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Cambridge IGCSE0610

Asexual reproduction

Biology 0610 Chapter Notes

What this chapter covers

Asexual reproductionSexual reproductionSexual reproduction in plantsSexual reproduction in humansSex hormones in humansSexually transmitted infections
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In this chapter

  1. 1What asexual reproduction is
  2. 2Spotting asexual reproduction
  3. 3A species in the wild
  4. 4Crop production
  5. 5What sexual reproduction is
  6. 6Fertilisation
  7. 7Haploid and diploid nuclei
  8. 8Sexual reproduction in the wild
  9. 9Sexual reproduction in crop production
  10. 10Parts of a flower and what they do
  11. 11Insect-pollinated flowers
  12. 12Wind-pollinated flowers
  13. 13Pollen grains: insect and wind
  14. 14Pollination and fertilisation
  15. 15Conditions for germination
  16. 16Self-pollination and cross-pollination
  17. 17The pollen tube
  18. 18The male reproductive system
  19. 19The female reproductive system
  20. 20Fertilisation
  21. 21Adaptations of sperm and egg cells
  22. 22Comparing male and female gametes
  23. 23Zygote, embryo and implantation
  24. 24Supporting the fetus
  25. 25The placenta: exchange and its limits
  26. 26Puberty: testosterone and oestrogen
  27. 27The menstrual cycle: ovary and uterus lining
  28. 28Where oestrogen and progesterone are made
  29. 29FSH and LH
  30. 30Oestrogen and progesterone in the cycle
  31. 31Hormones in pregnancy
  32. 32What an STI is
  33. 33HIV and AIDS
  34. 34How HIV is transmitted
  35. 35Controlling the spread of STIs

1. What asexual reproduction is

Asexual reproduction is a process resulting in the production of genetically identical offspring from one parent.

There is no fusion of gametes, so a second parent is not needed. Say "identical", not "similar".

Examiner insight

“The majority of candidates knew that the offspring produced in asexual reproduction are genetically identical but some thought that asexual reproduction involved the fusion of gamete nuclei.” “The candidates often described the daughter cells as being genetically similar, rather than genetically identical.”

Worked example 11 mark

A spider plant has a daughter spider plant growing from it. Which statement is correct?

A Fertilisation was involved in this type of reproduction. · B Only one parent is needed in this type of reproduction. · C The parent and daughter plant are genetically different. · D This is an example of sexual reproduction.

  1. 1

    B: only one parent is needed. [1]

2. Spotting asexual reproduction

To identify asexual reproduction, look for one parent and offspring that are genetically identical to it. A daughter plant growing from a spider plant is an example. So are plantlets, which are produced by mitosis.

Bacterial cells that divide into daughter cells with the same DNA are another. Each daughter cell has the same number of DNA loops. Many invertebrates also use asexual reproduction.

One parent alone is not enough. If gametes fuse, the reproduction is not asexual.

Examiner insight

“A common misconception was that asexual reproduction only happens in plants and bacteria and sexual reproduction only happens in animals.” “Most candidates scored at least one mark but many thought that insect-pollination was an example of asexual reproduction.”

Worked example 12 marks

A bacterial cell divides into two daughter cells. Each daughter cell has the same DNA as the parent cell. State the type of reproduction and give a reason.

  1. 1

    asexual reproduction [1]

  2. 2

    one parent, and the daughter cells are genetically identical to it [1]

3. A species in the wild

Asexual reproduction needs only one parent and is a faster process. Both are advantages to a population in the wild.

The disadvantage is the lack of genetic variation, because the offspring are genetically identical. If the parent is not resistant to a disease, none of the offspring are resistant, and the disease could kill them all.

Self-pollination happens within the same plant, but it is a form of sexual reproduction. Do not describe it as asexual.

Examiner insight

“It was evident that many confused self-pollination with asexual reproduction and gave the advantages of asexual reproduction.” “Some stated that if the parent did not have resistance to a particular disease, then none of the offspring would be resistant and they would all be killed if the disease spread.”

Worked example 12 marks

State one advantage and one disadvantage of asexual reproduction to a population of a species in the wild.

  1. 1

    advantage: only one parent is needed, or it is a faster process [1]

  2. 2

    disadvantage: offspring are genetically identical, so there is no genetic variation [1]

4. Crop production

In crop production, the advantages of asexual reproduction must be applied to the crop. Asexual reproduction needs one parent, is a faster process and gives genetically identical offspring.

So all the offspring inherit the desired characteristic of the parent plant. Variation in the crop does not increase.

Examiner insight

“Many candidates knew the advantages of asexual reproduction, but some did not consider the context of crop production in their answers.” “Most knew that the process only involved one parent, that it was a faster process and resulted in genetically identical offspring, but many did not apply their knowledge to this specific context.”

Worked example 11 mark

Which statement describes an advantage of using asexual reproduction in crop production?

A All offspring inherit the desired characteristic. · B Germination occurs quickly. · C There is increased variation within the crop species. · D The production of adult plants is slower.

  1. 1

    A: all offspring inherit the desired characteristic. [1]

Recap

  • Asexual reproduction is a process resulting in the production of genetically identical offspring from one parent.
  • It has no fusion of gametes. Insect-pollination and self-pollination are not asexual reproduction.
  • Examples: a daughter spider plant, plantlets, and bacterial cells dividing into identical daughter cells.
  • Extended: for a species in the wild, only one parent is needed and it is faster, but there is no genetic variation, so one disease could kill all the offspring.
  • Extended: in crop production, all offspring inherit the desired characteristic.

5. What sexual reproduction is

Sexual reproduction is a process involving the fusion of the nuclei of two gametes to form a zygote. It produces offspring that are genetically different from each other.

The same stages occur in flowering plants and in humans: gametes are made by meiosis, the male gamete moves to the female gamete, and the gametes fuse to form a zygote.

Examiner insight

“Although the vast majority of candidates recalled the definition of sexual reproduction, some incorrectly believed that sexual reproduction results in genetically identical offspring.” “Some candidates incorrectly believed that in sexual reproduction, offspring are genetically identical to the parent.”

Worked example 11 mark

Which stages in sexual reproduction occur in both flowering plants and humans?

1 Gametes are made by meiosis. · 2 Gametes fuse to form a zygote. · 3 The male gamete moves to the female gamete.

A 1, 2 and 3 · B 1 and 2 only · C 1 only · D 2 and 3 only

  1. 1

    A: all three stages occur in both. [1]

6. Fertilisation

Fertilisation is the fusion of the nuclei of gametes.

It is the nuclei that fuse. A sperm entering an egg cell is not enough, and alleles do not fuse. Fertilisation forms a zygote, not an embryo.

Examiner insight

“Some candidates incorrectly believed that fertilisation is the fusion of alleles to form a zygote.” “A few candidates were unsure about the difference between a zygote and an embryo.”

Worked example 11 mark

What is fertilisation?

A a sperm entering an egg cell · B a sperm nucleus fusing with an egg cell nucleus · C the division of a zygote · D the movement of a sperm towards an egg cell

  1. 1

    B: a sperm nucleus fusing with an egg cell nucleus. [1]

7. Haploid and diploid nuclei

The nuclei of gametes are haploid. The nucleus of a zygote is diploid.

A haploid nucleus has one set of unpaired chromosomes. A diploid nucleus has two copies of each type of chromosome, a full set of paired chromosomes.

Two haploid nuclei fuse at fertilisation, so the diploid number is restored.

Examiner insight

“Some candidates incorrectly believed that the nucleus of a human gamete is diploid and contains 46 chromosomes.” “Many knew that the haploid nucleus contains chromosomes but did not qualify their statement further to distinguish it from a diploid nucleus.”

Worked example 13 marks

Compare the nucleus of a gamete with the nucleus of the zygote that it helps to form.

  1. 1

    the nucleus of the gamete is haploid [1]

  2. 2

    the nucleus of the zygote is diploid [1]

  3. 3

    haploid: one set of unpaired chromosomes; diploid: two copies of each type of chromosome [1]

8. Sexual reproduction in the wild

Offspring of sexual reproduction show genetic variation. In a wild population this gives increased survival in new or changing environments, and a disease is less likely to affect the whole population.

The disadvantages are the need for two parents and the additional time and energy it takes.

Examiner insight

“Occasionally candidates were vague in their responses, referring to an increase in survival in the environment rather than increased survival in new or changing environments.” “Sometimes candidates required more detail when explaining that increased genetic variation decreased susceptibility to disease. It was evident there was some confusion between transmissible diseases and genetic diseases.”

Worked example 11 mark

Which statement about sexual reproduction in the wild is correct?

A A disease is likely to affect the whole population. · B There is genetic variation in the offspring. · C Only one parent is required. · D The species is less able to adapt to changes in the environment.

  1. 1

    B: there is genetic variation in the offspring. [1]

9. Sexual reproduction in crop production

In plant breeding, the main disadvantage is that sexual reproduction takes a long time. Variation can also have a negative effect on the desired outcome of selective breeding.

The advantage is the same genetic variation as in the wild: the plants are genetically different from each other, so a disease is less likely to affect the whole crop. Always tie the answer to the crop.

Examiner insight

“Some candidates incorrectly focused on the idea that variation in the population would be reduced.” “For candidates who had revised the disadvantages these were very accessible marks; however, some did not relate their answers to plant breeding.”

Worked example 14 marks

A farmer breeds a new variety of wheat using sexual reproduction. Discuss one advantage and one disadvantage of using sexual reproduction to produce a crop.

  1. 1

    advantage: the offspring are genetically different from each other [1]

  2. 2

    so a disease is less likely to affect the whole wheat crop [1]

  3. 3

    disadvantage: it takes a long time to breed the new variety [1]

  4. 4

    variation can have a negative effect on the desired outcome of selective breeding [1]

Recap

  • Sexual reproduction: fusion of the nuclei of two gametes to form a zygote, giving offspring that are genetically different from each other.
  • Fertilisation: the fusion of the nuclei of gametes.
  • Extended: gamete nuclei are haploid. The zygote nucleus is diploid.
  • Extended: haploid means one set of unpaired chromosomes. Diploid means two copies of each type of chromosome.
  • Extended: in the wild, genetic variation gives survival in new or changing environments. The costs are two parents, time and energy.
  • Extended: in crops, variation means a disease is less likely to affect the whole crop, but sexual reproduction takes a long time and variation can harm the desired outcome.

10. Parts of a flower and what they do

An insect-pollinated flower has sepals and petals on the outside. Each stamen is made of an anther and a filament. The carpel has a stigma, a style and an ovary. The ovules are inside the ovary.

The anther produces pollen grains. The stigma receives pollen during pollination. Petals and nectar attract insects for pollination. Fertilisation occurs in an ovule.

Of the anther, the filament and the stigma, only the anther produces gametes.

Examiner insight

“Most candidates identified the ovule or ovary. Fewer candidates knew the filament, and very few knew the sepal (which was usually called a leaf).” “Most candidates were aware of which structure attracts insects but the structures that produce and receive pollen were often confused.”

Worked example 12 marks

State the names of the two parts of a stamen.

  1. 1

    filament [1]

  2. 2

    anther [1]

11. Insect-pollinated flowers

An insect-pollinated flower has structures that attract insects. It has large petals, often with a spotted pattern, and a nectary.

The anthers and the stigma are inside the flower, among the petals. Its pollen is sticky.

If a question asks for features you can see, give visible ones. Scent and nectaries cannot be seen in a photograph.

Examiner insight

“The best responses generally referred to large petals and the presence of a spotted pattern on petals to attract insects.” “Some described how insects are adapted for pollination rather than how flowers are adapted.”

Worked example 12 marks

Describe two structural adaptations of an insect-pollinated flower.

  1. 1

    has (large) petals, to attract insects [1]

  2. 2

    anthers and stigma are inside the flower / contains a nectary / sticky pollen [1]

12. Wind-pollinated flowers

A wind-pollinated flower has small petals. Its anthers hang outside the flower on long filaments, so the pollen can be blown by the wind.

Its stigmas are feathery. It is the stigmas that are feathery, not the stamens and not the pollen.

Examiner insight

“Many candidates understood that a structural adaptation of wind-pollinated flowers is the presence of long filaments so that the anthers hang outside of the flower. Some candidates incorrectly believed that they have feathery stamens.”

Worked example 12 marks

Describe the anthers and the stigmas of a wind-pollinated flower.

  1. 1

    anthers hang outside the flower, on long filaments [1]

  2. 2

    stigmas are feathery [1]

13. Pollen grains: insect and wind

Insect-pollinated pollen is sticky. The flower makes less pollen, and the grains are larger and heavier.

Wind-pollinated pollen is lighter and smaller, and the flower makes more of it. Compare the pollen, not the flowers.

Examiner insight

“Many candidates described differences between wind-pollinated and insect-pollinated flowers rather than differences between the pollen.”

Worked example 12 marks

Distinguish between the pollen grains of an insect-pollinated flower and a wind-pollinated flower.

  1. 1

    insect-pollinated pollen is sticky; wind-pollinated pollen is not sticky [1]

  2. 2

    insect-pollinated pollen grains are larger / heavier than wind-pollinated pollen grains [1]

14. Pollination and fertilisation

Pollination is the transfer of pollen grains from an anther to a stigma.

Fertilisation occurs when a pollen nucleus fuses with a nucleus in an ovule. It happens after pollination, in the ovule.

Examiner insight

“Some candidates understood that fertilisation is when a pollen nucleus fuses with the nucleus in an ovule. Some candidates incorrectly believed that this process is pollination.” “Some candidates incorrectly believed that pollination occurs when pollen grains are transferred from the anther to the ovary.”

Worked example 11 mark

In pollination, where are pollen grains transferred to after they leave the anther?

A ovules

B stamen

C stigma

D style

  1. 1

    pollination moves pollen from an anther to a stigma

  2. 2

    answer C: stigma [1]

15. Conditions for germination

Seeds need water, oxygen and a suitable temperature to germinate. Light is not needed.

To investigate, keep seeds in different conditions and change one condition at a time. Count the seeds that germinate.

Examiner insight

“When describing temperature, it’s important that it is qualified as a ‘suitable temperature’ as seeds have different temperature requirements for germination.” “The question specifies conditions for the germination of seeds, not the growth of seedlings.”

Worked example 14 marks

A student investigated the conditions required for seed germination. Sets of seeds were kept in different conditions in separate tubes. Seeds germinated in only two of the tubes. The table shows the conditions for each tube.

(i) Predict in which tubes the seeds would germinate. Explain your answer.

(ii) State one condition not given in the table that is required for germination.

conditionstube Atube Btube Ctube Dtube E
oxygenpresentpresentabsentpresentpresent
waterabsentabsentpresentpresentpresent
lightabsentpresentpresentpresentabsent
  1. 1

    (i) seeds need both oxygen and water: only tubes D and E have both

  2. 2

    tube D [1]

  3. 3

    tube E [1]

  4. 4

    germination requires oxygen and water / does not require light [1]

  5. 5

    (ii) a suitable temperature [1]

16. Self-pollination and cross-pollination

Self-pollination is the transfer of pollen grains from the anther of a flower to the stigma of the same flower or a different flower on the same plant.

Cross-pollination is 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.

Self-pollination gives little wastage of pollen and independence from pollinators, but little variation. Cross-pollination relies on pollinators and gives more variation. A population with little variation has less capacity to respond to changes in the environment.

Examiner insight

“Some candidates did not realise that self-pollination can occur between different flowers on the same plant as well as within a single flower.” “Few candidates gave positive aspects of self-pollination, such as the little wastage of pollen and the independence from pollinators.”

Worked example 11 mark

What describes cross-pollination?

A the transfer of pollen from the anther of a flower to the stigma of a different flower on the same plant

B the transfer of pollen from the anther of a flower to the stigma of a flower on a different plant of the same species

C the transfer of pollen from the anther of a flower to the stigma of the same flower

D the transfer of pollen from the stigma of a flower to the anther of a flower on a different plant of the same species

  1. 1

    A is self-pollination: the other flower is on the same plant

  2. 2

    C is self-pollination: the same flower

  3. 3

    D is the wrong way round: pollen goes from an anther to a stigma

  4. 4

    answer B: a flower on a different plant of the same species [1]

17. The pollen tube

After pollination, a pollen tube grows and enters the ovule. The pollen nucleus moves down the tube. It fuses with the nucleus in the ovule. This is fertilisation.

The pollen tube grows after pollination, not before. The pollen grain does not travel down the tube.

Examiner insight

“A common misconception was that it is the pollen grain that travels down the pollen tube.”

Worked example 13 marks

A pollen grain lands on a stigma. Describe how fertilisation then happens.

  1. 1

    a pollen tube grows [1]

  2. 2

    the pollen tube enters the ovule, and the pollen nucleus moves down the tube [1]

  3. 3

    the pollen nucleus fuses with the nucleus in the ovule [1]

Recap

  • Stamen = anther + filament. Carpel: stigma, style and ovary. Ovules are in the ovary.
  • The anther produces pollen grains. The stigma receives them.
  • Insect-pollinated: large petals, nectary, anthers and stigma inside the flower, sticky pollen.
  • Wind-pollinated: small petals, anthers outside on long filaments, feathery stigmas.
  • Pollination = pollen grains from an anther to a stigma.
  • Fertilisation = a pollen nucleus fuses with a nucleus in an ovule.
  • Germination needs water, oxygen and a suitable temperature. Light is not needed.
  • Extended: self-pollination is the same flower or the same plant. Cross-pollination is a different plant of the same species.
  • Extended: after pollination a pollen tube grows and enters the ovule. The pollen nucleus fuses with the nucleus in the ovule.

18. The male reproductive system

The testes produce sperm (male gametes). The scrotum holds the testes outside the body.

The sperm duct transports sperm from the testis to the urethra. The prostate gland secretes seminal fluid for the sperm to use.

The urethra is the tube for urine and semen to pass through. The penis delivers sperm into the vagina.

Examiner insight

“The prostate was often linked to the formation of sperm rather than seminal fluid and some thought it was involved in urination.” “The commonest incorrect response was to identify a ureter rather than the urethra.”

Worked example 13 marks

State the function of each of these parts of the male reproductive system.

prostate gland · sperm duct · penis

  1. 1

    prostate gland: secretes seminal fluid for sperm cells to use [1]

  2. 2

    sperm duct: transports sperm from the testis to the urethra [1]

  3. 3

    penis: delivers sperm / semen into the vagina [1]

19. The female reproductive system

The ovaries produce egg cells and secrete oestrogen. Fertilisation normally occurs in the oviduct.

The embryo implants in the lining of the uterus. Sperm are deposited in the vagina and pass through the cervix and the uterus on their way to the oviduct.

Examiner insight

“The most common error was confusing the oviduct with the uterus.” “Generally well-answered but some candidates stated oviduct instead of ovaries.”

Worked example 11 mark

What is the order of structures that a sperm usually travels through to fertilise an egg cell?

A cervix, then oviduct, then ovary · B cervix, then uterus, then oviduct · C uterus, then cervix, then ovary · D uterus, then cervix, then oviduct

  1. 1

    B: cervix, then uterus, then oviduct [1]

  2. 2

    Sperm are deposited in the vagina first and fertilisation normally occurs in the oviduct.

20. Fertilisation

Fertilisation is the fusion of the nuclei from a male gamete (sperm) and a female gamete (egg cell).

The fusing nuclei matter, not just the cells meeting. Fertilisation normally occurs in the oviduct.

Examiner insight

“Many candidates understood that fertilisation is the fusion of nuclei to form a zygote. Some candidates incorrectly believed that fertilisation is the fusion of alleles to form a zygote.” “Whilst many candidates understood that fertilisation usually occurs in the oviducts, some candidates incorrectly opted for the uterus lining.”

Worked example 12 marks

Describe what is meant by the term fertilisation.

  1. 1

    the fusion of the nuclei [1]

  2. 2

    of (male and female) gametes [1]

21. Adaptations of sperm and egg cells

A sperm swims to the egg cell using its flagellum. Its mitochondria release energy, and there should be a large number of them. Enzymes in the acrosome digest the jelly coat of the egg cell.

An egg cell has an energy store. It provides nutrients for the zygote and the embryo until implantation, so the cells can divide.

The jelly coat changes at fertilisation. It then stops more than one sperm from entering.

Examiner insight

“Common misconceptions included that the mitochondria produce energy rather than release energy and that the acrosome is a digestive enzyme.” “Few stated that to do this the jelly coat goes through changes, often described as a hardening, which makes the coat impenetrable to more sperm.”

Worked example 12 marks

Explain why the egg cell contains a large energy store.

  1. 1

    to provide nutrients for the zygote / embryo [1]

  2. 2

    until implantation, so cells can divide [1]

22. Comparing male and female gametes

Compare gametes on four points: size, structure, motility and numbers. Motility is the ability to move independently.

A sperm cell is smaller than an egg cell. It is more motile and made in greater numbers. An egg cell is larger, less motile and fewer in number.

Structure differs too. A sperm has a flagellum, mitochondria and an acrosome. An egg cell has an energy store and a jelly coat.

Examiner insight

“The general pattern was for candidates to state an adaptive feature of a sperm (usually that it had a flagellum/tail or that it had a streamlined shape) but be unable to state an adaptive feature of an egg cell.” “Few candidates understood that motility refers to the ability to move independently.”

Worked example 11 mark

How do sperm cells compare with egg cells?

A larger, less motile, fewer in number than egg cells · B larger, more motile, greater in number than egg cells · C smaller, less motile, fewer in number than egg cells · D smaller, more motile, greater in number than egg cells

  1. 1

    D: smaller, more motile, greater in number than egg cells [1]

23. Zygote, embryo and implantation

Fertilisation forms a zygote. In early development the zygote forms an embryo, which is a ball of cells.

The embryo implants into the lining of the uterus. The order of the terms is zygote, embryo, then fetus.

Examiner insight

“Some candidates were unsure about the terms zygote, embryo and fetus, stating that an embryo divides many times to form a zygote or assuming the terms embryo and fetus are synonymous.” “Occasionally candidates were not specific enough and referred to the uterus as the site of implantation rather than the lining or wall of the uterus.”

Worked example 11 mark

Which statement is correct for reproduction in humans?

A A zygote embeds in the lining of the uterus. · B The female gamete has a large store of energy. · C Fertilisation is the fusion of two gametes. · D The male deposits sperm in the uterus of the female.

  1. 1

    B: The female gamete has a large store of energy. [1]

  2. 2

    A is wrong: the embryo implants, not the zygote. C is wrong: the nuclei fuse. D is wrong: sperm are deposited in the vagina.

24. Supporting the fetus

The umbilical cord connects the fetus to the placenta. The placenta is where substances are exchanged between the mother and the fetus.

The amniotic sac (amnion) surrounds the fetus and holds the amniotic fluid. The fluid is a shock absorber that protects the fetus from injury. It does not supply nutrients or oxygen.

Examiner insight

“Widespread confusion exists between the functions of the amnion and of the placenta.” “The most common error was incorrectly identifying the umbilical cord.”

Worked example 12 marks

A fetus develops inside a fluid-filled sac in the uterus. Name the sac and state the function of the fluid.

  1. 1

    amniotic sac / amnion [1]

  2. 2

    the fluid is a shock absorber and protects the fetus from injury [1]

25. The placenta: exchange and its limits

The placenta passes dissolved nutrients such as glucose and amino acids, and oxygen, from the mother’s blood to the fetus’s blood. It removes carbon dioxide and urea from the fetus to the mother. The exchange takes place in the placenta, not in the umbilical cord.

The placenta keeps maternal and fetal blood separate and forms a barrier to toxins and pathogens. Even so, some pathogens and toxins can pass across it and affect the fetus.

Examiner insight

“One of the main functions of the placenta is to keep the blood of the mother and fetus separate.” “There was a misconception that a pregnant woman’s blood gains dissolved oxygen and loses urea as it passes through the placenta.”

Worked example 11 mark

How will the composition of a pregnant woman’s blood change as it passes through the placenta?

concentration of dissolved oxygenconcentration of urea
Adecreasesdecreases
Bdecreasesincreases
Cincreasesdecreases
Dincreasesincreases
  1. 1

    B: dissolved oxygen decreases and urea increases [1]

  2. 2

    Oxygen leaves her blood for the fetus. Urea enters her blood from the fetus.

Recap

  • Testes make sperm. Prostate gland secretes seminal fluid. Urethra carries urine and semen. Penis delivers sperm into the vagina.
  • Ovaries make egg cells. Fertilisation is in the oviduct. The embryo implants in the lining of the uterus.
  • Fertilisation = fusion of the nuclei of a male gamete and a female gamete.
  • Sperm: flagellum, many mitochondria, enzymes in the acrosome that digest the jelly coat. Egg: energy store, jelly coat that changes at fertilisation.
  • Sperm are smaller, more motile and greater in number than egg cells.
  • Order: gamete, zygote, embryo (a ball of cells), fetus.
  • Extended: the placenta exchanges dissolved nutrients, oxygen, carbon dioxide and urea, and keeps the two blood supplies separate. Some pathogens and toxins still cross.

26. Puberty: testosterone and oestrogen

Secondary sexual characteristics appear at puberty. Oestrogen causes the development of the female ones, such as widening of the hips.

Testosterone causes the development of the male ones: pubic, body and facial hair develops, the voice deepens, the shoulders widen, muscles develop, and the penis and testes enlarge.

Examiner insight

“The most common incorrect answer was progesterone, but it could not be credited because it is not responsible for the development of secondary sexual characteristics.” “A large number of candidates stated changes which occurred in one sex only, and others referred to psychological changes.”

Worked example 11 mark

Which row shows the roles of oestrogen and testosterone in the development of secondary sexual characteristics in humans?

production of gameteswidening of hipswidening of shoulders
Atestosteronetestosteroneoestrogen and testosterone
Btestosteroneoestrogenoestrogen and testosterone
Coestrogen and testosteroneoestrogentestosterone
Doestrogen and testosteroneoestrogen and testosteronetestosterone
  1. 1

    answer C: oestrogen widens the hips and testosterone widens the shoulders [1]

27. The menstrual cycle: ovary and uterus lining

In a 28-day menstrual cycle, an egg cell is released from the ovary at ovulation, around day 14.

The uterus lining changes through the cycle. It breaks down and is lost at menstruation. Then it builds up again, and its thickness is maintained.

Examiner insight

“Many answers incorrectly stated that menstruation occurred during days 8 to 14.”

Worked example 13 marks

Describe the changes to the lining of the uterus during the menstrual cycle.

  1. 1

    lining breaks down / is lost OR menstruation occurs [1]

  2. 2

    then lining builds up again / thickens / repairs [1]

  3. 3

    lining thickness is maintained [1]

28. Where oestrogen and progesterone are made

In the menstrual cycle, oestrogen and progesterone are made in the ovary.

In pregnancy, the placenta makes both progesterone and oestrogen. Once the placenta is established, it produces most of the progesterone.

Examiner insight

“Incorrect responses included the pituitary gland, uterus, and uterus lining.” “Most candidates were not aware that in pregnancy, once the placenta is established, it produces most of the progesterone.”

Worked example 11 mark

Which hormones are released by the placenta during pregnancy?

hormones
AFSH and LH
BFSH and progesterone
Coestrogen and LH
Dprogesterone and oestrogen
  1. 1

    answer D: the placenta releases progesterone and oestrogen [1]

29. FSH and LH

FSH causes a follicle to develop and the egg cell to mature. It also stimulates oestrogen production.

LH causes ovulation, the release of the egg cell. It also stimulates progesterone production.

On a graph of hormone concentration, LH has only one peak, just before ovulation.

Examiner insight

“For LH, there is only one peak, and it occurs just before ovulation.” “Few candidates understood that FSH and LH are required for the development and release of an egg cell.”

Worked example 11 mark

The diagram shows the relative concentrations of two hormones that are involved in controlling the human menstrual cycle. Which hormone is represented by X?

hormone
AFSH
BLH
Coestrogen
Dprogesterone
  1. 1

    answer B: X has only one peak, and it is just before ovulation at day 14, so it is LH [1]

30. Oestrogen and progesterone in the cycle

Oestrogen, from the ovary, causes the uterus lining to increase in thickness.

Progesterone, also from the ovary, causes the maintenance of the lining and prepares it for implantation of a fertilised egg.

If the egg is not fertilised, progesterone decreases. The drop in progesterone causes the lining to break down at menstruation.

Examiner insight

“The idea of progesterone thickening the lining was often given, which on its own was insufficient for credit.” “Quite a number of candidates used the term uterine wall when they meant the lining.”

Worked example 14 marks

Figure 3.1 shows the thickness of the uterus lining during one menstrual cycle. Explain how oestrogen and progesterone are involved in the changes shown in Figure 3.1.

  1. 1

    the decrease in thickness is menstruation [1]

  2. 2

    it is caused by a drop in progesterone / oestrogen [1]

  3. 3

    the increase in thickness of the lining is caused by oestrogen [1]

  4. 4

    maintenance of the lining is caused by progesterone [1]

  5. 5

    also credited: oestrogen and progesterone are secreted by the ovary

31. Hormones in pregnancy

If the egg cell is fertilised, the placenta produces progesterone and oestrogen.

Progesterone maintains the lining of the uterus. Its level increases and then remains high. Progesterone also prevents the release of FSH and LH.

Examiner insight

“Fewer were able to articulate that the level of progesterone release remains high.” “Some also stated that progesterone prevents the release of FSH and LH.”

Worked example 13 marks

Describe how the level of progesterone changes during pregnancy and state one role of progesterone.

  1. 1

    the level of progesterone increases [1]

  2. 2

    and then remains high [1]

  3. 3

    progesterone maintains the lining of the uterus [1]

Recap

  • Testosterone causes male secondary sexual characteristics. Oestrogen causes female ones.
  • Ovulation: an egg cell is released from the ovary, around day 14 of a 28-day cycle.
  • The uterus lining breaks down at menstruation, builds up again, and its thickness is maintained.
  • Oestrogen and progesterone are made in the ovary. In pregnancy the placenta makes them.
  • FSH: follicle develops, egg cell matures, oestrogen production stimulated.
  • LH: ovulation, and progesterone production stimulated.
  • Oestrogen increases the thickness of the lining. Progesterone maintains it.
  • A drop in progesterone causes menstruation. In pregnancy progesterone increases, then stays high.

32. What an STI is

A sexually transmitted infection (STI) is an infection that is transmitted through sexual contact.

The best answers specified that STIs spread through body fluids. An unqualified 'fluid' lacked sufficient detail.

Examiner insight

“The best responses specified that STIs spread through body fluids.” “‘Fluid’ unqualified was not accepted as it lacked sufficient detail.”

Worked example 12 marks

Describe what is meant by a sexually transmitted infection (STI).

  1. 1

    an infection [1]

  2. 2

    that is transmitted through sexual contact [1]

33. HIV and AIDS

HIV (human immunodeficiency virus) is a pathogen that causes an STI. HIV is a virus, not a bacterium.

HIV infection may lead to AIDS. AIDS is a group of diseases caused by HIV. HIV is the pathogen. AIDS is not.

Examiner insight

“Some were unclear of the difference between HIV which is an STI and AIDS which is a group of diseases caused by HIV.” “Many candidates were aware that the organism that causes AIDS is a virus.”

Worked example 12 marks

HIV is a pathogen that causes an STI.(a) State what the letters HIV stand for.(b) State the disease that HIV infection may lead to.

  1. 1

    (a) human immunodeficiency virus [1]

  2. 2

    (b) AIDS [1]

34. How HIV is transmitted

HIV is transmitted by unprotected sexual intercourse with an infected person, and by blood from an infected person passing into another person's blood.

Only infected body fluids transmit the virus. Sharing needles is a route: a needle used by an infected person passes infected blood into the next person's blood. Holding hands and sharing a drinking cup do not transmit HIV.

Examiner insight

“Many candidates stated that body fluids were the medium of transmission, but omitted to state clearly that only infected body fluids would transmit the virus.” “Many responses stated that blood can transmit the virus but did not state that the blood has to mix with other blood for transmission.”

Worked example 11 mark

Methods of disease transmission are listed.

1 blood transferred from an infected person · 2 holding hands with an infected person · 3 sharing a drinking cup with an infected person · 4 unprotected sexual intercourse with an infected person

By which methods of disease transmission can the human immunodeficiency virus (HIV) be transmitted between people?

A 1 and 2 · B 1 and 4 · C 2 and 3 · D 3 and 4

  1. 1

    B: blood from an infected person and unprotected sexual intercourse. [1]

35. Controlling the spread of STIs

The spread of STIs is controlled by using a condom. A condom is a barrier method of contraception.

Other methods of birth control do not protect against STIs. That includes the contraceptive pill, the IUD, female sterilisation and the contraceptive injection. Not sharing needles also controls the spread of HIV.

Examiner insight

“One misconception was that the contraceptive pill would protect against STIs.” “Unqualified references to contraception, regular checks or using clean needles were ignored.”

Worked example 12 marks

Explain how the spread of STIs can be controlled.

  1. 1

    use a condom during sexual intercourse [1]

  2. 2

    a condom is a barrier method, so infected body fluids do not pass from one person to the other [1]

Recap

  • An STI is an infection that is transmitted through sexual contact.
  • HIV is a pathogen (a virus) that causes an STI.
  • HIV infection may lead to AIDS. AIDS is not the pathogen.
  • HIV is transmitted by unprotected sexual intercourse and by infected blood, including a shared needle.
  • Holding hands and sharing a drinking cup do not transmit HIV.
  • A condom is a barrier method that helps control the spread of STIs. The contraceptive pill does not.

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. State how many parents are needed for asexual reproduction.1 mark
  2. Describe asexual reproduction.2 marks
  3. A student says: "In asexual reproduction, two gametes fuse." Correct the student.2 marks
  4. One sea anemone, an invertebrate animal, splits into two. Each has the same DNA as the original. A student says this cannot be asexual reproduction because a sea anemone is an animal. State whether the student is correct and give a reason.2 marks
  5. The table gives information about three types of reproduction, P, Q and R. Identify which of P, Q and R is asexual reproduction. Give a reason why R is not. | type | number of parents | gametes fuse | offspring genetically identical | | --- | --- | --- | --- | | P | 1 | no | yes | | Q | 2 | yes | no | | R | 1 | yes | no |2 marks
  6. A student says that an insect carrying pollen between flowers is an example of asexual reproduction. Explain why the student is wrong.2 marks
  7. A student writes: "An advantage of asexual reproduction to a species in the wild is that it gives genetic variety in the offspring." Correct the student, and state two advantages the student could have given.3 marks
  8. A farmer has one crop plant that gives a high yield. Explain two advantages of using asexual reproduction to produce next year's crop from this plant.2 marks
  9. Duckweed is a small plant that floats on ponds. One duckweed plant is put into a new pond. New plants grow from the side of each plant and break off. No flowers form. After six weeks the plants cover the pond. A disease then kills every plant. (a) Explain how this type of reproduction let one plant cover the pond in six weeks. (b) Explain why the disease killed every plant.5 marks
  10. A blackcurrant bush has very large fruit. A grower can make new bushes from it in two ways. Method 1: pieces of stem cut from the bush grow into new bushes. No gametes are involved. Method 2: pollen from the bush pollinates its own flowers. She sows the seeds that form. A student says: "Both methods are asexual reproduction, because each uses one bush." (a) Discuss whether the student is correct. (b) The grower wants every new bush to have very large fruit, quickly. State which method she should use and explain why. (c) State one risk to a field of bushes made by this method.6 marks
  11. Name the cell formed when two gametes fuse.1 mark
  12. State which parts of the two gametes fuse in fertilisation.1 mark
  13. A student describes sexual reproduction: “Two gametes fuse to form a zygote. The offspring are identical to each other.” The description does not earn full marks. Rewrite it so that it is correct.2 marks
  14. In a flowering plant, a pollen nucleus fuses with a nucleus in an ovule. Name this process and the cell that it forms.2 marks
  15. Describe sexual reproduction.3 marks
  16. State whether the nucleus of a gamete is haploid or diploid, and whether the nucleus of a zygote is haploid or diploid.2 marks
  17. A wild plant species reproduces by sexual reproduction. State two advantages and one disadvantage of this to the population.3 marks
  18. The nucleus of a human zygote contains 46 chromosomes. Explain why the nucleus of a human sperm cannot contain 46 chromosomes as well.2 marks
  19. In one species of plant, the nucleus of a male gamete contains 14 chromosomes. A student writes: “The nucleus of the female gamete is diploid, so it contains 28 chromosomes. The zygote is diploid, so its nucleus contains 46 chromosomes.” Identify the two mistakes. State the correct number of chromosomes in each of these two nuclei and explain your answers.4 marks
  20. A wild species of rice reproduces by sexual reproduction. A plant breeder uses sexual reproduction to breed a crop variety from it. A student writes: “In the wild, sexual reproduction reduces variation, so the plants survive in the environment. For the breeder it is a fast method, and all the offspring have the desired features.” Discuss the advantages and disadvantages of sexual reproduction to the wild population and to crop production, correcting the student’s mistakes.6 marks
  21. Name the part of a flower that (a) produces pollen grains and (b) receives pollen grains.2 marks
  22. State one difference between pollination and fertilisation.2 marks
  23. The nucleus of a pollen grain fuses with a nucleus inside a structure in the ovary. Name this structure.1 mark
  24. A grass flower is wind-pollinated. Describe the position of its anthers and the structure of its stigmas.2 marks
  25. Pollen is collected from two flowers. Flower X is wind-pollinated. Flower Y is insect-pollinated. State two ways in which the pollen grains of flower Y differ from the pollen grains of flower X.2 marks
  26. A flower has large petals with a spotted pattern. Its anthers and stigma are inside the petals. State how it is pollinated and give two reasons from this description.3 marks
  27. Seeds are put on damp cotton wool in an open dish, so air reaches them. The dish is kept in a dark cupboard at a suitable temperature. Predict whether the seeds germinate and explain your answer.3 marks
  28. Describe how to investigate whether temperature affects the germination of seeds.3 marks
  29. Plants P and Q are the same species. Plant R is a different species. Pollen is moved in four ways. 1: from an anther of a flower on P to the stigma of that flower 2: from an anther of a flower on P to the stigma of another flower on P 3: from an anther of a flower on P to the stigma of a flower on Q 4: from an anther of a flower on P to the stigma of a flower on R (a) State every transfer that is self-pollination. (b) State the transfer that is cross-pollination. Explain why the remaining transfer is not cross-pollination. (c) One population of this species reproduces only by transfer 1 for many generations. Another reproduces by transfer 3. Compare the two populations in terms of variation and their capacity to respond to changes in the environment.5 marks
  30. A grower keeps one plant alone under a fine net. No insects and no pollen from other plants can reach its flowers. Each flower has its anthers and stigma inside the petals. The plant still produces seeds. (a) Name the type of pollination that takes place and describe it. (b) A student writes: ‘Before pollination, a pollen tube grows. The pollen grain travels down the tube and fuses with the ovary. This is called pollination.’ Identify and correct the four mistakes. (c) State one advantage of this type of pollination that the grower’s result shows.7 marks
  31. State the function of the scrotum.1 mark
  32. (a) Name the tube that carries sperm from the testis. (b) Name the tube that carries both urine and semen out of the body.2 marks
  33. State the part of the female reproductive system where each of these happens. (a) Sperm are deposited. (b) Fertilisation normally occurs. (c) The embryo implants.3 marks
  34. (a) Write these four words in the order in which they appear: embryo, fetus, gamete, zygote. (b) State one function of the amniotic fluid.3 marks
  35. A sperm cell has a flagellum, many mitochondria and enzymes in its acrosome. Explain how each feature helps the sperm to fertilise an egg cell.3 marks
  36. An egg cell is adapted for its function. (a) State two adaptive features of an egg cell. (b) Explain how each feature helps.4 marks
  37. The table gives data for two human gametes, X and Y. (a) Identify the egg cell. Give two reasons from the table. (b) Calculate how many times wider X is than Y. | | cell X | cell Y | | --- | --- | --- | | width of cell / µm | 120 | 5 | | moves independently | no | yes | | number in one sample | 1 | 5 000 000 |4 marks
  38. A fetus exchanges substances with its mother. (a) State the direction in which oxygen passes: mother to fetus, or fetus to mother. (b) State the direction in which urea passes. (c) Name the structure in which this exchange takes place.3 marks
  39. Blood flows between a fetus and the placenta in the umbilical cord. Doctors test the blood of a pregnant woman and the blood of her fetus for a toxin, T, and for two pathogens, V and M. The table gives the results. (a) State where the toxin in the fetus’s blood came from. Describe the route it took to reach the fetus. (b) A student says: ‘T is in the fetus’s blood, so the mother’s blood must flow into the fetus.’ Use the table to explain why the student is wrong. (c) State what the results for V and M tell you about the placenta and pathogens. | | toxin T | pathogen V | pathogen M | | --- | --- | --- | --- | | mother’s blood | present | present | present | | fetus’s blood | present | present | absent |7 marks
  40. Blood samples are taken from four places near a placenta. W and X are the mother’s blood. Y and Z are the fetus’s blood. In each pair, one sample was taken before the blood passed through the placenta and one after. The table gives the concentration of four substances in each sample, in arbitrary units. (a) Identify the sample of the mother’s blood taken after it passed through the placenta. Explain your choice using two substances. (b) Identify the sample of the fetus’s blood taken after it passed through the placenta. (c) Calculate the percentage decrease in the concentration of oxygen in the mother’s blood as it passes through the placenta. (d) A student says the two blood supplies mix in the placenta. Give one piece of evidence from the table against this. State the function of the placenta that explains it. | | W | X | Y | Z | | --- | --- | --- | --- | --- | | oxygen | 72 | 96 | 30 | 54 | | carbon dioxide | 46 | 40 | 54 | 48 | | urea | 4.6 | 4.0 | 5.4 | 4.8 | | glucose | 4.4 | 5.0 | 3.6 | 4.2 |8 marks
  41. Name the hormone that causes the development of female secondary sexual characteristics.1 mark
  42. State two effects of testosterone during puberty in boys.2 marks
  43. Name the process in which the uterus lining breaks down and is lost.1 mark
  44. State the day of a 28-day menstrual cycle when an egg cell is most likely to be released, and name the organ that releases it.2 marks
  45. A student writes: "At ovulation the uterus lining breaks down." Explain what is wrong with this.2 marks
  46. The table lists four changes at puberty. For each change, state whether it is caused by testosterone or by oestrogen. | Change at puberty | | --- | | voice deepens | | hips widen | | shoulders widen | | facial hair develops |4 marks
  47. Name the organ that makes progesterone during the menstrual cycle, and the organ that makes most of the progesterone later in pregnancy.2 marks
  48. A woman's ovaries do not release egg cells. In fertility treatment she is given FSH, then LH. Explain how these two hormones lead to an egg cell being released and to the uterus lining being ready for implantation.5 marks
  49. A doctor measures the concentrations of two hormones, J and K, in a woman's blood during one 28-day menstrual cycle. One is LH and the other is progesterone. Identify J and K, giving a reason from the table for each. Then explain the effect of the change in K between day 21 and day 28. | day | J / arbitrary units | K / arbitrary units | | --- | --- | --- | | 1 | 4 | 1 | | 7 | 5 | 1 | | 13 | 48 | 2 | | 21 | 4 | 16 | | 28 | 4 | 2 |5 marks
  50. A woman is 20 weeks pregnant. A blood test finds a high concentration of progesterone and very low concentrations of FSH and LH. A student writes: "Her ovaries make most of this progesterone. It thickens the uterine wall." Correct the student's two sentences. Then use the blood test results to explain why the woman does not menstruate and does not release egg cells during the pregnancy.6 marks
  51. Describe what is meant by an STI.2 marks
  52. HIV causes an STI. State the type of pathogen that HIV is.1 mark
  53. A newspaper says: 'AIDS is spread by sexual contact.' Name the pathogen that is transmitted, and state what it may lead to.2 marks
  54. Which methods can transmit HIV from an infected person? 1 sharing a needle · 2 sharing a drinking cup · 3 shaking hands · 4 unprotected sexual intercourse A 1 and 4 · B 1 and 2 · C 3 and 4 · D 2 and 31 mark
  55. Explain why shaking hands with an infected person does not transmit HIV.2 marks
  56. Two people share a needle to inject a drug. One of them is infected with HIV. Describe how HIV may be transmitted to the other person.3 marks
  57. A student writes: 'HIV is spread through body fluids.' Explain what is missing from this answer.2 marks
  58. A clinic leaflet lists four methods of birth control: A condom, B contraceptive pill, C IUD, D female sterilisation. State which method protects against STIs and name the type of method it is.2 marks
  59. A student drafts a health poster: 'AIDS is a bacterium that causes an STI. It is passed on by sharing food and by coughs and sneezes. People who inject drugs must not share injections. Any method of birth control stops it spreading.' Identify five mistakes in the poster and correct each one.5 marks
  60. A clinic surveys 400 adults in each of two towns. Each adult names the one method of birth control they use. The clinic also counts the new HIV infections among these adults in one year. The table gives the results. (a) Calculate the percentage of the adults in Town X who use a method that protects against STIs. (b) A student concludes: 'Town X has more new HIV infections because fewer people there use birth control.' Use the data to explain why the student is wrong, and give a better explanation. (c) One adult in Town Y always uses a condom but is infected with HIV. Describe how HIV could have been transmitted to this adult, and state how this route is controlled. | | Town X | Town Y | | --- | --- | --- | | condom | 60 | 220 | | contraceptive pill | 180 | 90 | | IUD | 100 | 50 | | female sterilisation | 60 | 40 | | new HIV infections in one year | 24 | 6 |7 marks

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