Reproduction

• Anchor: “One parent copies. Two gametes combine.” Every advantage, disadvantage and consequence in 16.1 and 16.2 — and most of the reasoning in 16.3 — unpacks from those six words.
• Anchor for the pair that costs the most marks: pollination is a journey; fertilisation is a fusion. One moves a grain across a flower; the other joins two nuclei inside an ovule. If you can keep those two apart under pressure, you have protected more marks in Topic 16 than any other single habit will win you.
• Anchor: the flower is built in four rings from the outside in — sepals, petals, stamens, carpel. Working from the outside inwards means you never miss one when labelling, and it is also the order to draw them in.
• Anchor: ovule → seed and ovary → fruit. Or, physically: the ovule is the small thing inside, and the seed is the small thing inside the fruit. What was inside stays inside. The numbers match too, which makes a quick check possible: a fruit with five seeds grew from an ovary that contained five fertilised ovules.
• Anchor: the gonad makes the hormone that shapes the body it belongs to — testes → testosterone, ovaries → oestrogen. Both words even start with the same letters as their gland.
• One last thing. If you remember nothing else from this chapter, remember these three sentences. “One parent copies; two gametes combine.” “Pollination moves pollen; fertilisation fuses nuclei.” And “every control measure breaks a route.” Between them they protect a remarkable proportion of the Core marks in Topic 16.

• Describe asexual reproduction as a process resulting in the production of genetically identical offspring from one parent. 16.1.1
• Identify examples of asexual reproduction in diagrams, images and information provided. 16.1.2
• Describe sexual reproduction as a process involving the fusion of the nuclei of two gametes to form a zygote, and the production of offspring that are genetically different from each other. 16.2.1
• Describe fertilisation as the fusion of the nuclei of gametes. 16.2.2
• Identify and draw the ten named parts of an insect-pollinated flower: sepals, petals, stamens, filaments, anthers, carpels, style, stigma, ovary and ovules. 16.3.1
• State the functions of all ten of those structures. 16.3.2
• Identify and describe the anthers and stigmas of a wind-pollinated flower. 16.3.3
• Distinguish between the pollen grains of insect-pollinated and wind-pollinated flowers. 16.3.4
• Describe pollination as the transfer of pollen grains from an anther to a stigma. 16.3.5
• State that fertilisation occurs when a pollen nucleus fuses with a nucleus in an ovule. 16.3.6
• Describe the structural adaptations of insect-pollinated and wind-pollinated flowers. 16.3.7
• Investigate and describe the environmental conditions that affect germination of seeds, limited to water, oxygen and a suitable temperature. 16.3.8
• Identify and state the functions of the testes, scrotum, sperm ducts, prostate gland, urethra and penis. 16.4.1
• Identify and state the functions of the ovaries, oviducts, uterus, cervix and vagina. 16.4.2
• Describe fertilisation as the fusion of the nuclei from a male gamete (sperm) and a female gamete (egg cell). 16.4.3
• Explain the adaptive features of sperm, limited to flagellum, mitochondria and enzymes in the acrosome. 16.4.4
• Explain the adaptive features of egg cells, limited to energy stores and the jelly coat that changes at fertilisation. 16.4.5
• Compare male and female gametes in terms of size, structure, motility and numbers. 16.4.6
• State that in early development the zygote forms an embryo, a ball of cells that implants into the lining of the uterus. 16.4.7
• Identify and state the functions of the umbilical cord, placenta, amniotic sac and amniotic fluid in the development of the fetus. 16.4.8
• Describe the roles of testosterone and oestrogen in the development and regulation of secondary sexual characteristics during puberty. 16.5.1
• Describe the menstrual cycle in terms of changes in the ovaries and in the lining of the uterus. 16.5.2
• Describe a sexually transmitted infection (STI) as an infection that is transmitted through sexual contact. 16.6.1
• State that human immunodeficiency virus (HIV) is a pathogen that causes an STI. 16.6.2
• State that HIV infection may lead to AIDS. 16.6.3
• Describe the methods of transmission of HIV. 16.6.4
• Explain how the spread of STIs is controlled. 16.6.5
• Discuss the advantages and disadvantages of asexual reproduction: (a) to a population of a species in the wild, and (b) to crop production. 16.1.3
• State that the nuclei of gametes are haploid and that the nucleus of a zygote is diploid. 16.2.3
• Discuss the advantages and disadvantages of sexual reproduction, in the same two contexts. 16.2.4
• Describe self-pollination as the transfer of pollen grains from the anther of a flower to the stigma of the same flower, or of a different flower on the same plant. 16.3.9
• Describe cross-pollination as 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. 16.3.10
• Discuss the potential effects of self-pollination and cross-pollination on a population, in terms of variation, capacity to respond to changes in the environment, and reliance on pollinators. 16.3.11
• Describe the growth of the pollen tube and its entry into the ovule, followed by fertilisation (details of endosperm production and development are not required). 16.3.12
• Describe the function of the placenta and umbilical cord in relation to the exchange of dissolved nutrients, gases and excretory products between the blood of the mother and the blood of the fetus. 16.4.9
• State that some pathogens and toxins can pass across the placenta and affect the fetus. 16.4.10
• Describe the sites of production of oestrogen and progesterone in the menstrual cycle and in pregnancy. 16.5.3
• Explain the role of hormones in controlling the menstrual cycle and pregnancy, limited to FSH, LH, progesterone and oestrogen. 16.5.4

Notice the shape of the list. Every measure does one of four things: stop the pathogen leaving an infected person, block the route between two people, protect the person who would be infected, or reduce how much pathogen is present in the population at all. If you can name the route, you can usually construct the control yourself rather than recalling it — which is exactly what an unfamiliar-context question is testing.

• “The pollen grain is a sperm cell.” Say instead The pollen grain contains the pollen nucleus — the male gamete nucleus. The grain is not itself the gamete, and it stays on the stigma while the nucleus travels down the tube.
• “The style turns into the pollen tube.” Say instead The style is part of the flower and stays where it is. The pollen tube is a new structure, grown by the pollen grain, that travels through the style.
• “Any pollen will do.” Say instead The pollen must be compatible — from the same species. Incompatible pollen may land on the stigma but will not produce a tube that reaches an ovule.
• “Asexual reproduction produces variation.” Core 16.1.1 Why wrong It contradicts the definition. One parent, no fusion of gamete nuclei, so nothing is combined and no new combination of genes can appear. Say instead The offspring are genetically identical to the parent and to each other. Any differences you can see between them are caused by the environment, not by their genes.
• “Sexual reproduction guarantees that the offspring survive environmental change.” Core 16.2.1 Why wrong Variation is not foresight. Nothing about being different makes an individual suited to a change that has not happened yet. Say instead It increases the chance that some individuals survive, because they are not all the same. The environment does the selecting, afterwards.
• “The offspring of sexual reproduction are better than the parents.” Core 16.2.1 Why wrong It smuggles a purpose into the process. Variation is neutral until an environment selects. Say instead They are genetically different — from each other and from both parents. Different is the word the syllabus uses, and it is the word that is true.
• “Pollination is the fusion of nuclei.” Core 16.3.5 Why wrong It gives pollination fertilisation’s definition, and then there is no word left for the transfer. Say instead Pollination is the transfer of pollen grains from an anther to a stigma. Fertilisation is the fusion of nuclei, later, inside an ovule.
• “Fertilisation happens on the stigma.” Core 16.3.6 Why wrong The stigma is where the journey starts, not where it ends. Say instead Fertilisation occurs when a pollen nucleus fuses with a nucleus in an ovule, inside the ovary.
• “A pollen grain is a sperm cell.” Supplement 16.3.12 Why wrong The grain contains the male gamete nucleus; it is not itself the gamete, and it does not travel to the ovule. Say instead The grain stays on the stigma. A pollen tube grows from it, and the nucleus travels down the tube.
• “The style becomes the pollen tube.” Supplement 16.3.12 Why wrong The style is part of the flower and stays where it is. Say instead The pollen tube is a new structure, grown by the pollen grain, that travels through the style.
• “Self-pollination is asexual reproduction.” Supplement 16.3.9 Why wrong Two gamete nuclei still fuse, so it meets the definition of sexual reproduction exactly. Say instead It is sexual reproduction with less variation, because both nuclei came from the same plant — not no variation, and not a different kind of reproduction.
• “A wind-pollinated plant has no flowers.” Core 16.3.3 Why wrong Grasses and many trees are wind-pollinated, and their flowers are small and green rather than absent. Say instead It has flowers with anthers, stigmas and ovules — but small, dull petals or none, because there is no animal to attract.
• “Light is required for germination.” Core 16.3.8 Why wrong 16.3.8 is limited to three conditions, and light is not one of them. Most seeds germinate in the dark, buried. Say instead Water, oxygen and a suitable temperature. Light is needed for photosynthesis, which is what the seedling does after germination.
• “A seed that has not germinated is dead.” Core 16.3.8 Why wrong A dry seed is alive but barely ticking over, and it can stay that way for years. Say instead “It did not germinate within seven days.” If you want to know whether it was viable, move it to full conditions and see.
• “The seeds in tube C died because there was no oxygen.” Core 16.3.8 Why wrong Removing a condition prevents germination; it does not establish that anything was killed. Say instead Without oxygen the seeds could not respire aerobically, so insufficient energy was released for growth and they were prevented from germinating.
• “Fertilisation normally occurs in the uterus.” Core 16.4.3 Why wrong It confuses fertilisation with implantation, which happens days later and in a different organ. Say instead Fertilisation occurs in an oviduct. Implantation occurs in the lining of the uterus.
• “The sperm and the egg join to make a zygote.” Core 16.4.3 Why wrong “Join” describes the geography and misses the definition. Two cells do not merge. Say instead The nuclei of the two gametes fuse. That single word is the mark.
• “The zygote implants in the uterus.” Core 16.4.7 Why wrong The zygote is one cell, and it is still in the oviduct. It divides on the way. Say instead The zygote forms an embryo — a ball of cells — and it is the embryo that implants, in the lining of the uterus.
• “Sperm mitochondria give the sperm energy.” Core 16.4.4 Why wrong Mitochondria do not contain or supply energy; they are where aerobic respiration happens. Say instead Many mitochondria release energy in aerobic respiration, which is used for the movement of the flagellum.
• “The sperm’s tail is its adaptation.” Core 16.4.4 Why wrong Half right, and half an answer. 16.4.4 says explain, so naming a structure is only the first half of each mark — and 0610 calls it a flagellum. Say instead “A flagellum, which beats so that the sperm can swim to the egg cell in the oviduct.” Feature plus advantage.
• “Amniotic fluid supplies the fetus with nutrients.” Core 16.4.8 Why wrong It gives the fluid the placenta’s job, and usually costs the placenta mark as well. Say instead The fluid cushions the fetus against mechanical shock and allows it to move. Nutrients cross the placenta and travel along the umbilical cord.
• “Maternal and fetal blood mix in the placenta.” Supplement 16.4.9 Why wrong The statement itself says exchange happens between the blood of the mother and the blood of the fetus — two bloods, not one. Say instead The two circulations stay separate. Substances diffuse across the thin barrier between them; blood does not cross.
• “The placenta blocks every harmful substance.” Supplement 16.4.10 Why wrong A barrier thin enough to let oxygen and glucose through quickly cannot be selective about everything else. Say instead Some pathogens and some toxins can cross the placenta and affect the fetus. The word some is the statement.
• “You cannot describe the menstrual cycle without naming hormones.” Core 16.5.2 Why wrong 16.5.2 says in terms of changes in the ovaries and in the lining of the uterus, and names no hormone at all. A Core candidate is not taught them. Say instead Lining lost → lining repaired and thickened while an egg develops → egg released about halfway through → lining maintained → lost again if no embryo implants. Four marks, no hormones.
• “FSH triggers ovulation.” Supplement 16.5.4 Why wrong FSH is follicle-stimulating — the name states the job. It develops the follicle and stimulates oestrogen secretion. Say instead A sharp rise in LH triggers ovulation.
• “Progesterone causes the follicle to develop.” Supplement 16.5.4 Why wrong Progesterone appears only after ovulation, from the tissue left behind in the ovary. It cannot have caused what happened before it existed. Say instead FSH develops the follicle; progesterone maintains the thickened lining afterwards.
• “The menstrual cycle is 28 days long.” Core 16.5.2 Why wrong 28 is a convenient average. Real cycles vary between people and from one cycle to the next in the same person. Say instead Read the days from the data you are given. If ovulation is shown on day 17, it is day 17.
• “An STI is a kind of disease caused by a virus.” Core 16.6.1 Why wrong It defines the class by the pathogen instead of by the route, and the definition is about the route. Say instead An STI is an infection that is transmitted through sexual contact. Some are bacterial and some, such as HIV, are viral.
• “HIV and AIDS are the same thing.” Core 16.6.3 Why wrong One is a pathogen, the other is a condition, and they are separated by a progression that may take years. Say instead HIV is the virus; AIDS is a condition that HIV infection may lead to.
• “Antiviral treatment cures HIV.” Core 16.6.5 Why wrong Treatment reduces how much virus is present; it does not remove it from the body. Say instead It is treatment, not a cure. It keeps the immune system working for far longer and lowers the chance of the infection being passed on.
• “Condoms, testing, screening, education.” Core 16.6.5 Why wrong Four correct measures and no explanation. 16.6.5 says explain, so a list scores almost nothing. Say instead Attach a route to every measure: “barrier protection prevents infected fluids reaching the other person, so the pathogen does not enter a new host”.

• One warning specific to this chapter. Three sections — E (pollination), G (fertilisation in the flower) and R (the menstrual cycle) — contain both Core and Supplement content, and in each case the Core half is much shorter than the Supplement half that follows it. A Core candidate who reads straight through will do a great deal of work that will never be examined; an Extended candidate who stops at the first natural pause will miss most of the section. Read the tier label at the top of each block.
• Where the tiering bites in Topic 16. Unlike some topics, the split here is not a clean break at a section boundary. Three sections carry both tiers: E — Pollination. Core is the definition of pollination, and it is four lines long. Self-pollination, cross-pollination and their effects on a population are Supplement, and they are most of the section. G — Fertilisation in the flower. Core is one statement: fertilisation occurs when a pollen nucleus fuses with a nucleus in an ovule. The pollen tube — germination of the grain, growth down the style, entry into the ovule — is Supplement. R — The menstrual cycle. Core describes the cycle through changes in the ovaries and in the uterus lining, with no hormone named. FSH, LH, oestrogen and progesterone are Supplement. In each case the Core part comes first and closes cleanly before the Supplement block opens. You will not find a Core sentence you need trapped inside a Supplement block.
• One habit worth building. Before writing any answer that mentions pollination or fertilisation, say the definition silently to yourself first. The confusions this chapter guards against are not gaps in knowledge — they are the right idea attached to the wrong word.
• Answering a “discuss” question. Give a point, its cost, and then tie the judgement to the context in the question. If the context is a wild population facing a changing environment, variation wins. If the context is a grower who wants a uniform crop of a known variety, uniformity wins. The same two facts support opposite conclusions in different contexts, and saying so is what a “discuss” mark scheme is looking for.
• Ten parts, and two of them are groups. 16.3.1 names sepals, petals, stamens, filaments, anthers, carpels, style, stigma, ovary and ovules. Notice that stamen and carpel are on that list in their own right, alongside the parts they are made of — so 16.3.2 wants a function for each of those too, not only for the individual structures. Both are in the table below. Notice also the verb: 16.3.1 says identify and draw. A labelled sketch is something you can be asked to produce, not only to read.
• Neither one is “better”. 16.3.11 says discuss, and a discussion that only praises cross-pollination has answered half the statement. Self-pollination guarantees a seed set on an isolated plant in a year when no insect comes; cross-pollination gambles on a pollinator arriving and, if it does, produces offspring that are not all alike. Which is the advantage depends entirely on the situation the question puts in front of you.
• Three separate Core statements live in this one section, and it is worth knowing which is which. 16.3.3 is narrow and specific: identify and describe the anthers and stigmas of a wind-pollinated flower. Those two structures only. 16.3.4 is about the pollen grains: distinguish the insect-pollinated kind from the wind-pollinated kind. 16.3.7 is the wide one: describe the structural adaptations of both flower types. The rows carrying 16.3.3 and 16.3.4 are marked in the table, because those two are the ones a question is most likely to ask on their own.
• Turn description into explanation. A question worth four marks rarely wants four descriptions. Pair each feature with its reason: “the stigma is large and feathery, which gives a large surface area to catch pollen grains carried in the air”. That single sentence usually scores both the description mark and the explanation mark.
• 16.3.8 is Core, it is limited, and its verb is unusual. The statement reads: investigate and describe the environmental conditions that affect germination of seeds, limited to the requirement for: water, oxygen and a suitable temperature. Limited to three. Water, oxygen, a suitable temperature. Nothing else is a required condition — and the one students most often add is dealt with below. Investigate is in the statement itself, so the practical in section I is subject content for both routes, not an optional extra.
• Three statements, and two of them are tightly limited. 16.4.4 — explain the adaptive features of sperm, limited to: flagellum, mitochondria and enzymes in the acrosome. Three features, and the verb is explain, so each one needs its advantage attached. 16.4.5 — explain the adaptive features of egg cells, limited to: energy stores and the jelly coat that changes at fertilisation. Two features. 16.4.6 — compare male and female gametes in terms of size, structure, motility and numbers. Four named points, and they are the four numbered rows of the table below. The two rows after them carry the adaptive features of 16.4.4 and 16.4.5 instead, and are tagged accordingly. Notice that 0610 says flagellum. “Tail” describes the same structure but is not the term the statement uses, and in a question that asks for the adaptive features it is worth writing the word the syllabus writes.
• “The sperm and egg join” is not the definition. The mark is for nuclei fusing. Two cells touching, meeting or joining describes the geography, not the event. Write: “the nucleus of the sperm fuses with the nucleus of the egg”.
• 16.4.7 is a single sentence, and its verb is state. In early development, the zygote forms an embryo, which is a ball of cells that implants into the lining of the uterus. Three things to get right: what is formed, what it is, and where it goes. The statement deliberately does not name the kind of cell division involved, and you are not asked for it — that is Chapter 17.
• Two circulations, one wall. If asked why the two bloods are kept separate, a safe syllabus-level answer is that the mother's and the fetus's blood may differ — for example in blood pressure, which is much higher in the mother and could damage the delicate fetal vessels. Keep the emphasis on the fact itself: they remain separate, and substances pass across.
• 16.5.1 has two verbs in it, and both earn marks. The statement asks for the roles of testosterone and oestrogen in the development and regulation of secondary sexual characteristics during puberty. The hormones do not only switch the changes on; they keep them going and maintain the characteristics afterwards. An answer that says only “causes them to develop” has answered half the statement.
• This is the section where the Core and Supplement boundary matters most in the whole chapter, so it is worth reading this box before anything else. Core 16.5.2 asks you to describe the menstrual cycle in terms of changes in the ovaries and in the lining of the uterus. That phrase is a limit, not a decoration: no hormone is named in the Core statement, and none is needed to answer it. A Core candidate who can say what happens to the egg and to the lining, and roughly when, has answered 16.5.2 completely. The hormones are 16.5.3 and 16.5.4, both Supplement. They are in the tier-block further down, and the Core account below does not depend on them.
• This is why the placenta appears in a hormone question. Section P treated the placenta as an exchange surface, which is 16.4.9. It is also an endocrine organ, and that is 16.5.3. If a question asks where progesterone comes from in a woman who is five months pregnant, “the ovary” is the wrong answer and “the placenta” is the right one.
• Read the statement carefully: it says STIs, not HIV. 16.6.5 asks you to explain how the spread of sexually transmitted infections in general is controlled. HIV is the example 0610 names elsewhere in the subtopic, and it is used below as the worked case — but an answer that only ever mentions HIV has answered a narrower question than the one asked. Note the verb too. 16.6.5 says explain, and the other four statements in 16.6 say describe or state. Explaining a control measure means saying which route it breaks and how. Listing measures is worth very little.
• Do not answer “day 14” from memory. Cycle length varies between people and between cycles, and a question that gives you a graph has given you the data precisely so that you read the day off it. If the spike is on day 17 of a 31-day cycle, ovulation is on day 17.

• Topic 16's statements are heavy on definitions, labelled diagrams and ordered sequences, and light on calculation — you can see that in the command words in the syllabus map. That shapes how to revise it: precision of wording matters more here than breadth of reading.
• The commonest shape in this topic. Label four parts of a flower or of the male or female reproductive system for four marks, then explain one structure’s function or adaptation for two or three more. 16.3.1 also says draw, so you may be asked to produce the diagram rather than annotate one.
• “Describe what happens from the moment a pollen grain lands on a stigma until fertilisation occurs” is a classic four-to-five mark item — and on a Core paper it becomes a two-mark version, because the tube is Supplement. Matched comparison of insect and wind pollination, and of sperm and egg, are Core on both routes.
• Germination percentages from an investigation, on either route. On the Extended papers, a menstrual-cycle hormone graph as well. You will be asked to use the values printed, identify a control, or explain an anomalous result — not to recall a number.
• Germination is the standard practical here, and 16.3.8 makes it subject content as well: identify the independent variable, name two variables to control, state the germination criterion, choose the right graph, and explain why replicates are needed.
• Three Supplement statements use this verb — 16.1.3, 16.2.4 and 16.3.11. It wants both sides, and in the first two it also wants the named context: a wild population, or crop production. Half of a discuss statement is not an answer to it.