Reproduction: Question 3
Syllabus 16.4
A midwife runs a workshop for trainee health workers, using models of the human reproductive system and the developing fetus.
(a) State the function of each of these structures in human reproduction. (i) testes (ii) prostate gland (iii) oviduct [3]
(b) State one adaptive feature of a sperm cell, and state one adaptive feature of an egg cell (ovum). [2]
(c) A trainee asks why millions of sperm cells are released during ejaculation, yet only one egg cell matures and is released roughly each month. State the difference in number and the difference in motility between sperm cells and egg cells, and suggest why these differences are useful for successful fertilisation. [2]
(d) The midwife explains that once an embryo implants, the placenta forms in the wall of the uterus. Explain how the placenta allows a fetus to obtain oxygen and nutrients from its mother's blood, and state one other type of substance, besides oxygen and nutrients, that can also cross the placenta and may harm the fetus. [3]
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Worked solution
Part (a): Functions of reproductive structures
- (i) Testes: produce sperm cells (the male gametes) and produce the hormone testosterone.
- (ii) Prostate gland: produces a fluid that nourishes sperm cells and helps transport them, forming part of semen.
- (iii) Oviduct: carries the egg cell released from the ovary towards the uterus; it is also where fertilisation normally takes place.
Part (b): Adaptive features of gametes
A sperm cell is adapted for reaching and fertilising the egg cell: its flagellum lets it swim, its mitochondria release the energy needed for swimming, and enzymes in its acrosome digest a path through the egg cell’s jelly coat.
An egg cell is adapted to support development after fertilisation: it has large stores of energy and nutrients to nourish the early embryo, and a jelly coat that changes structure immediately after fertilisation, preventing any further sperm from entering.
Part (c): Why gamete numbers and motility differ
Sperm cells are tiny, motile and released in the millions with each ejaculation, whereas an egg cell is far larger, non-motile, and typically only one is released roughly each month.
This difference is useful because the journey from the vagina to the oviduct is long and hazardous for a sperm cell, and most will not survive it. Releasing huge numbers of motile sperm greatly increases the chance that at least one reaches the oviduct and fertilises the single egg cell, which itself has no means of swimming towards the sperm.
Part (d): The placenta and substance exchange
The placenta develops in the wall of the uterus once the embryo has implanted. It provides a large, thin surface area positioned close to the mother’s blood supply, which allows oxygen and dissolved nutrients to diffuse from the mother’s blood into the fetus’s blood, while carbon dioxide and other waste products diffuse in the opposite direction, all without the two blood supplies actually mixing.
This same large surface area, however, also allows some substances other than oxygen and nutrients to cross. Certain pathogens (for example some viruses) and toxins (for example from alcohol, nicotine or some other drugs) can pass from the mother’s blood across the placenta into the fetus’s blood, and can harm the developing fetus.
Final answers
- (a) (i) Testes – produce sperm and testosterone. (ii) Prostate gland – produces fluid for semen. (iii) Oviduct – carries the egg cell; site of fertilisation.
- (b) Sperm cell – flagellum for swimming. Egg cell – large energy/nutrient stores.
- (c) Sperm cells are motile and numerous; the egg cell is non-motile and single. This makes fertilisation more likely to succeed.
- (d) The placenta allows diffusion of oxygen and nutrients between mother and fetus; some pathogens or toxins can also cross and harm the fetus.