are much smaller, can move, and are made in far greater numbers; eggs are large, cannot move themselves, and one is released each month

After fertilisation, the zygote divides to form an embryo, a ball of cells, which implants into the lining of the uterus.

A fetus inside the uterus, surrounded by amniotic fluid within the amniotic sac. The umbilical cord joins the fetus to the placenta on the uterus wall. The cervix is at the base.
The placenta exchanges substances; the umbilical cord connects it to the fetus; amniotic fluid protects the fetus.

The amniotic sac encloses the amniotic fluid, which cushions and protects the fetus. The umbilical cord joins the fetus to the placenta. EXTENDED In the placenta the blood of the mother and of the fetus come very close but do not mix. Dissolved nutrients (glucose, amino acids) and oxygen diffuse from mother to fetus; carbon dioxide and urea diffuse from fetus to mother. Some pathogens (such as the rubella virus) and toxins (such as nicotine and alcohol) can also cross the placenta and harm the fetus.

Sex hormones and the menstrual cycle

At puberty, testosterone (from the testes) and oestrogen (from the ovaries) cause the secondary sexual characteristics: in boys, growth of facial and body hair, a deeper voice, more muscle; in girls, breast development, wider hips, and the start of the menstrual cycle; in both, pubic hair and a growth spurt.

A 28-day cycle. Top: the uterus lining breaks down during the period, days 1 to 5, then thickens until about day 21 and stays thick. Ovulation is at about day 14. Bottom, schematic hormone levels: FSH is high early in the cycle with a small peak before ovulation; LH has a sharp peak just before day 14; oestrogen rises to a peak just before ovulation; progesterone rises after ovulation, peaking around day 21.
Lining breaks down (period), rebuilds, ovulation at about day 14, lining maintained. The hormone panel is Supplement.

During the menstrual cycle, an egg matures in the ovary and is released at ovulation; the uterus lining thickens ready to receive an embryo. If no embryo implants, the lining breaks down and is lost in the period.

  • EXTENDED FSH (from the pituitary gland) stimulates an egg to mature in the ovary and the ovary to make oestrogen.
  • Oestrogen (from the ovaries) makes the uterus lining thicken.
  • LH (from the pituitary) causes ovulation.
  • Progesterone is made in the ovary after ovulation (by the empty follicle, the corpus luteum); it maintains the lining. If no embryo implants, progesterone falls and the lining breaks down.
  • In pregnancy, progesterone and oestrogen stay high — made first by the ovary and then by the placenta — so the lining is maintained and no more eggs are released.

Sexually transmitted infections

A sexually transmitted infection (STI) is an infection that is transmitted through sexual contact. HIV (human immunodeficiency virus) is a pathogen that causes an STI; HIV infection may lead to AIDS, as the virus destroys lymphocytes and weakens the immune system. HIV is transmitted in body fluids: unprotected sex, sharing needles, blood transfusions with unscreened blood, and from mother to baby across the placenta, at birth or in breast milk. The spread of STIs is controlled by using condoms, limiting the number of sexual partners, testing and treatment, screening donated blood, and not sharing needles.

✏️Worked example

(a) State where fertilisation normally takes place, and where the embryo implants. [2] (b) Explain how the structure of a sperm cell helps it to reach and fertilise an egg. [3] (c) EXTENDED Explain why a pregnant woman is advised not to smoke. [2] (d) EXTENDED Explain why the uterus lining breaks down if no egg is fertilised. [2]

(a) In the oviduct; in the lining of the uterus.

(b) The flagellum lets it swim; many mitochondria release energy for swimming; enzymes in the acrosome digest the jelly coat so the sperm can reach the egg’s membrane.

(c) Toxins such as nicotine and carbon monoxide can cross the placenta into the fetus’s blood and harm its development.

(d) Without an embryo, progesterone levels fall, and progesterone is needed to maintain the lining, so the lining breaks down.

Check it. Every feature in (b) is paired with what it achieves — the pattern examiners reward for “explain the adaptive features”.
The placenta does not mix the blood. The two bloods are separated by thin membranes, and substances diffuse across; the mother’s blood never enters the fetus.

📝Practise

In the style of the multiple-choice and theory papers. EXTENDED marks Supplement content.

1. (Multiple choice.) Which structure adds fluid to sperm to form semen? A: testis. B: scrotum. C: prostate gland. D: urethra.
C.
2. (Theory.) State the function of (a) the oviduct, (b) the cervix. [2]
(a) Carries the egg to the uterus; site of fertilisation. (b) Holds the fetus in the uterus during pregnancy, and widens during birth.
3. (Theory.) State the function of the amniotic fluid. [1]
It cushions and protects the fetus.
4. (Theory.) Give two secondary sexual characteristics caused by testosterone. [2]
Any two: facial and body hair; deeper voice; increased muscle; growth of the penis and testes.
5. (Theory.) Describe two ways HIV is transmitted and two ways its spread is controlled. [4]
Transmission (any two): unprotected sex; sharing needles; unscreened blood transfusions; mother to baby. Control (any two): condoms; screening blood; not sharing needles; testing and treatment; fewer sexual partners.
6. (Theory.) EXTENDED Name two substances that pass from the mother to the fetus across the placenta, and two that pass the other way. [2]
To the fetus: oxygen, glucose (or amino acids). To the mother: carbon dioxide, urea.
7. (Theory.) EXTENDED State the hormone that causes ovulation and the gland that secretes it. [2]
LH; the pituitary gland.
8. (Theory.) EXTENDED Explain why a woman does not have periods during pregnancy. [2]
Progesterone (and oestrogen) stay high, made by the placenta, so the uterus lining is maintained and does not break down.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • NHS — pregnancy week by week
  • World Health Organization — HIV fact sheet