Human Reproductive Systems and Fertilisation
Human reproductive systems and fertilisation, at IGCSE, require precise, structure-by-structure knowledge of the human male and female reproductive systems and a detailed account of fertilisation as an ordered sequence, going beyond the GCSE spec's brief mention of sperm meeting an egg. Students must name and give the function of every major structure (testis, scrotum, sperm duct/vas deferens, prostate gland, urethra, penis; ovary, oviduct/fallopian tube, uterus, cervix, vagina), describe how the sperm cell and egg cell are each adapted to their role, and give fertilisation as a sequence culminating in fusion of the sperm and egg nuclei to form a diploid zygote.
Before you start
No specific prerequisites - this is a good place to start.
Method
- Label the male reproductive system and give each structure's function: testes (produce sperm and testosterone), scrotum (holds the testes outside the body at a slightly lower temperature for sperm production), sperm duct/vas deferens (carries sperm from the testis towards the urethra), prostate gland (adds fluid to the sperm to form semen, nourishing and protecting the sperm), urethra (carries semen, or separately urine, out through the penis), penis (deposits semen in the vagina during sexual intercourse).
- Label the female reproductive system and give each structure's function: ovary (produces egg cells/ova and the hormones oestrogen and progesterone), oviduct/fallopian tube (the site of fertilisation; carries the egg towards the uterus, lined with cilia that help move the egg), uterus (womb; where the embryo implants and develops), cervix (the ring of muscle at the base of the uterus, which dilates during birth and produces mucus that changes with the cycle), vagina (receives the penis/semen during intercourse and is the birth canal).
- Describe the adaptations of each gamete precisely: a sperm cell has a tail (flagellum) for swimming, many mitochondria in the mid-piece to release energy for movement, and an acrosome at the head containing enzymes to digest a path through the egg's outer layers; an egg cell (ovum) is large with cytoplasm containing nutrients to support the early embryo, and has a jelly-like outer coat that changes structure immediately after one sperm has entered, to prevent any further sperm entering (polyspermy).
- Give fertilisation as an ordered sequence: (1) semen containing millions of sperm is deposited in the vagina during intercourse; (2) sperm swim through the cervix, uterus and into the oviduct; (3) if an egg is present, sperm surround it and the acrosome of one sperm releases enzymes that digest a path through the egg's outer coat; (4) the head of one sperm enters the egg and the egg's outer coat changes immediately to block any other sperm; (5) the nucleus of the sperm fuses with the nucleus of the egg to form a single diploid nucleus, forming a zygote.
- State the chromosome numbers involved and why they matter: sperm and egg cells are haploid (23 chromosomes each in humans), so fusion of their nuclei restores the diploid number (46 chromosomes) in the zygote, one set inherited from each parent.
- For 'why fertilisation normally happens in the oviduct' questions, link the site to the timing: the egg is only viable for around 24 hours after release, and sperm can survive for several days in the female tract, so fertilisation occurs where and while the egg is travelling towards the uterus.
- When answering exam questions on structure and adaptation together, always pair the named structure with its function in the same sentence (never a bare list) since marks are awarded for the linked pair, not the name alone.
Worked example
Describe the sequence of events that occurs from sperm being deposited in the vagina to the formation of a zygote, naming the structures involved.
- Start at the beginning of the sequence: sperm (in semen) are deposited in the vagina during intercourse.
- Give the route travelled: sperm swim through the cervix and uterus, then into the oviduct (fallopian tube).
- Describe what happens when sperm meet an egg: sperm cluster around the egg in the oviduct, and the acrosome of a sperm releases enzymes that digest a path through the egg's outer coat.
- Describe entry and the block to further sperm: the nucleus-containing head of one sperm enters the egg, and the egg's outer coat immediately changes so no other sperm can enter.
- Describe fusion: the nucleus of the sperm fuses with the nucleus of the egg.
- Final answer: this fusion of the haploid sperm nucleus and haploid egg nucleus forms a single diploid nucleus, creating a zygote with the full 46 chromosomes (23 from each parent).
Practice questions
Type your answer and press Check to be marked straight away, or reveal the answer and mark yourself.
Q1Name the structure that carries sperm from the testis towards the urethra.Show answer
Answer: The sperm duct (vas deferens).
Q2Name the site in the female reproductive system where fertilisation normally occurs.Show answer
Answer: The oviduct (fallopian tube).
Q3State the function of the acrosome.Show answer
Answer: It contains enzymes that digest a path through the outer layers of the egg cell, allowing the sperm nucleus to enter.
Q4State why sperm cells contain a large number of mitochondria.Show answer
Answer: To release energy (by respiration) for the tail (flagellum) to move, so the sperm can swim towards the egg.
Q5State the number of chromosomes in a human sperm cell, and explain why it is this number.Show answer
Answer: 23; sperm cells are haploid (contain half the number of chromosomes of a normal body cell), so that when the sperm nucleus fuses with the egg nucleus at fertilisation, the diploid number (46) is restored.
Q6Explain how the egg cell prevents more than one sperm from fertilising it.Show answer
Answer: Immediately after the first sperm nucleus enters, the egg's outer coat/membrane changes structure so it can no longer be penetrated by further sperm.
Exam-style questions
Written in the style of a IGCSE Science exam paper, with a full mark scheme.
State the function of the prostate gland and the function of the cervix.
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A couple record that intercourse took place 3 days before the woman released an egg. The egg survived for 24 hours after release before fertilisation would no longer be possible. Given that sperm can survive in the female reproductive tract for up to 5 days, explain whether fertilisation was possible in this case, showing your reasoning.
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Describe the structures and adaptations of the sperm cell and the egg cell, and explain how these adaptations allow fertilisation to occur successfully.
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Free printable worksheet
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This topic is chapter 3 of IGCSE Science Workbook, the whole course as one free printable PDF.
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