The Human Reproductive Systems

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 3.8 — How the male and female reproductive systems are adapted for their functions
• 3.13 — The roles of oestrogen and testosterone in secondary sexual characteristics

Specification section 3(a) Reproduction — also taught in Sexual and Asexual Reproduction, Flowers, Pollination and Fertilisation, Seeds and Germination, The Menstrual Cycle.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

Both reproductive systems are solving the same three problems: make gametes, get them to meet, and — in one of the two systems — house and supply an embryo for nine months afterwards. Almost every structure on this page is an answer to one of those, and if you can say which one, you can work out most of the adaptations rather than memorising them.

The male system, structure by structure

1. TESTES — produce sperm, and produce the hormone testosterone. They are held OUTSIDE the body cavity in the scrotum, because sperm production requires a temperature slightly below normal body temperature.
2. SCROTUM — the sac holding the testes outside the body, which is what keeps them at that lower temperature.
3. SPERM DUCT — carries sperm from the testis towards the urethra.
4. GLANDS (including the prostate) — add fluid to the sperm, forming semen. The fluid supplies the sperm with nutrients for respiration and a medium to swim in.
5. URETHRA — the tube through which semen leaves the body. It also carries urine, but never at the same time.
6. PENIS — delivers the semen into the vagina, so that the sperm are released close to the female gametes rather than outside the body.

The female system, structure by structure

1. OVARIES — contain the ova (egg cells) and release one roughly every month; they also produce oestrogen and progesterone.
2. OVIDUCT (fallopian tube) — carries the egg from the ovary towards the uterus. It is lined with cilia that sweep the egg along, and it is where fertilisation normally happens.
3. UTERUS — a muscular organ with a thick lining, in which the embryo implants and develops. Its muscular wall contracts during birth.
4. CERVIX — the ring of muscle at the base of the uterus, which holds the developing fetus in place and dilates during birth.
5. VAGINA — receives the semen, and is the passage through which the baby is born.

Now compare the two gametes, because the differences are not arbitrary — each one follows from the job that cell has to do. A SPERM CELL has to travel a considerable distance under its own power, so it is tiny, it has a tail (flagellum) to swim with, and it is packed with mitochondria, which is where aerobic respiration releases the energy that swimming requires. It also carries an enzyme-filled tip that digests through the egg's outer membrane. An EGG CELL does not travel at all — it is swept along by cilia — so it can afford to be enormous, and it is, because it carries the food store that will supply the embryo until implantation.

Think of it like sending a letter against sending a shipping container

Millions of letters, each stripped to nothing but an address and a stamp, are dispatched in the hope that one arrives. A single shipping container goes nowhere on its own, is loaded with everything the destination will need, and is moved by somebody else's machinery. Neither design is better; they are answers to opposite questions. That is why a man produces sperm continuously in enormous numbers and a woman releases roughly one egg a month, and why one gamete is among the smallest cells in the body and the other among the largest.

None of this machinery works from birth. It is switched on at PUBERTY, and it is switched on by hormones. In males the testes begin to release TESTOSTERONE; in females the ovaries begin to release OESTROGEN. Both hormones do two jobs at once: they cause the reproductive organs to mature and start producing gametes, and they cause the SECONDARY SEXUAL CHARACTERISTICS — the physical changes that are not directly part of reproduction.

What each hormone causes at puberty

1. TESTOSTERONE, from the testes: sperm production begins; the voice deepens; facial, body and pubic hair grows; muscle mass and body size increase; the reproductive organs mature.
2. OESTROGEN, from the ovaries: the menstrual cycle begins; the breasts develop; the hips widen; pubic and underarm hair grows; the reproductive organs mature.
3. Notice what the two lists share: both include the maturing of the reproductive organs and the growth of body hair, and both mark the start of gamete release. The differences are what the term 'secondary' picks out.

It is worth being clear about how a hormone can do any of that, because the mechanism is the same one every time. The gland releases the hormone into the BLOOD, which carries it to every organ in the body — it is not aimed. Only cells that can RESPOND to that particular hormone are affected by it, and those organs are its target organs; every other cell simply ignores it, however much arrives. The effect then continues for as long as the hormone's CONCENTRATION in the blood is maintained, which is why puberty takes years rather than minutes and why the changes it produces do not reverse. And once the effect has been achieved, the response feeds back to reduce further release — the same negative feedback shape as every other control system on this course.

The word secondary is doing real work there, and it is worth being precise about. PRIMARY sexual characteristics are the reproductive organs themselves, present from birth. SECONDARY sexual characteristics are the changes that appear at puberty and are not directly involved in producing or delivering gametes — a deeper voice does not help sperm to swim. They develop because the hormone is now circulating in the blood, and they persist as long as it does.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Learn this definition · Secondary sexual characteristic

A physical feature that develops at puberty under the influence of testosterone or oestrogen, and which is not directly involved in producing or delivering gametes.

How a hormone produces an effect — the three stages, which every hormone question needs

1 · Released into the blood

A gland releases the hormone into the bloodstream, which is its only means of transport. The blood carries it to every organ in the body; it is not aimed at anything.

2 · Only target organs respond

Only cells able to respond to that particular hormone are affected. Every other cell ignores it, however much of it arrives.

3 · The effect lasts while the concentration does

The response continues for as long as the hormone's concentration in the blood is maintained, which is why hormonal changes are slow and long-lasting.

Negative feedback

Once the effect has been achieved, the response reduces further release of the hormone.

Statement 3.8 — the male system, and the function of each part

Testes

Produce sperm and the hormone testosterone. Held outside the body in the scrotum, because sperm production needs a temperature slightly below body temperature.

Scrotum

The sac that holds the testes outside the body cavity, keeping them cooler than 37 °C.

Sperm duct

Carries sperm from the testis to the urethra.

Prostate and other glands

Add fluid containing nutrients to the sperm, forming semen — the nutrients supply energy for respiration in the sperm.

Urethra

Carries semen out of the body through the penis. It also carries urine, but not at the same time.

Penis

Delivers semen into the vagina, so sperm are released close to the female gametes.

Statement 3.8 — the female system, and the function of each part

Ovaries

Contain the ova and release one about every month; also produce oestrogen and progesterone.

Oviduct (fallopian tube)

Lined with cilia that sweep the egg towards the uterus. Fertilisation normally occurs here.

Uterus

Muscular organ with a thick lining in which the embryo implants and develops; its wall contracts during birth.

Cervix

The ring of muscle at the base of the uterus; holds the fetus in place and dilates during birth.

Vagina

Receives semen, and is the passage through which the baby is born.

Sperm cell against egg cell — every difference follows from the job

Statement 3.13 — the two hormones at puberty

Primary against secondary sexual characteristics

A. PRIMARY: the reproductive organs themselves, present from birth and directly involved in producing or delivering gametes.

B. SECONDARY: the changes that appear at puberty under the influence of testosterone or oestrogen, and which are NOT directly involved in producing or delivering gametes — a deeper voice, wider hips, body hair.

Not this: The testes are outside the body because there is no room for them inside.

This: They are outside because sperm production requires a temperature slightly below normal body temperature. It is a functional adaptation, not a matter of space.

Model answer [4 marks]

Explain two ways in which a sperm cell is adapted for its function. [4]

A sperm cell has a tail, which allows it to swim towards the egg cell in the oviduct. It contains many mitochondria, which release the energy from respiration needed for that swimming. It also carries enzymes in its tip, which digest through the outer membrane of the egg cell so that the nuclei can fuse.

Model answer [2 marks]

Explain why the testes are held outside the body cavity. [2]

Sperm production requires a temperature slightly lower than normal body temperature. Holding the testes outside the body cavity in the scrotum keeps them cooler than 37 °C, so sperm can be produced successfully.

Model answer [4 marks]

Describe the roles of testosterone and oestrogen at puberty. [4]

Testosterone is produced by the testes. It causes sperm production to begin and causes the male secondary sexual characteristics to develop, including a deeper voice, the growth of facial and body hair, and increased muscle mass. Oestrogen is produced by the ovaries. It causes the menstrual cycle to begin and causes the female secondary sexual characteristics to develop, including breast development and widening of the hips.

Mark-losing trap. Name the ORGAN that makes each hormone: testosterone from the testes, oestrogen from the ovaries.

Mark-losing trap. Fertilisation happens in the OVIDUCT, not in the uterus. The uterus is where the embryo implants afterwards.

Mark-losing trap. Give a reason with every adaptation. 'Sperm have a tail' is one mark; 'a tail, so it can swim to the egg' is two.

Mark-losing trap. Secondary characteristics are NOT directly involved in making or delivering gametes. That is what distinguishes them from primary ones.

Mark-losing trap. The oviduct moves the egg using CILIA. The egg cell cannot swim — only the sperm can.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — In which structure does fertilisation normally take place?
2. Grade 7 · Explain [2 marks] — A sperm cell contains far more mitochondria than most cells of its size. Explain why.
3. Grade 8 · Explain [5 marks] — Explain how the structure of an egg cell and the structure of a sperm cell each suit the job that cell has to do. Select every statement that belongs in a full-mark explanation.
4. Grade 9 · Calculate [3 marks] — A sample of semen contains 3.2 × 10⁸ sperm. Of these, 0.05 % reach the oviduct containing the egg cell. Calculate how many sperm reach that oviduct.
5. 9+ · Analyse [5 marks] — A young man has testes that produce testosterone normally, but the cells of his body carry a fault meaning they cannot respond to it. He has not developed a deeper voice, facial hair or increased muscle mass at puberty. Blood tests show his testosterone concentration is normal, in fact slightly higher than average. Select every statement that belongs in a full-mark explanation.

The people behind this science

Two ways into the same idea — the one who explained how a hormone reaches a distant organ, and the one who first watched an embryo take shape. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Ernest Starling — the one who explained how a hormone reaches a distant organ

This page claims that a chemical released by the testes changes a person's voice, and that a chemical released by the ovaries changes the shape of their hips — organs with no connection to the source at all. Starling is the reason that is a mechanism rather than a mystery. In 1902 he showed that an organ can command another with every nerve between them cut, because the message travels in the blood, and in 1905 he named that class of messenger a hormone. Puberty is one of the clearest cases of what he described: one gland, many target organs, and effects that last for years.

• “How can a chemical from one organ change a completely different part of the body?”
• “Why does a hormone affect some organs and not others?”
• “Why do hormonal changes last so much longer than nervous ones?”
• “What made you certain the message was chemical and not nervous?”

Marcello Malpighi — the one who first watched an embryo take shape

Everything on this page exists to produce a fertilised egg cell, and Malpighi is the first person who set out to watch what one becomes. He opened chick eggs at intervals over successive days and drew what he found under the lens, tracing the development of the heart, the blood vessels and the nervous system from a structureless start. It was the first systematic account of an embryo developing rather than being assumed to exist fully formed and merely growing — which was the prevailing view he was arguing against.

• “What did you see on the first day, and what on the fourth?”
• “Did the embryo appear to be built, or already present and growing?”
• “What was the hardest structure to make out under your lens?”
• “Why examine a chick rather than a mammal?”

Carry on through the course

• Previous lesson: Seeds and Germination
• Next lesson: The Menstrual Cycle
• Sexual and Asexual Reproduction — Two ways to make a new organism — one produces copies, the other produces something that has never existed before. Why any species would want both.
• Flowers, Pollination and Fertilisation — Two completely different solutions to one problem — a plant that cannot move has to get its male gamete to another plant's female gamete.
• Seeds and Germination — What a seed needs to start growing, what it lives on before it has a single leaf — and why light is not on the list.
• The Menstrual Cycle — Four hormones, two organs and one repeating question — should the uterus lining be built up, held, or let go?
• The Placenta and the Amniotic Fluid — An exchange surface with a constraint no other one has: two bloodstreams that must trade materials without ever mixing.
• All of Biology · Biology key terms · required practicals · Incandio Science · The 4BI1 specification