S2 Human Reproduction

There are two broad routes. In asexual reproduction a single parent produces offspring with no fusion of nuclei at all, so the offspring are genetically identical to that parent. In sexual reproduction the nuclei of two sex cells — one contributed by each parent — fuse, producing offspring that are genetically different from both parents and from one another. Humans reproduce sexually.

Human sexual reproduction requires two sets of reproductive organs and one cell from each:

Ask three questions of the two gametes and the whole comparison falls out: what must each one carry, how does it get to the meeting point, and how much raw material does it bring?

Trace the path a sperm takes and the function of each organ becomes obvious — it is produced, stored, fitted out with fluid, and delivered.

The female system does two jobs at once: it produces mature egg cells, and it provides somewhere for a fertilised egg to grow. Both jobs are under hormonal control.

• Height and weight increase quickly.
• Body odour develops as the sweat glands become more active.
• Hair begins to grow in the pubic region and under the arms.

• The testes enlarge and begin producing sperm; the penis and scrotum grow larger with them.
• The voice box enlarges, so the voice deepens.
• Shoulders and chest broaden, and muscle strength increases.
• Hair grows on the face and chest, and on other parts of the body.

• The ovaries begin releasing mature egg cells, and menstruation starts.
• The uterus enlarges and the hips widen.
• The breasts develop.

Every month the uterus builds a soft, blood-rich lining in case an egg is fertilised and implants there. If no pregnancy begins, that lining is not needed and is removed, and the preparation starts over. The sequence repeats roughly every 28 days, although its length varies from person to person and from cycle to cycle — stress, diet, nutrition and illness all shift it.

It is easier to remember the cycle as four stages named after what the uterus is doing than as a string of day numbers. The day ranges below describe a typical 28-day cycle.

Fertilisation needs a live egg and live sperm in the oviduct at the same time, so the fertile window is simply where the two lifespans overlap:

1. During sexual intercourse the penis enters the vagina and ejaculation deposits semen there, releasing sperm.
2. Sperm swim out of the vagina, through the cervix, across the uterus and into the oviducts.
3. One sperm reaches the egg cell in the oviduct and fuses with it. The rest never succeed and die — millions are released precisely because so few complete the journey.

• The egg cell is protected by a layer of follicle cells and a jelly-like matrix. Enzymes released from the acrosome at the tip of the sperm head break through those coverings.
• The cell membranes of sperm and egg fuse, and the sperm nucleus passes into the cytoplasm of the egg cell.
• Once one sperm has entered, the jelly-like matrix thickens and hardens, sealing the egg against every other sperm. This is why only one sperm ever fertilises an egg.
• If no egg is present in the oviduct, fertilisation cannot occur at all, however many sperm have been released.

Sexual reproduction only works if the uterus is ready at the moment an embryo arrives. The body solves this by repeating the same preparation every month: it builds a thick, blood-rich uterine lining, releases one ovum in the middle of that build-up, then — if nothing has been fertilised — takes the lining down and starts over. This is the menstrual cycle, and it ends only at menopause, when the ovaries stop releasing ova.

Read the cycle as four stages, each defined by what the ovary is doing and what the uterine lining is doing at the same time.

An ovum survives only about a day after release, while sperm can stay alive in the female reproductive system for three to five days. Fertilisation is therefore possible only in a window around ovulation — roughly days 10 to 15 of a typical cycle. That window is a matter of timing, not a guarantee.

Four hormones drive the cycle — two from the pituitary gland, two from the ovary — each switching the next one on at the right moment.

During sexual intercourse semen is deposited in the vagina. Sperm swim through the cervix and across the uterus into the oviduct, driven by the tail and powered by the mitochondria in the middle piece. Many reach the ovum; only one succeeds.

The zygote divides repeatedly as it is carried along the oviduct by the beating of cilia and the tube's gentle contractions, becoming a ball of cells — an embryo. About a week after fertilisation it reaches the uterus and sinks into the prepared lining: implantation, the point at which pregnancy begins.

Implantation changes the hormonal picture. The corpus luteum, which would otherwise wind down and let the lining break down, is kept active, so oestrogen and progesterone stay high. The early embryo is maintained by the ovary in this way until the placenta forms and takes over the job of producing progesterone.

By about the eighth week the embryo has the beginnings of all its major organs and is known as a foetus. From then on it grows and matures rather than forming new structures.

A foetus cannot feed itself, breathe or remove its own waste. Three structures cover these needs: the placenta, the umbilical cord and the amniotic sac.

The placenta. Soon after implantation the embryo sends finger-like projections called villi (singular: villus) into the uterine lining, growing into the maternal blood spaces there. The placenta is formed from these foetal villi together with the maternal tissue they invade. Substances cross the thin boundary between the two blood supplies by diffusion, so maternal and foetal blood always stay separated.

The umbilical cord. The cord joins the foetus to the placenta and is the route by which everything is carried to and from it. It contains two umbilical arteries and one umbilical vein.

The amniotic sac and amniotic fluid. The embryo produces a thin but strong membrane that encloses it completely, forming a fluid-filled sac in which the foetus floats. The fluid:

• absorbs shock, protecting the foetus from mechanical injury;
• holds a steady temperature and a constant internal environment;
• lets the foetus move, helping its muscles and bones to develop;
• keeps the uterine wall stretched apart, so the foetus is not pressed on;
• lubricates the vagina during childbirth.

Population size is a balance of two rates and two flows. Births and immigration add people; deaths and emigration remove them. When the additions outnumber the removals the population grows, and when they are fewer it falls. The world’s population has risen over the past two centuries mainly because deaths have fallen faster than births have, not because families have grown larger.

A pregnancy needs four events in order: an egg is released from an ovary (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 uterine lining (implantation). Every contraceptive breaks one of those links, so grouping methods by the link they break is more useful than grouping them by brand or shape — the group tells you what the method can and cannot do.

Two approaches sit outside that table. Withdrawal removes the penis before ejaculation; it depends entirely on the man’s timing and gives no protection from infection. Abstinence avoids the act altogether and is the only approach that removes both the risk of pregnancy and the risk of infection.

Effectiveness is reported as the share of couples who still conceive within a year of ordinary use, so a method always looks better in a trial than in real life: a pill is forgotten, a cap is not refitted, a condom is used carelessly. Used correctly and consistently, the hormonal and barrier groups are the most reliable, timing the cycle is the least reliable, and the surgical group is intended to be permanent. Since only the barrier group reduces the risk of infection, a couple who want protection from both pregnancy and infection must combine a condom with a second method.

Count the marks first: a two-mark question wants two separate points, not one long sentence. Then read the command word, because it fixes the kind of point that earns the mark.

1. Use the biological term. Write gamete, zygote, embryo, foetus, ovulation, fertilisation, implantation, uterine lining. Everyday words such as “seed” earn nothing.
2. Match the term to the stage. The cell released at ovulation is an ovum; once the nuclei fuse it is a zygote; after division and implantation it is an embryo, and only after about eight weeks is it a foetus.
3. Give structures in the order the gamete travels. Sperm: vagina, cervix, uterus, oviduct. Ovum: ovary, oviduct (fertilisation), uterus (implantation).

Question. Over 30 years a country’s population rose from 40 million to 58 million, while the average number of children per woman fell from 5 to 2.5.

1. Explain how the population could still rise while the average number of children per woman fell. [3]
2. A couple in that country use a combined hormonal pill. Explain how it prevents pregnancy. [2]
3. Another couple rely on the rhythm method. Explain why this is less reliable than the pill. [2]
4. A man in the same country has had a vasectomy. State what the operation does to his reproductive system and explain why he still produces the male sex hormone. [2]
5. Suggest one further change that would slow the rate of population growth in this country, and explain your answer. [1]

1. The birth rate is still higher than the death rate, so each year more people are added than removed. Medical care and clean water have cut the number of deaths, so far more children survive to become adults, which leaves more women of childbearing age — the total can keep rising even though each woman has fewer children. Net migration also adds people. (3 marks — three separate points; the first must refer to both rates.)
2. The pill supplies synthetic oestrogen and progesterone, which keep the levels of these hormones high enough that the ovary is not stimulated to release an egg; ovulation is prevented, so there is no egg to fertilise. The same hormones also thicken the mucus at the cervix and thin the uterine lining, so an embryo is less likely to implant. (2 marks — prevention of ovulation, plus one further effect.)
3. The rhythm method does not act on the body at all: it predicts the fertile days from past cycles and relies on the couple avoiding intercourse then, but cycles vary with stress, illness and diet, so the predicted day of ovulation can be wrong, and sperm can survive several days in the female reproductive system. The pill prevents ovulation whatever the length of the cycle. (2 marks — why the timing can be misjudged, and the contrast with a method acting directly on ovulation.)
4. The two tubes carrying sperm away from the testes are cut and the ends sealed, so sperm cannot leave the testes and cannot reach an ovum. The testes are not removed and are unaffected by the blockage, so they keep producing testosterone, together with sperm that are simply reabsorbed. (2 marks — the blocked tubes, and the point that the testes remain intact.)
5. For example, improving education and access to contraception lets couples plan a smaller family, which lowers the birth rate and so slows population growth. (1 mark — any one reasoned change that lowers the birth rate or raises the death rate.)