Hi Tara. This topic is written the way your future biology teachers will write it and the way Cambridge examines it: plainly, with the proper anatomical words and no fuss. The correct words are the marks here — there is no way to score well on Topic 16 while dancing round the vocabulary, so we will use it from the first line.
Here is the shape of it. 16.1 is reproduction with one parent and no gametes at all. 16.2 is the definition of sexual reproduction and the one sentence at the centre of the topic — fertilisation is the fusion of the nuclei of two gametes. 16.3 is the whole of plant reproduction, from the parts of a flower to the pollen tube to what a seed needs in order to germinate. 16.4 is human reproduction: the organs, the two gametes, and the placenta. 16.5 is puberty and the menstrual cycle, which is the hardest thing here because it is a chain of four hormones and you have to be able to rebuild the chain rather than recite it. 16.6 is sexually transmitted infections and HIV. 16.7 is the checklist for the night before.
Three warnings that will each save you marks. First: pollination and fertilisation are two different events, days apart, and swapping them is the single commonest error in this topic. Pollination is pollen grains moving from an anther to a stigma; fertilisation is two nuclei fusing. Second: the mother’s blood and the fetus’s blood never mix. The placenta is an exchange surface, not a join. Third: say oviduct, not the other word — and remember that fertilisation happens in the oviduct while implantation happens in the lining of the uterus. Two events, two organs, several days apart.
One Parent, No Gametes, Identical Offspring
Cambridge gives you a definition and expects it back almost word for word:
genetically identical offspring from one parent
Think about what that definition rules out. If there are no gametes, nothing has to travel and nothing has to meet. There is no mate to find, no pollen to move, no timing to get right. A single organism, on its own, in one place, can produce a whole population. That is the entire advantage of asexual reproduction, and every other advantage on the list is a version of it.
It also rules out something else, and this is where the marks are. There is no source of genetic difference. The offspring are copies. If a new disease arrives that the parent cannot resist, none of the offspring can resist it either — not because they were unlucky, but because they are the same organism repeated. That is the entire disadvantage, and again everything else on the list follows from it.
You will be shown organisms you have never met and asked whether they are reproducing asexually. You do not need to recognise the organism. Ask three questions of whatever you are given:
1. How many parents? One means asexual. 2. Are any nuclei fusing? If two nuclei fuse, it is sexual, whatever it looks like. 3. Are the offspring genetically identical to the parent? If the question says so, that settles it.
Any one of those three is enough to answer the question. You are being tested on the definition, not on the organism.
The Examples You Should Recognise
Cambridge does not ask you to memorise a list — it asks you to identify examples in diagrams, images and information provided. But it helps to have met the usual six, so that when one turns up you recognise the shape of it rather than starting from nothing.
When a question describes an unfamiliar organism, certain phrases are the examiner telling you the answer. “A single individual”, “without a mate”, “divides into two”, “an outgrowth that detaches”, “buds develop on the leaf margin and drop off”, “a piece of the parent grows into a whole new organism” — all asexual.
And one phrase that always means sexual, however plant-like or unfamiliar the organism: “two nuclei fuse”. If the stem says that, nothing else in the question can make it asexual.
Advantages and Disadvantages — and Why the Answer Changes
This is a discuss objective, which means Cambridge wants both sides and it wants them applied to a stated situation. The trick is that the same fact — the offspring are genetically identical — is an advantage in one context and a disaster in another. A grower who has bred a perfect apple wants copies. A wild population facing a new fungus wants anything but copies.
| Advantages | Disadvantages | |
|---|---|---|
| To a population of a species in the wild | Only one parent is needed, so no time or energy is spent finding a mate and no pollinator or other carrier is required. Reproduction is fast, so numbers build quickly and a favourable habitat can be colonised rapidly. If the parent is well adapted and conditions are not changing, every offspring inherits that successful set of features. | There is no genetic variation. If the environment changes — a new disease, a change in temperature, a new predator — then either all of them can cope or none of them can, so the whole population may be wiped out. The offspring also grow up right next to the parent, so they compete with it and with each other for light, water, mineral ions and space. |
| To crop production | Every plant is genetically identical, so the crop is uniform: the same size, the same quality, ripening at the same time, which makes harvesting and selling far easier. A grower who has a variety worth keeping can reproduce it exactly instead of gambling on what seeds would produce. It is quicker than growing from seed, and needs no pollinator and no second plant. | A whole field is genetically identical, so one new disease or pest, or one change in conditions, can destroy all of it — there is no plant in the field with any resistance the others lack. There is also no variation for the grower to select from, so the variety cannot be improved this way. |
Weak version: “there is no variation so the plants might all die.” That is worth about half a mark because it never says why.
Full version: “All the plants are genetically identical, so if a new disease appears none of them will have resistance to it, and the whole crop can be lost.” Two linked ideas — identical, therefore no resistant individuals — and a consequence. That is a two-mark answer written properly.
Bananas of the type sold in most supermarkets are grown from suckers, not seeds, so essentially every one of them is genetically identical. A fungal disease called Panama disease destroyed the previously dominant variety worldwide in the mid-twentieth century, and a new strain is now doing the same to its replacement. Nothing about that is on the syllabus — but if a question asks you to discuss a risk of asexual crop production, having one concrete case in mind makes the argument much easier to write.
The Sentence the Whole Topic Rests On
If you learn one thing from Topic 16, learn this. It is the definition of sexual reproduction, it is the definition of fertilisation, and it is the answer to a surprising number of questions about flowers and about humans that look as though they are asking something else.
to form a zygote, producing offspring that are genetically different from each other
Notice how little the definition says about bodies. It does not mention flowers, or oviducts, or anything you might expect. That is deliberate: exactly the same event happens in a rose, a cod, a fungus and a human being. Two gametes, two nuclei, one fusion, one zygote. Everything else in this topic — petals, pollen tubes, flagella, placentas — exists either to get the two nuclei together or to keep the zygote alive afterwards. Hold that idea and the rest of the topic organises itself.
“Fertilisation is when the sperm meets the egg” earns nothing. Meeting is not fusing, and the mark scheme wants nuclei.
“Fertilisation is when the sperm and the egg join together” earns nothing or at best one. It is the nuclei that fuse.
“Fertilisation is the fusion of the nucleus of a male gamete with the nucleus of a female gamete” earns the mark every time, in plants and in animals. Write it that way even when the question is about a flower.
Why the Offspring Come Out Different
You do not need the mechanism for this syllabus — that comes later in the course. What you do need is the logic, and the logic is simple. In asexual reproduction there is one source of genetic material, so the output is a copy. In sexual reproduction there are two sources, combined in a way that comes out differently every time, so the offspring differ from their parents and from each other. Brothers and sisters are the everyday proof: same two parents, and no two of them identical.
Say genetically different, not “different”. Two seedlings from the same fruit may look identical and still be genetically different; two cuttings of the same plant may look completely different, because one was kept in the dark, and still be genetically identical. Appearance is not the thing being asked about.
Haploid and Diploid
Two words, one rule, and a precision trap.
- A haploid nucleus contains one set of chromosomes.
- A diploid nucleus contains two sets of chromosomes.
- The nuclei of gametes are haploid. The nucleus of a zygote is diploid.
That is genuinely all the syllabus asks for here. You are not required at this point to explain how a haploid gamete is produced — that comes later in the course — only to state which is which.
The arithmetic is worth seeing once, because it explains why it has to work this way. If two diploid nuclei fused, the zygote would have four sets, its offspring eight, and the number would double every generation for ever. Because each gamete nucleus is haploid, one set plus one set gives a diploid zygote, and the number stays constant generation after generation. Halving before fusing is what keeps the number stable.
Haploid and diploid describe nuclei, not cells and not organisms. The safest answer uses Cambridge’s wording, “the nucleus of a gamete is haploid”, but “a haploid gamete” is also accepted. What loses the mark is saying the gamete is half a cell.
And a sperm is not “half a cell”. It is a complete cell, with a membrane, cytoplasm, mitochondria and a nucleus. It is the number of sets of chromosomes in its nucleus that is halved, and nothing else about it.
Advantages and Disadvantages of Sexual Reproduction
Once again this is a discuss objective, and once again the whole argument runs off one fact — this time that the offspring are genetically different. Notice that the table below is almost exactly the mirror image of the one in 16.1. If you can write one properly you can write the other by turning every line over.
| Advantages | Disadvantages | |
|---|---|---|
| To a population of a species in the wild | It produces genetic variation. If the environment changes — a new disease, a new predator, a shift in climate — it is likely that some individuals will have features that let them survive, so the population is not wiped out and can recover. Variation gives a population the capacity to respond to change, and that is the phrase Cambridge uses. | It is slower, and usually needs two parents, so time and energy are spent finding a mate. It can fail entirely if no mate, pollinator or carrier is available, or if the numbers in the population are already very low. A well-adapted combination of features in a parent is broken up rather than passed on intact. |
| To crop production | The variation gives the grower something to select from, so varieties can be improved — higher yield, better flavour, resistance to a disease. The crop as a whole is less likely to be destroyed by a single disease, because the plants are not all identical. Seeds are also easy to store and to transport. | The crop is not uniform: plants vary in size, quality and the time at which they ripen, which makes harvesting and selling harder. The grower cannot be certain what she will get from a batch of seed. It may depend on pollinators, and growing from seed usually takes longer. |
The Comparison Table, Side by Side
| Asexual reproduction | Sexual reproduction | |
|---|---|---|
| Number of parents | One | Usually two |
| Gametes | None | Two, one from each parent |
| Fusion of nuclei | Does not happen | Happens — this is fertilisation |
| Zygote formed? | No | Yes |
| Offspring | Genetically identical to the parent and to each other | Genetically different from the parents and from each other |
| Variation | None | Produced every time |
| Speed | Fast; numbers build quickly | Slower |
| Best when | Conditions are stable and the parent is already well suited to them | Conditions are changing, or may change |
Identical and different. Every advantage and disadvantage in 16.1 and 16.2, for wild populations and for crops, is one of those two words plus a consequence. If you get stuck in the exam, write the word first and then ask yourself “so what?” twice.
Identical → so no plant differs → so a new disease affects all of them. Different → so some may differ usefully → so the population survives a change. That is the whole argument.
What a Flower Is For
A flower is a delivery system. The plant cannot move, so it cannot carry its gametes anywhere, and it has to arrange for something else to do the carrying — an insect, or the wind. Everything about a flower follows from that one problem. The bright petals, the scent, the nectar, the sticky pollen, the dangling anthers, the feathery stigma: none of these are decoration, they are all solutions to the problem of moving a male gamete to a female one without moving.
You are asked to identify and draw the parts of an insect-pollinated flower and to state their functions. Ten names, and Cambridge will use every one of them.
| Part | Where it is | Function — the wording Cambridge wants |
|---|---|---|
| Sepals | The outermost ring, usually small and green | Protect the flower while it is still a bud |
| Petals | Inside the sepals | In an insect-pollinated flower they are large, brightly coloured and scented to attract insects |
| Stamen | The male part — there are usually several | Made of an anther and a filament. The word “stamen” means both of them together |
| Anther | The head of the stamen | Produces pollen grains, which contain the male gametes, and releases them |
| Filament | The stalk of the stamen | Supports the anther and holds it in a position where an insect will brush against it |
| Carpel | The female part, in the centre | Made of a stigma, a style and an ovary. Say carpel, not pistil |
| Stigma | The top of the carpel | A sticky surface that receives pollen grains |
| Style | The stalk joining stigma to ovary | Supports the stigma; the pollen tube grows down through it |
| Ovary | The swollen base of the carpel | Contains the ovules; after fertilisation it becomes the fruit |
| Ovules | Inside the ovary | Contain the female gametes; after fertilisation each becomes a seed |
Stamen = anther + filament. Carpel = stigma + style + ovary. If a question labels a bracket down the side of a diagram rather than a single arrow, it is asking for one of those two collective names, and a single part will not do.
And the pairing worth burning in: anther makes pollen; stigma receives pollen. Anther and stigma, A and S, start and finish. Getting those the wrong way round wrecks the definition of pollination, which is the most-used sentence in this section.
Pollination Is Not Fertilisation
This is the most valuable page in the whole topic, so read it slowly. Two events, two definitions, two places, and often days between them.
“Pollination is when the pollen fertilises the egg.” That sentence contains both words and scores zero, because it merges two separate events.
Test yourself with a question: a bee carries pollen from one flower to another and it lands on the stigma. Has fertilisation occurred? No. Pollination has occurred. Fertilisation may follow hours or days later, if a pollen tube grows all the way down to an ovule — and it may not happen at all.
Insect-Pollinated and Wind-Pollinated Flowers
Now that you know what a flower is for, the two designs stop being lists to memorise and become two answers to the same question. An insect is a good courier — it flies deliberately from flower to flower of the same species — but it has to be paid, so the plant invests in advertising and in nectar. The wind is a terrible courier — it goes wherever it goes and most of the pollen is wasted — but it is free, so the plant invests in quantity and in catching.
| Feature | Insect-pollinated | Wind-pollinated | Why |
|---|---|---|---|
| Petals | Large, brightly coloured | Small and green, or absent | Insects must be attracted; wind cannot see |
| Scent and nectar | Present | Absent | The insect has to be paid for the journey |
| Anthers | Inside the flower, firmly attached | Large, loosely attached and hanging outside the flower | Insects brush past them inside; the wind must shake pollen out of them |
| Stigmas | Small, sticky, inside the flower | Large, feathery and hanging outside the flower | A large branched net catches grains drifting past |
| Pollen grains | Larger, heavier, sticky or spiky | Smaller, lighter, smooth | Sticky and spiky grains cling to an insect; light smooth grains float on air |
| Quantity of pollen | Less | Very much more | Wind pollination wastes most of the pollen, so the plant compensates with numbers |
“Wind-pollinated flowers have feathery stigmas” is a description, and gets one mark at best. Attach the reason to every feature: “the stigmas are large and feathery and hang outside the flower, so they have a large surface area to catch pollen grains carried in the air.”
Feature, then position, then consequence. Three parts to every sentence, and adaptation questions in this section are almost always marked in exactly those three pieces.
Self-Pollination and Cross-Pollination
| Term | Definition — learn these exactly |
|---|---|
| Self-pollination | The transfer of pollen grains from the anther of a flower to the stigma of the same flower, or to the stigma of a different flower on the same plant. |
| Cross-pollination | The transfer of pollen grains from the anther of a flower to the stigma of a flower on a different plant of the same species. |
Read the second half of the self-pollination definition again, because it is where the marks hide. A second flower on the same plant still counts as self-pollination. Students lose this constantly by assuming that two flowers must mean cross-pollination.
This is the misconception this sub-topic is built around. Self-pollination is still sexual reproduction: pollen is still transferred, a pollen tube still grows, two nuclei still fuse, a zygote is still formed. So the offspring are still genetically different from each other — just less different than they would be from cross-pollination, because both sets of genetic material came from the same plant.
Less variation is not no variation. If you write “self-pollination produces identical offspring” you have contradicted the definition of sexual reproduction and the mark is gone.
Cambridge asks you to discuss the effects of the two on a population, in terms of three named things: variation, capacity to respond to changes in the environment, and reliance on pollinators. Use those three headings and the answer writes itself.
| Self-pollination | Cross-pollination | |
|---|---|---|
| Variation in the population | Less variation, because both gametes came from the same plant | More variation, because the two gametes came from two different plants |
| Capacity to respond to environmental change | Lower. With little variation, a new disease or a change in conditions is more likely to affect all the plants in the same way | Higher. More variation means it is more likely that some individuals have features allowing them to survive a change |
| Reliance on pollinators | Little or none, so reproduction is more reliable — a plant growing alone, or in a year with few insects, can still produce seed | Depends on a pollinator or on the wind, and on another plant of the same species being near enough, so it can fail |
Self-pollination is reliable but limited. Cross-pollination is risky but rich. Neither is “better”. Which one pays depends on whether the plant’s problem this year is finding a partner or surviving a change.
The Pollen Tube
Here is the journey between the two events. Cambridge asks you to describe the growth of the pollen tube and its entry into the ovule followed by fertilisation. It does not ask about what happens to the seed afterwards, so stop when the nuclei have fused.
- A pollen grain lands on the stigma. This is pollination.
- The pollen grain grows a pollen tube, which grows down through the style towards the ovary.
- The pollen nucleus travels down inside the pollen tube — it is carried, it does not swim.
- The tube enters the ovule through a small opening in its wall.
- The pollen nucleus fuses with a nucleus in the ovule. This is fertilisation, and a zygote is formed.
Germination: What a Seed Needs
Three conditions, and only three. Cambridge names them and the syllabus says “limited to”, which is examiner language for this is the whole list.
| Condition | Why it is needed |
|---|---|
| Water | The seed is very dry. Water is absorbed, the seed swells and softens, and the water allows enzymes to work and chemical reactions to take place. It is also the medium in which dissolved food is transported to the growing parts. |
| Oxygen | For aerobic respiration, which releases the energy the seed needs for growth. A germinating seed cannot photosynthesise yet — it is underground and has no leaves — so respiring its stored food is the only supply it has. |
| A suitable temperature | Germination depends on enzymes, and enzymes have an optimum temperature. Too cold and reactions are far too slow; too hot and the enzymes are denatured. |
Seeds germinate perfectly well in complete darkness, which makes sense once you notice that most seeds are buried. The seedling lives on the food stored inside the seed until it breaks the surface and grows leaves; only then does it need light.
So if a question offers you “light” as one of the conditions needed for germination, it is wrong — and if a question asks you to explain why the seeds in the dark tube still germinated, the answer is that light is not required for germination, only later for photosynthesis. Do not confuse the two. This is a favourite examiner trap and it catches a lot of people.
Investigating the Three Conditions
You need to be able to design this, criticise it, and read results from it. The principle is the one you have used all through this course: change one variable at a time and keep everything else the same. Five tubes, four of which differ from the first in exactly one way.
Why the water in tube 3 is boiled and then covered with oil. Boiling removes the dissolved oxygen; the oil layer stops oxygen from the air dissolving back into it. Both steps are needed and a question may ask for either.
Why tube 4 is a weaker piece of evidence than the others. Seeds in a fridge often germinate eventually, just far more slowly, so “a suitable temperature” is about rate as much as about all-or-nothing. Say “fewer seeds germinated, and more slowly” rather than “none germinated” unless the data actually say none.
How to measure the result. Count the number or percentage of seeds that germinate in a fixed time, using the same number of seeds in every tube. Ten seeds per tube is far better than one, because seeds vary and one dead seed would otherwise ruin a whole tube. Repeat the whole experiment.
The Organs and What Each One Does
This part of the syllabus is straightforward marks: eleven named organs, each with a function, and Cambridge asks for them directly. Learn the function alongside the name from the start — a label on its own is worth much less than a label with a job attached, and “identify and state the functions of” is exactly how the objective is written.
| Male part | Function |
|---|---|
| Testes (one testis, two testes) | Produce sperm and the hormone testosterone |
| Scrotum | The sac that holds the testes outside the body, where it is slightly cooler — sperm are produced best a little below body temperature |
| Sperm ducts | Carry sperm from the testes to the urethra |
| Prostate gland | Adds fluid to the sperm to make semen. The fluid contains nutrients and provides a medium in which the sperm can swim |
| Urethra | Carries semen out through the penis; at other times it carries urine. One tube, two jobs, never both at once |
| Penis | Transfers semen into the vagina |
| Female part | Function |
|---|---|
| Ovaries | Produce egg cells and the hormones oestrogen and progesterone |
| Oviducts | Carry the egg cell from the ovary towards the uterus. This is where fertilisation happens |
| Uterus | A thick, muscular organ where the embryo implants and the fetus develops; its muscular wall contracts during birth |
| Cervix | The ring of muscle at the neck of the uterus; it holds the fetus in place during pregnancy |
| Vagina | Receives semen; it is also the birth canal |
Cambridge uses oviduct. Other books say fallopian tube and other people say egg tube; in a Cambridge paper, use the Cambridge word, because a mark scheme is written from the syllabus.
And keep these two apart, because a great many marks turn on it: fertilisation happens in the oviduct. Implantation happens in the lining of the uterus. Two different events, two different organs, and several days between them while the embryo travels down.
The Two Gametes
The sperm and the egg cell are both gametes, both have haploid nuclei, and in almost every other respect they are opposites. That is not an accident. They have two completely different jobs: one has to travel, and the other has to supply.
| Sperm cell — adaptive features | What the feature is for |
|---|---|
| Flagellum | Beats from side to side so the cell can swim to the egg cell. Call it a flagellum, never a tail |
| Many mitochondria | Aerobic respiration in the mitochondria releases the energy needed for the flagellum to keep beating |
| Enzymes in the acrosome | The acrosome is the sac at the tip of the head. Its enzymes digest a path through the jelly coat of the egg cell so the sperm can reach the membrane |
| Egg cell — adaptive features | What the feature is for |
|---|---|
| Energy stores in the cytoplasm | Supply the embryo with energy and materials for the several days between fertilisation and implantation, during which it has no other source |
| A jelly coat that changes at fertilisation | The moment one sperm nucleus fuses, the coat changes so that no other sperm can enter. Without it the zygote would receive too many sets of chromosomes |
Cambridge also asks you to compare the two gametes, and it names the four headings it wants. Use them as headings and you cannot go wrong.
| Male gamete (sperm) | Female gamete (egg cell) | |
|---|---|---|
| Size | Very small — about 55 µm long, with a head only about 5 µm across | Very much larger — about 120 µm across, and roughly ten thousand times the volume |
| Structure | Streamlined head, acrosome, very little cytoplasm, many mitochondria, a flagellum | Rounded, a large volume of cytoplasm containing energy stores, surrounded by a jelly coat; no flagellum |
| Motility | Motile — it swims using its flagellum | Not motile — it cannot move itself and is moved along the oviduct |
| Numbers | Very many — hundreds of millions released at a time | Very few — usually one released about every 28 days |
The sperm has to get there: so it is small, streamlined, motile, has almost no cargo, and is produced in enormous numbers because most will not make it.
The egg cell has to keep the zygote alive: so it is large, full of stores, immobile, and produced one at a time because each one is expensive.
If you are asked to explain any difference between them, answer with the job, not the shape.
Fertilisation, and What Happens Next
Fertilisation in humans is the same event as everywhere else: the fusion of the nucleus of a sperm with the nucleus of an egg cell, producing a zygote with a diploid nucleus. It happens in the oviduct.
What comes next is a short sequence that Cambridge states almost word for word, so it is worth learning as a sequence rather than as loose facts. The zygote divides to form a ball of cells called an embryo, and the embryo implants into the lining of the uterus.
Zygote = the single cell produced by fertilisation. Embryo = the ball of cells it divides into, and the name used through the early weeks. Fetus = the name used once the organs have formed and it is recognisably developing as a human.
Cambridge uses fetus in the placenta objectives and embryo in the implantation objective, so match the word to the stage the question is describing.
The Structures That Keep the Fetus Alive
| Structure | Function |
|---|---|
| Placenta | The organ where exchange between the blood of the mother and the blood of the fetus takes place |
| Umbilical cord | Carries blood between the fetus and the placenta — it contains the blood vessels that connect the two |
| Amniotic sac | The membrane that encloses the fetus and contains the amniotic fluid |
| Amniotic fluid | Supports the fetus and protects it from bumps and knocks, and keeps its temperature steady. It does not feed the fetus |
What the Placenta Actually Does
Cambridge asks you to describe the function of the placenta and umbilical cord in the exchange of dissolved nutrients, gases and excretory products between the blood of the mother and the blood of the fetus. Three categories, and it is worth being able to fill each of them with examples.
| Direction | What crosses | By what process |
|---|---|---|
| Mother → fetus | Dissolved nutrients: glucose, amino acids, water, mineral ions, vitamins. Gas: oxygen. Also some of the mother’s antibodies | Mostly diffusion down concentration gradients; some substances, such as certain mineral ions and amino acids, by active transport against the gradient, using energy from respiration |
| Fetus → mother | Excretory products: urea. Gas: carbon dioxide | Diffusion, down concentration gradients — the fetus makes them, so its blood always has more of them than the mother’s does |
The umbilical cord is the transport link. It contains the blood vessels joining the fetus to the placenta. The umbilical vein carries blood rich in oxygen and dissolved nutrients (glucose, amino acids) from the placenta to the fetus. The umbilical arteries carry blood containing carbon dioxide and urea from the fetus to the placenta, where those wastes diffuse into the mother’s blood. No exchange with the mother happens in the cord: exchange happens only in the placenta.
You already know how an efficient exchange surface is built, because you met the argument with the alveolus and with the villus. The placenta is the same design solving the same problem: a large surface area, a thin barrier so the diffusion distance is short, and a good blood supply on both sides to keep the concentration gradients steep. When a question asks why the placenta is efficient, that is the answer, and it is worth three marks in one sentence each.
“The mother’s blood mixes with the baby’s in the placenta” is the single commonest wrong sentence in this section. The two circulations come very close, separated by a thin barrier, and everything crosses that barrier by diffusion or active transport — but they never join.
Two reasons to be able to give: the mother’s blood pressure is much higher and would damage the delicate fetal capillaries, and her immune system could attack the fetus’s cells, which are not genetically identical to her own.
Cambridge states directly that some pathogens and some toxins can pass across the placenta and affect the fetus. The placenta is a thin exchange surface, not a sieve that inspects what goes through.
Examples worth having: the rubella virus and HIV can cross the placenta, and so can toxic substances including nicotine and alcohol, and some medicines. That is why a doctor is careful about what is prescribed during pregnancy, and it is also why HIV can be transmitted from a mother to her baby — a link you will use again in 16.6.
Testosterone and Oestrogen at Puberty
You already know what a hormone is: a chemical substance, produced by a gland, carried by the blood, that alters the activity of one or more specific target organs. Two of them run puberty. Testosterone is produced by the testes; oestrogen is produced by the ovaries. Between them they cause and then regulate the secondary sexual characteristics — the changes to the body that develop during puberty and that are not the reproductive organs themselves.
| Testosterone — from the testes | Oestrogen — from the ovaries |
|---|---|
| Growth of facial and body hair | Growth of underarm and pubic hair |
| The voice deepens | The breasts develop |
| Growth of the penis and testes; sperm production begins | The hips widen |
| Increased muscle development | The menstrual cycle begins |
| Both also cause a growth spurt and the growth of underarm and pubic hair. | |
Secondary sexual characteristics are the ones that develop at puberty. The reproductive organs themselves are present from birth, so they are not on this list.
And the objective says the hormones are involved in the development and regulation of these characteristics — not just in triggering them. The hormones do not switch off after puberty; they keep being produced and keep maintaining what they built.
The Menstrual Cycle — the Core Version
Before any hormones, learn the events. The Core objective asks you to describe the cycle in terms of changes in two places: the ovaries, and the lining of the uterus. Two tracks, one timeline, about 28 days. Day 1 is the first day of bleeding, which is a convention worth remembering because graph questions always start there.
| Days | In the ovaries | In the lining of the uterus |
|---|---|---|
| 1 – 5 | A follicle begins to develop, with an egg cell inside it | Menstruation: the thickened lining breaks down and is lost through the vagina |
| 5 – 14 | The follicle continues to grow and mature | The lining is repaired and thickens, and grows a rich blood supply |
| 14 | Ovulation — an egg cell is released from the ovary into the oviduct | The lining is now thick |
| 14 – 28 | What remains of the follicle develops into a structure that produces progesterone, then shrinks away if there is no pregnancy | The lining is maintained thick, ready to receive an embryo |
| 28 / 1 again | The cycle begins again | If no embryo has implanted, the lining breaks down: menstruation |
The uterus lining is rebuilt every month for an embryo that usually does not arrive, and is then thrown away and rebuilt again. Everything in the cycle is either building that lining, releasing an egg cell to be fertilised, keeping the lining while it waits, or losing it and starting over.
If you can say which of those four things is happening on a given day, you can answer almost any Core question on this cycle.
The Four Hormones, and Where They Come From
Now the harder half. Four hormones control the cycle: FSH and LH from the pituitary gland, and oestrogen and progesterone from the ovary. Cambridge asks specifically for the sites of production of oestrogen and progesterone, so learn those first.
| Hormone | Produced by | Its role |
|---|---|---|
| FSH | The pituitary gland | Causes a follicle in the ovary to develop, and stimulates the ovary to produce oestrogen |
| Oestrogen | The ovary — by the developing follicle. In pregnancy, later also by the placenta | Repairs and thickens the lining of the uterus. It inhibits FSH, and when its level becomes high it stimulates a surge of LH |
| LH | The pituitary gland | A surge of LH causes ovulation at about day 14, and causes what is left of the follicle to develop into the structure that produces progesterone |
| Progesterone | The ovary — by the remains of the follicle after ovulation. In pregnancy, later by the placenta | Maintains the thick lining of the uterus. It inhibits FSH and LH, so no new follicle develops while it is high |
Sites of production: the two-by-two you must know
| During the menstrual cycle | During pregnancy | |
|---|---|---|
| Oestrogen | The ovary (the developing follicle) | The placenta |
| Progesterone | The ovary (what remains of the follicle after ovulation) | First the ovary (what remains of the follicle), then the placenta takes over |
During pregnancy the placenta makes both hormones. Their levels stay high, so the uterus lining is maintained and FSH and LH stay inhibited: no new follicle develops and no egg cell is released.
Reconstructing the Chain
Do not memorise this as eight separate facts. Memorise it as a chain in which each step causes the next, and then you can rebuild it in the exam even if you have forgotten a piece. Start on day 1, when everything is at its lowest.
- Progesterone and oestrogen are low, so the lining is not being maintained — it breaks down. This is menstruation.
- Because progesterone is low, FSH is no longer inhibited. FSH rises.
- FSH makes a follicle develop in the ovary and makes the ovary produce oestrogen.
- Rising oestrogen repairs and thickens the uterus lining, and inhibits FSH so no more follicles start.
- When oestrogen reaches a high level it triggers a surge of LH from the pituitary gland.
- The LH surge causes ovulation — the egg cell is released, at about day 14.
- The remains of the follicle now produce progesterone, which maintains the thick lining and inhibits FSH and LH, so no new follicle develops.
- If no embryo implants: the remains of the follicle break down, progesterone falls, the lining is no longer maintained, and it breaks down. That is step 1 again, and the cycle repeats.
- If an embryo does implant: the remains of the follicle keep producing progesterone, and from a few months into pregnancy the placenta takes over producing it. Progesterone therefore stays high throughout pregnancy, so the lining is maintained, menstruation does not occur, and FSH and LH stay inhibited so no new egg cell is released.
F is for follicle. FSH is follicle stimulating hormone — the name tells you what it does. L is for letting go. LH is the surge that releases the egg cell.
And a rule that resolves most of the difficult questions: progesterone maintains, oestrogen builds. Oestrogen does the repairing and thickening in the first half; progesterone does the holding in the second half. If a level falls, ask what that hormone was doing — and then the consequence is simply that it stops.
Anyone can learn that FSH develops a follicle. The answers that separate grades are the ones about hormones switching each other off: oestrogen inhibits FSH, and progesterone inhibits both FSH and LH.
This is why no second follicle starts developing during the second half of the cycle, and it is why nothing new is released during a pregnancy. If a question asks “explain why FSH stays low from day 16 to day 26”, the answer is progesterone is high and progesterone inhibits FSH — one sentence, two marks.
What the Syllabus Actually Asks For
This sub-topic is short and entirely factual, and it is examined as public health biology. There are five things to know: what an STI is, that HIV is a pathogen causing an STI, that HIV infection may lead to AIDS, how HIV is transmitted, and how the spread of STIs is controlled. Nothing else is on the syllabus, and there is no Supplement material here.
an infection that is transmitted through sexual contact
You met transmissible disease earlier in the course, so you already have the framework for this. A pathogen is an organism that causes disease; a transmissible disease is one in which the pathogen can be passed from one host to another. An STI is simply a transmissible disease whose main route of transmission is sexual contact — that is, the transfer of infected body fluids between people.
HIV and AIDS Are Not the Same Thing
This distinction is examined directly and is worth learning as two sentences.
| HIV | AIDS |
|---|---|
| The human immunodeficiency virus. It is a pathogen — a virus — and it causes a sexually transmitted infection. | Acquired immune deficiency syndrome. It is a condition that HIV infection may lead to, often many years later. |
| You can be infected with HIV and be perfectly well, sometimes for a decade or more. | AIDS is the stage at which the immune system has been damaged so far that the person can no longer fight off other infections. |
The word Cambridge uses is may. HIV infection may lead to AIDS — it is not automatic and it is not immediate, and with treatment many people never reach that stage at all. Writing “HIV is AIDS” or “HIV causes AIDS immediately” both lose the mark.
This is where earlier work pays off. HIV infects and destroys lymphocytes — the white blood cells that produce antibodies. As the number of lymphocytes falls, fewer antibodies can be produced, so the immune system can no longer defend the body against other pathogens.
The consequence is the part people miss: someone with AIDS usually becomes seriously ill from other infections — ones a healthy immune system would deal with easily. The virus does not attack the lungs or the gut itself; it removes the defence, and something else does the damage.
And one more sentence you already know from your work on drugs: antibiotics kill bacteria and have no effect on viruses, so no antibiotic will ever treat HIV. Antiviral drugs can reduce the amount of virus in the body and keep a person well for many years, but they do not remove the virus.
How HIV Is Transmitted
Every route below is a route by which infected body fluids get from one person into another. If you remember that single principle, you can work the list out rather than memorise it — and you can also work out what is not a route.
| Route of transmission | What is happening |
|---|---|
| Sexual contact with an infected person | Infected body fluids are exchanged directly. This is the reason HIV is classed as an STI |
| Sharing needles or syringes | Blood from an infected person remains in the needle and is injected directly into the next person |
| Transfusion of infected blood or blood products | Infected blood is put straight into the circulation. This is why donated blood is screened |
| From an infected mother to her child | Across the placenta during pregnancy, during birth, or in breast milk. This is the link back to 16.4: the placenta is a thin exchange surface, not a filter, and some pathogens cross it |
HIV is not transmitted by touching, hugging or shaking hands; by sharing cups, plates or cutlery; by coughs and sneezes; by toilet seats; by swimming pools; or by insect bites. The virus does not survive well outside the body and none of those routes carries infected body fluid into another person’s blood.
That is not just reassurance — it is examinable. Questions ask which of a list is a genuine route, and the wrong options are drawn from exactly these misconceptions.
Controlling the Spread of STIs
Cambridge asks you to explain how the spread of STIs is controlled, and “explain” means each measure needs the mechanism attached. The organising idea is the same one you used for cholera and for other transmissible diseases: you can act on the infected person, on the route of transmission, or on the people not yet infected.
| Measure | How it reduces spread |
|---|---|
| Using a barrier method, such as a condom | The condom stops body fluids (semen, vaginal fluid) passing between partners during sex. The pathogen travels in those fluids, so this blocks the main route of transmission |
| Reducing the number of sexual partners (or having one partner who is not infected, or not having sex at all) | Each new partner is another chance of contact with an infected person. Fewer partners means fewer chances for the pathogen to be passed on |
| Education and public awareness | People who know how an infection is and is not transmitted can avoid the routes that matter. It also reduces the stigma that stops people coming forward to be tested |
| Screening and testing | Many STIs cause few or no symptoms at first, so people transmit them without knowing. Testing identifies infected people so that they can be treated and can avoid passing it on |
| Treating infected people | STIs caused by bacteria can be cured with antibiotics, which removes that person as a source of infection. HIV cannot be cured, but antiviral drugs greatly reduce the amount of virus in the body and so reduce the chance of transmitting it |
| Contact tracing | The partners of an infected person are traced, informed and offered testing, so that people who do not know they are infected are found and treated |
| Not sharing needles, and providing clean ones | Removes the direct blood-to-blood route entirely |
| Screening donated blood | Infected donations are identified and not used, so transfusions cannot transmit the virus |
| Treating infected women during pregnancy and birth | Reduces the amount of virus in the mother’s blood, which greatly reduces the chance of transmission to the baby across the placenta or during birth |
Most of these measures work by blocking the route (condoms, clean needles, screened blood); the others find infected people and reduce how much pathogen they can pass on (testing, treatment, contact tracing).
Listing measures earns very little. Each line needs a because.
“Screening donated blood” — one mark at best. “Donated blood is screened, so infected donations are identified and not given to patients, which removes blood transfusion as a route of transmission” — the full answer.
“Contact tracing” — one mark at best. “The partners of an infected person are traced and tested, because many infected people have no symptoms and would otherwise continue to transmit the infection without knowing” — the full answer.
The Ten Sentences That Cost the Most Marks
Almost every mark lost in Topic 16 is lost to one of these. None of them is a knowledge problem — in each case the student knew the biology and wrote it in a form the mark scheme could not accept.
| What people write | Why it scores nothing | What to write instead |
|---|---|---|
| “Pollination is when the pollen fertilises the ovule.” | It merges two separate events that happen days apart. | “Pollination is the transfer of pollen grains from an anther to a stigma.” |
| “Fertilisation is when the sperm and egg join.” | Joining is not fusing, and the mark is for nuclei. | “Fertilisation is the fusion of the nuclei of two gametes.” |
| “The offspring are the same as the parent.” | “The same” could mean the same size or colour. | “The offspring are genetically identical to the parent.” |
| “The mother’s blood and the baby’s blood mix in the placenta.” | They never mix. The placenta is an exchange surface. | “Substances diffuse across a thin barrier; the two bloodstreams stay separate.” |
| “The embryo implants in the oviduct.” | Fertilisation is in the oviduct; implantation is not. | “The embryo implants into the lining of the uterus.” |
| “The sperm uses its tail to swim.” | Cambridge names the structure a flagellum. | “The sperm swims using its flagellum.” |
| “Seeds need water, warmth and light to germinate.” | Light is not required, and “warmth” is imprecise. | “Water, oxygen and a suitable temperature.” |
| “Self-pollination produces identical offspring.” | It is still sexual reproduction, so nuclei still fuse. | “Self-pollination produces less variation than cross-pollination.” |
| “Progesterone builds up the uterus lining.” | Oestrogen builds it; progesterone holds it. | “Oestrogen repairs and thickens the lining; progesterone maintains it.” |
| “HIV is a disease that antibiotics can treat.” | Two errors: HIV is a pathogen, and antibiotics do not affect viruses. | “HIV is a virus; infection with it may lead to AIDS. Antibiotics have no effect on viruses.” |
Nuclei. Genetically. Oviduct. Maintains.
If the question was about fertilisation, does your answer say nuclei? If it was about offspring, does it say genetically identical or different? If it was about where fertilisation happens, does it say oviduct? If it was about the second half of the cycle, does it say maintains? Four checks, about ten seconds, and they are worth several marks a paper.
Reading the Command Word
State or name — one short fact. “Name the part of the flower that receives pollen grains.” Answer: the stigma. One word. Writing a paragraph cannot gain more and may contradict you.
Describe — say what happens, in order, with figures if there are figures. “Describe the changes in the lining of the uterus during the cycle” wants the sequence and the days, not the hormones.
Explain — say why. In this topic almost every explain answer should contain one of: so that the nuclei can fuse, because it is genetically identical / different, down the concentration gradient, because the surface area is large and the barrier is thin, or because it inhibits. If none of those appears, you are probably describing.
Compare — both sides of every point, in the same sentence, using a comparative word. “Insect-pollinated pollen is larger and stickier than wind-pollinated pollen” scores; two separate lists often do not.
Discuss — advantages and disadvantages, applied to the situation in the stem. A discuss question that only gets one side rarely gets more than half the marks.
How to Attack a Topic 16 Data Question
Two kinds turn up again and again. Here is how to take each one apart.
1. Find day 14 first. Ovulation is the anchor of the whole chart, and every other event is described relative to it.
2. Identify the lines by shape, not by guessing. One sharp spike just before day 14 is LH. A hump that peaks just before day 14 and rises again slightly afterwards is oestrogen. A broad hill in the second half only is progesterone. A low line that rises early and is then held down is FSH.
3. Read the uterus lining track downwards from the hormone track. Thickening under rising oestrogen; maintained under high progesterone; lost when progesterone falls.
4. If a line does not do what it normally does, ask what is different. Progesterone that never falls means pregnancy. FSH that never rises means it is being inhibited.
5. Quote days and values when you describe. “It rises” is worth little; “it rises from day 5, peaks at day 13 and falls sharply by day 15” is the full answer.
1. Find the control — the tube or group in which nothing was removed. Every other result is only meaningful compared with it.
2. Check that exactly one variable was changed in each other tube. If two were changed, say so; that is usually a mark.
3. List what was kept the same if asked: same number of seeds, same species, same volume of water, same temperature, same time.
4. Give percentages, not counts, when the numbers of seeds differ, and remember that a percentage of a small sample is unreliable.
5. Never claim more than the data show. Seeds that did not germinate in a fridge in seven days may germinate in twenty-one. “Fewer germinated, and more slowly” is safer and usually more accurate than “none germinated”.
6. Improvements are always the same three: more seeds, repeat the whole experiment, and control the variable you have just been criticising.
The Night-Before Checklist
The definition of asexual reproduction, word for word, including “no gametes and no fusion of nuclei”. The three-question test for recognising it. Six examples and what makes each one recognisable. Advantages and disadvantages of asexual reproduction to a wild population and to crop production, with the “so what?” attached to each. The definition of sexual reproduction and of fertilisation, both with the word nuclei. Zygote and gamete, defined separately. Haploid and diploid, and that they describe nuclei. Why a sperm is not half a cell. Advantages and disadvantages of sexual reproduction, both contexts. The ten parts of an insect-pollinated flower with a function each, and which two are collective names. The definition of pollination and the definition of fertilisation in a plant, and the fact that they are different events. Six differences between insect- and wind-pollinated flowers, each with its reason. The difference between insect and wind pollen grains. Self- and cross-pollination defined exactly, including the same-plant clause, and why self-pollination does not produce clones. The three effects on a population: variation, capacity to respond to change, reliance on pollinators. The five steps of the pollen tube, with pollination as step one and fertilisation as step five. The three conditions for germination, and that light is not one of them. The five-tube experiment, why the water is boiled, why oil is added, and how you would measure the result. Six male organs and five female organs with a function each. Where fertilisation happens and where implantation happens. Three sperm adaptations and two egg adaptations, each with its purpose. The four comparison headings: size, structure, motility, number. Zygote, embryo, fetus in order. Placenta, umbilical cord, amniotic sac, amniotic fluid, with functions. What crosses the placenta in each direction and by which process. Why the two bloodstreams do not mix, with two reasons. That some pathogens and toxins cross. Testosterone and oestrogen, their glands, and the secondary sexual characteristics. The menstrual cycle in terms of the ovary and the uterus lining, with the four day-ranges. The four hormones, their sites of production, and their roles. The two inhibition relationships. What happens to progesterone and to FSH in pregnancy, and why. The definition of an STI. HIV against AIDS. What HIV destroys and what that leads to. Four routes of transmission and four things that are not routes. Nine ways the spread of STIs is controlled, each with a because.
That list is the whole topic. If you can say it out loud in about eight minutes, you are ready.