Chapter 16: Human Sexual Reproductive System (Summary Sheet)

Part A · Reproduction, Gametes and the Reproductive Systems

1 Sexual Reproduction and Heredity

Humans reproduce sexually: the nucleus of a sperm and the nucleus of an egg cell fuse, so the new individual receives genetic material from two parents.
Heredity is the passing of genetic material from one generation to the next. It is held in the nucleus of every cell, so a child receives one set of instructions from each parent.
Which version of each trait a particular child receives is settled at random, so every new individual gets a unique combination — one half from the mother (via the egg) and one half from the father (via the sperm).
That is why children are similar to their parents and to each other, yet never identical: brothers and sisters differ from one another and from their parents. Identical twins are the exception — they come from one fertilised egg, so they share the same combination.
The gamete is the cell that carries that half-share: a sperm from the male and an egg cell from the female.

2 The Two Gametes, Built on Opposite Plans

	Sperm	Ovum (egg cell)
Made in	The two testes	The two ovaries
Shape	Long and streamlined: a head, a middle piece and a whip-like tail	Rounded, wrapped in two outer coverings
Getting there	The tail beats, so the sperm swims to the egg	Non-motile — carried along the oviduct by the tube's muscles and its cilia
Size	Tiny	Comparatively huge — many times wider
Extra cargo	Many mitochondria in the middle piece, releasing the energy the tail needs	Cytoplasm packed with food and mitochondria — fuel for an embryo
Numbers	Released in millions; each one has only a slim chance of reaching the egg	Released one at a time, about one per cycle

A gamete carries half the genetic material needed — 23 chromosomes in humans.
Every feature links to the job: the sperm is built to travel and depends on being produced in vast numbers, while the egg is built to wait, carrying its own store of food.

3 The Male Reproductive System

Part	What it does
Testis (plural testes)	Makes sperm and the male sex hormone testosterone; the two are held in the scrotum, kept just below body temperature because sperm develop best when cool
Epididymis	A coiled tube against each testis where sperm are stored until release
Sperm duct	Carries sperm from the epididymis towards the urethra
Sex glands	Add seminal fluid, a nutrient-rich liquid, so sperm have a medium to swim in; sperm plus this fluid is semen
Urethra	The single tube through the penis; carries semen out of the body (and urine, but never at the same moment)
Penis	Contains the urethra; deposits semen in the vagina during sexual intercourse

The route semen takes out of the body: testis → epididymis → sperm duct (fluid added) → urethra → out.

4 The Female Reproductive System

Part	What it does
Ovary	Holds the immature eggs, releases one mature ovum at ovulation, and makes the female sex hormones oestrogen and progesterone
Oviduct (fallopian tube)	Muscular tube from the uterus to the ovary, lined with cilia; its muscles and cilia move the ovum towards the uterus. This is where fertilisation happens
Uterus (womb)	Muscular, elastic organ whose inner lining is rich in blood vessels; the embryo implants here and the foetus develops; its muscles contract during childbirth
Cervix	The opening at the lower end of the uterus, joining it to the vagina; it stays closed during pregnancy and widens during childbirth
Vagina	The passage from the cervix to the outside; receives semen during intercourse and is the birth canal

Exam tip: two locations carry most of the marks in this chapter — the oviduct is where fertilisation happens, and the uterine lining is where implantation happens. An answer that puts fertilisation in the uterus loses marks straight away.

Part B · Puberty and the Menstrual Cycle

5 Puberty: the Physical Changes

Puberty is the stage of rapid growth during which the body becomes physically able to reproduce. It normally begins between about 11 and 14 years old.
It is driven by hormones — chemical messengers released in one part of the body that travel in the blood and switch on the activity of other organs. The sex hormones are testosterone in males and oestrogen in females.

Group	Changes
Primary sex characteristics	The reproductive organs themselves — present from birth, they mature during puberty and are what makes reproduction possible
Secondary sex characteristics	The outward changes the sex hormones bring about; they do not produce gametes, but they distinguish the two sexes
Both sexes	Rapid growth in height and weight; body odour as sweat glands become more active; hair grows in the pubic region and under the arms
Males	Testes and penis enlarge and sperm production begins; the voice box enlarges so the voice deepens; shoulders and chest broaden, muscles strengthen; hair grows on the face and body
Females	Ovaries begin releasing mature eggs and menstruation starts; the uterus enlarges and the hips widen; the breasts develop

Sorting a list into primary and secondary is a test of the organ, not of the sex: a reproductive organ (ovary, uterus, testis, penis) is primary; anything else (voice, hair, hips, breasts) is secondary.
The syllabus note: the details of the hormonal system are not required — it is enough to know that hormones bring the changes about.

6 The Menstrual Cycle, Stage by Stage

Each month the uterus builds a thick, blood-rich lining in case an egg is fertilised and implants there. If no pregnancy begins, the lining is not needed, so it is removed and the build-up starts again.
Ovulation is the release of a mature egg from an ovary. Menstruation is the discharge, through the vagina, of blood and the shed uterine lining together with the unfertilised egg.
Day 1 is the first day of the flow, and the cycle runs to the day before the next flow begins. About 28 days is typical, but a healthy cycle can be shorter or longer — stress, diet, illness and body weight all shift the timing, which ends for good at menopause.

Stage	Days	What is happening
Shedding	1–5	Menstruation takes place: both hormones are at their lowest and the lining breaks down and leaves the body
Rebuilding	6–13	Oestrogen rises and peaks; an egg cell matures in the ovary and the lining is repaired and thickened with new blood vessels
Release	about 14	Ovulation: the mature egg leaves the ovary and is swept into the oviduct; progesterone starts to rise and the lining keeps thickening
Maintenance	15–28	Progesterone peaks, holding the lining at its thickest and best supplied with blood. If no fertilisation happens the levels fall away, the lining is shed, and a new cycle begins at day 1

Two hormones act on the uterus, and the pattern is simple: oestrogen builds the lining up after menstruation, and progesterone holds it thick and ready. When both fall sharply, the lining is no longer supported and is shed.
When reading a graph: while progesterone is high the lining is being maintained; the moment both hormones fall, menstruation is about to begin.
The fertile period runs from about day 10 to about day 15 of a typical cycle, because the two lifespans overlap: an egg lives only about one day after ovulation, while sperm can survive three to five days inside the female system — so sperm deposited before ovulation can wait for the egg. Pregnancy is unlikely on the other days, though not impossible, since cycle lengths vary.

Part C · Fertilisation, Implantation and the Foetus

7 Fertilisation: the Fusion of Two Nuclei

Fertilisation is the fusion of the nucleus of a sperm with the nucleus of an egg cell, forming a single cell — the fertilised egg, or zygote. It takes place in the oviduct, not the uterus.
Each gamete carries half the genetic material (23 chromosomes), so the zygote has the full set — 46 chromosomes, half from each parent.
The sperm reach it: during intercourse semen is deposited in the vagina, and sperm swim through the cervix and across the uterus into the oviduct. Millions set out; only one succeeds.
At the egg, enzymes from the tip of the sperm head digest a path through its outer coverings; the cell membranes fuse and the sperm nucleus passes into the egg cell.
The covering then hardens at once, so no further sperm can enter and the rest die of their own accord. The two nuclei fuse, and the new individual has begun.

Exam tip: say the nuclei fuse — not “join” or “mix”. And say the egg's covering blocks the other sperm; the egg does not kill them.

8 From Zygote to Implantation

The zygote divides repeatedly as it travels down the oviduct, becoming a ball of cells — an embryo.
About a week after fertilisation it reaches the uterus and sinks into the prepared lining. This is implantation, and it is the point at which pregnancy begins.
Implantation keeps the ovary's temporary gland active, so oestrogen and progesterone stay high and the lining is not shed.
If the egg is not fertilised, no such signal is sent: the hormone levels fall, the lining can no longer be maintained, and menstruation begins — day 1 of the next cycle.
By about the eighth week the embryo has the beginnings of all its major organs and is called a foetus; from then on it grows and matures.

Part D · Population, Contraception and Sexual Health

9 Why the Human Population Has Grown

Population size is a balance: births and immigration add people, deaths and emigration remove them. The world's population has risen mainly because fewer people die young, not because families have grown larger.

Reason	How it works
Advances in medicine	Vaccination, antibiotics, safe childbirth and emergency care cure or prevent conditions that were once fatal, so the death rate falls
Improved sanitation	Clean piped water and safe disposal of waste keep disease-causing organisms out of drinking water, so fewer people — especially young children — die
Reliable food supply	Higher yields, irrigation and better storage keep more people fed through the year
Education and contraception	People who stay in school longer tend to marry later and choose smaller families, so the birth rate falls and growth slows
Migration	People move for work, study or safety; net migration changes a country's total without changing either rate

10 Birth Control: the Link Each Method Breaks

A pregnancy needs four events in order: an egg is released (ovulation); sperm reach it and fuse with it (fertilisation); the fertilised egg divides as it travels to the uterus; and the embryo settles into the lining (implantation).
Every birth control method works by breaking one of those links, so conception — the start of a pregnancy — cannot happen. Name the link the method breaks, then say what follows.

Group	Examples	What it stops
Acts on the cycle, so no egg is released	Combined pill, progestogen-only pill, injection, implant	Ovulation. The hormones also thicken the mucus at the cervix and thin the uterine lining, so a sperm that does reach an egg is less likely to fertilise it and an embryo is less likely to implant. Must be taken or refitted on schedule.
Blocks the sperm's route to the egg	Male condom, female condom, diaphragm or cap	Fertilisation: the two gametes are kept apart physically. This is the only group that also reduces the risk of a sexually transmitted infection passing between partners.
Disables the sperm	Spermicidal cream, foam or jelly	Fertilisation: sperm are killed or made unable to swim. Too unreliable to use alone, so it is added to a cap or diaphragm.
Fitted inside the uterus	Copper and hormonal intra-uterine devices	Implantation, because the device alters the environment of the uterus; copper also harms sperm before they reach the egg. Fitted by a clinician and left in place for years.
Times intercourse to the cycle	Calendar counting; daily checks of body temperature or cervical mucus	Fertilisation, by avoiding intercourse on the fertile days. No device and no hormones, but it needs a regular cycle and careful records, so it is the least reliable group.
Permanent	Vasectomy in the male, tubal ligation in the female	Fertilisation, by blocking the tubes that carry sperm, or the oviducts, so the two gametes can never meet. The sex organs are not removed, so the sex hormones are still produced and the menstrual cycle continues.

Two approaches sit outside that table: withdrawal, which depends entirely on the man's timing and gives no protection from infection, and abstinence, which avoids the act altogether and so removes both the risk of pregnancy and the risk of infection.
Only the barrier group also protects against infection, so a couple who want protection from both pregnancy and infection must combine a condom with a second method.

11 Sexually Transmitted Infections (STIs)

An STI is an infection passed from one person to another mainly through sexual contact. It can be passed by a person who has no obvious symptoms.

STI	Symptoms	Harmful consequences	Can antibiotics cure it?
Syphilis bacteria	First a painless sore on the sex organs, which heals by itself; weeks later a rash and fever	The bacteria spread through the body and damage the heart, brain and other organs; it can also be passed from a mother to her baby	Yes — a bacterial STI, so antibiotics cure it if it is treated in time
Gonorrhoea bacteria	Discharge from the sex organs and pain when passing urine	Scarring of the tubes can leave the person infertile; it can infect a baby's eyes as it is born	Yes — a bacterial STI, so antibiotics cure it if it is treated in time
AIDS virus (HIV)	Often no symptoms for years, while the virus quietly multiplies	HIV attacks the body's defence system, so the person becomes vulnerable to infections that a healthy body would fight off	No — antibiotics do not work on viruses, so a viral STI cannot be cured by antibiotics

That is the one fact to carry away: bacterial STIs can be cured by antibiotics, viral STIs cannot.

12 Substance Abuse and the Foetus

What a pregnant woman takes in reaches the foetus through her own blood, so substance abuse harms a developing baby.

Substance	Effect on the foetus
Smoking	Carbon monoxide and nicotine reduce the oxygen and nutrients reaching the foetus, so it grows more slowly — babies are often born underweight or too early
Alcohol	Can damage the developing brain and organs, causing lifelong learning and growth problems
Drug abuse	Can cause premature birth, low birth weight and dependency, so the newborn baby may already need treatment for withdrawal

Because these effects cannot be undone after birth, avoiding them during pregnancy is a matter of prevention rather than cure.

13 Abortion and Pre-marital Sex: Weighing Consequences

These are evaluate questions: they want consequences on both sides and a reasoned conclusion, not a slogan. Answer neutrally and consider more than one viewpoint.

Issue	Points to weigh
Unplanned pregnancy	Ending a pregnancy is a medical procedure with risks to the woman's health, and a decision with emotional and ethical weight
Pre-marital sex	Increases the risk of an unplanned pregnancy and of STIs; consequences may include interruption of schooling
What reduces the risk	Accurate information, access to contraception and protection, and the freedom to make a considered choice rather than a pressured one

Part E · Say It Like This

14 Say It Like This

The question asks	The phrase that earns the mark
Where does fertilisation happen?	“Fertilisation takes place in the oviduct, where the nucleus of a sperm fuses with the nucleus of the egg cell to form a fertilised egg.”
What is implantation, and where?	“The embryo sinks into the uterine lining, which is richly supplied with blood vessels, and pregnancy begins.”
Why do brothers and sisters differ?	“Each new individual receives a unique combination of genetic material from the mother via the egg and the father via the sperm, so siblings are similar but not identical.”
What happens during the menstrual cycle?	“Oestrogen rebuilds the uterine lining after menstruation and progesterone keeps it thick and ready for an embryo; if no egg is implanted, both levels fall and the lining is shed as menstrual flow.”
How does a method prevent pregnancy?	“It stops ovulation / keeps the sperm from reaching the egg / prevents implantation — so fertilisation cannot take place.”
Why can gonorrhoea be treated but not AIDS?	“Gonorrhoea is caused by bacteria, which antibiotics can kill, whereas AIDS is caused by a virus, and antibiotics do not work on viruses.”