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IB Biology D3.1: Reproduction worked solutions

Worked solutions to every practice question in D3.1: Reproduction of Photon Biology: Study Guide for the IB Diploma (SL & HL). Try each question, then compare your working with the full solution.

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Basic practice

Question A1

[1 mark]

Strawberry plants send out runners that root and grow into new plants. Which statement about the new plants is correct? (A) Same genotype as the parent (B) New allele combinations made by meiosis (C) Haploid, because no fertilisation occurred (D) Formed by fusion of two gametes

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The runners form by mitosis from one parent, so the new plants are clones: they carry the parent's genotype apart from rare mutations. Options B and D describe sexual reproduction, and C is wrong because mitosis keeps the diploid number.

Final answer

Question A2

[1 mark]

Which row pairs a structure of the male reproductive system with its job? (A) Epididymis: makes sperm (B) Prostate gland: adds fluid to semen (C) Seminal vesicle: stores sperm (D) Scrotum: makes testosterone

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The prostate gland adds fluid to the sperm, and the fluid and sperm together are semen. Sperm are made in the testis, not the epididymis (A); the epididymis, not the seminal vesicle, stores sperm (C); testosterone comes from the testis, not the scrotum (D).

Final answer

Question A3

[1 mark]

Which hormone is secreted by the corpus luteum and maintains the thickness of the endometrium? (A) FSH (B) LH (C) Progesterone (D) Testosterone

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Progesterone from the corpus luteum thickens and maintains the endometrium. FSH and LH come from the pituitary gland, and testosterone is the main male sex hormone.

Final answer

Question A4

[1 mark]

In a regular 28-day menstrual cycle, between which days is the concentration of progesterone highest? (A) Days 1 to 5 (B) Days 8 to 12 (C) Days 13 to 15 (D) Days 19 to 23

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Progesterone rises after ovulation (about day 14) while the corpus luteum is active, peaks near day 21 and falls before day 28. Days 19 to 23 fall around the peak.

Final answer

Question A5

[1 mark]

Which event is an example of positive feedback in the menstrual cycle? (A) Rising oestradiol lowers FSH secretion (B) Progesterone lowers LH secretion (C) High oestradiol increases LH secretion (D) Falling progesterone sheds the lining

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A high oestradiol level stimulates more LH to be released, which produces the LH surge, so the change reinforces itself. A, B and D are inhibition or loss of a signal, so they are negative feedback or no feedback.

Final answer

Question A6

[2 marks]

Fertilisation takes place in the human female reproductive system. (a) State where fertilisation normally occurs. [1] (b) Name one part of the sperm that is destroyed after its nucleus has entered the egg. [1]

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(a) Fertilisation normally occurs in the oviduct (1). (b) After the sperm nucleus enters the egg, the sperm tail and mitochondria are destroyed (1 for either).

Final answer

(a) The oviduct. (b) The tail (or the mitochondria).

Question A7

[3 marks]

An insect-pollinated flower is cut in half from top to bottom. (a) Draw a half-view and label the stigma, anther, ovule and petal. [2] (b) State the function of the petal. [1]

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Award 1 for a recognisable half-view with the carpel in the centre and a stamen at the side (1). Award 1 for any two correctly placed labels and a further 1 for all four correct, with label lines that touch the structure (1). Petal function if asked: large and coloured to attract insects.

Final answer

Half-view with stigma, anther, ovule, petal labelled; petal attracts insects.

Question A8

[2 marks]

In flowering plants, pollination and dispersal are separate. Distinguish between pollination and seed dispersal.

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Pollination is the transfer of pollen from an anther to a stigma, and it happens before fertilisation (1). Seed dispersal is the transport of the seed away from the parent plant, after fertilisation (1).

Final answer

Pollination: transfer of pollen from anther to stigma, before fertilisation. Dispersal: movement of the seed away from the parent plant, after fertilisation.

Question A9

[1 mark]

Which set of conditions must be present for a bean seed to germinate? (A) Water, oxygen and a suitable temperature (B) Water, light and carbon dioxide (C) Oxygen, light and nitrate ions (D) Carbon dioxide, light and warmth

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A seed needs water to activate enzymes, oxygen for aerobic respiration and a suitable temperature for enzyme activity. Light is not needed until the leaves open, and carbon dioxide and nitrate are not needed to germinate.

Final answer

Question A10

[2 marks]

Men make about 10^{8} sperm a day; women release about one egg a month. Explain why many more sperm than eggs are made.

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Sperm must travel to the egg and most never reach it, so a very large number is needed (1). Each egg is large and contains a food store, so it is costly to make and only a few are produced (1). Accept the reverse argument that small sperm are cheap to make.

Final answer

Sperm travel and most do not arrive, so many are needed. Eggs are large and costly, so few are made.

Tutorial practice

Question B1

[5 marks]

In a menstrual cycle the ovary and the uterus change together. Outline how hormones cause ovulation and the thickening of the endometrium in the first half of the cycle.

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Any five points. FSH stimulates the growth of follicles (1). The follicle wall secretes oestradiol (1). Oestradiol repairs and thickens the endometrium (1). Oestradiol inhibits FSH by negative feedback (1). A high level of oestradiol stimulates the LH surge by positive feedback (1). LH completes meiosis in the oocyte and causes ovulation (1).

Final answer

FSH grows follicle; follicle makes oestradiol; oestradiol repairs and thickens lining; high oestradiol stimulates LH surge; LH causes ovulation (also oestradiol inhibits FSH).

Question B2

[4 marks]

A woman with blocked oviducts is treated by in vitro fertilisation. (a) State the purpose of injecting FSH. [1] (b) Explain why her natural hormone secretion is suspended before the injections begin. [2] (c) State why progesterone is given after the embryos are transferred. [1]

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(a) FSH makes several follicles mature at once, so several eggs are produced (superovulation) (1). (b) The natural secretion of FSH and LH is suspended so that her own feedback loops do not interfere (1) and the doctors control when the eggs mature (1). (c) Progesterone keeps the endometrium thick so the transferred embryo can implant (1).

Final answer

(a) Many follicles mature (superovulation). (b) So that doctors control timing / her own hormones do not interfere with the dose. (c) Progesterone maintains the lining so the embryo implants.

Question B3

[4 marks]

A plant species has male flowers and female flowers on the same plant. The male flowers release pollen two weeks before the female flowers open. (a) Identify two features that make self-pollination less likely. [2] (b) Explain one advantage to the species of cross-pollination. [2]

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(a) Male and female flowers are separate (1). The pollen and the stigma mature at different times (1). (b) Cross-pollination brings together alleles from different plants, giving new combinations and more variation for natural selection (1). It also avoids inbreeding, which lowers diversity and vigour (1).

Final answer

(a) Separate male and female flowers; different maturation times. (b) New allele combinations, more variation; avoids inbreeding.

Question B4

[5 marks]

Pollen grains were placed in sucrose solutions of different concentrations. The mean length of the pollen tubes was measured after three hours. The results are shown. Sucrose concentration (%): 0, 5, 10, 15, 20. Mean tube length (\mum): 15, 90, 140, 112, 38. (a) State the sucrose concentration that gave the longest pollen tubes. [1] (b) Calculate the percentage increase in mean tube length between 5 and 10 per cent sucrose. [2] (c) Suggest why the tubes were shorter at 20 per cent sucrose. [2]

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(a) 10 per cent sucrose (1). (b) \dfrac{140-90}{90}\times100=55.6\simeq56\%: the subtraction (1) and the answer (1). (c) At 20 per cent the solution outside has a much lower water potential (1), so water leaves the pollen tube by osmosis and the cell loses turgor, which slows tube growth (1).

Final answer

(a) 10%. (b) 56%. (c) High sucrose gives low water potential outside; water leaves the tube by osmosis; less turgor, less growth.

Question B5

HL[5 marks]

A primary spermatocyte in a human testis has 46 chromosomes. (a) State the number of chromosomes in a spermatid formed from it. [1] (b) Compare and contrast spermatogenesis and oogenesis. [4]

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(a) 23 (1). (b) Similarities, either: both involve mitosis, growth, the two divisions of meiosis and differentiation (1). Differences, any three: a primary spermatocyte gives four sperm but a primary oocyte gives one egg (1); the cytoplasm is shared equally in sperm formation but very unequally in egg formation (1); sperm formation starts at puberty, but egg formation starts in the fetus (1); the oocyte is paused in meiosis, but sperm formation is continuous (1). Maximum 4.

Final answer

(a) 23. (b) Both: mitosis, growth, two meiotic divisions, differentiation. Sperm: four per cell, equal cytoplasm, from puberty. Egg: one per cell, unequal cytoplasm, starts in the fetus.

Question B6

HL[5 marks]

Polyspermy is prevented in humans, and the embryo then implants in the uterus. (a) Distinguish between the acrosome reaction and the cortical reaction. [2] (b) Describe the development of the zygote up to implantation. [3]

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(a) The acrosome reaction releases enzymes from the sperm that digest a path through the zona pellucida (1). The cortical reaction releases enzymes from cortical granules in the egg that harden the zona pellucida so no other sperm can enter (1). (b) The zygote divides by mitosis (1) to form a hollow ball of cells, the blastocyst (1), which implants in the endometrium (1).

Final answer

(a) Acrosome reaction: enzymes digest the zona. Cortical reaction: egg releases enzymes that harden the zona. (b) Mitosis; hollow ball (blastocyst); sinks into endometrium.

Question B7

HL[4 marks]

Mammals with a placenta carry their young for longer before birth than mammals without one. Explain how the placenta supports the development of the foetus.

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Any four. The villi give a large surface area for exchange (1). Oxygen, glucose and amino acids pass to the foetus (1). Antibodies pass to the foetus (1). Carbon dioxide and urea pass to the mother (1). Maternal and foetal blood do not mix (1). The placenta secretes progesterone and oestradiol to maintain the pregnancy (1).

Final answer

Villi give large surface area; oxygen, nutrients, antibodies to foetus; carbon dioxide and urea to mother; blood does not mix; makes progesterone.

Question B8

HL[3 marks]

The onset of puberty is controlled by hormones. (a) Name the hormone released by the hypothalamus that triggers puberty. [1] (b) State the two hormones that the pituitary gland then releases. [1] (c) Outline one effect of testosterone at puberty. [1]

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(a) Gonadotropin-releasing hormone, GnRH (1). (b) The pituitary gland releases LH and FSH (1). (c) Any one effect of testosterone in a boy, such as a deeper voice or facial hair (1).

Final answer

(a) GnRH. (b) LH and FSH. (c) Testosterone: voice breaks, facial hair.

Advanced practice

Question S1

HL[6 marks]

A drug that blocks oxytocin receptors on uterine muscle is used to delay premature labour. (a) Explain why the drug delays labour. [2] (b) Describe the role of positive feedback in normal childbirth. [3] (c) State the change in progesterone that allows labour to begin. [1]

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(a) Oxytocin stimulates contractions of the uterine muscle (1). If the receptors are blocked, oxytocin cannot act and contractions are weaker or absent, so labour is delayed (1). (b) Oxytocin makes the uterus contract (1). Contractions push the foetus against the cervix, which signals the pituitary to release more oxytocin (1). The stronger contractions give more pressure, so the change reinforces itself until the baby is born (1). (c) A decrease in progesterone allows oxytocin to rise (1).

Final answer

(a) Contractions need oxytocin to act; blocked receptors, so no contractions. (b) Oxytocin causes contractions; head presses on the cervix; signals cause more oxytocin; loop until birth. (c) A fall.

Question S2

HL[5 marks]

An early observational study followed women who chose hormone replacement therapy (HRT) and women who did not. It found 2.1 cases of coronary heart disease (CHD) per 1000 women each year among HRT users and 3.4 among non-users. A later randomised controlled trial found 3.9 cases per 1000 women each year in the HRT group and 3.3 in the placebo group. (a) Calculate the percentage difference in the CHD rate of HRT users compared with the comparison group in each study. [2] (b) Suggest why the observational study gave a different result. [2] (c) State why random allocation makes the trial more reliable evidence of cause and effect. [1]

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(a) Observational: \dfrac{2.1-3.4}{3.4}\times100=-38.2\simeq38\% lower (1). Trial: \dfrac{3.9-3.3}{3.3}\times100=18.2\simeq18\% higher (1). (b) The women who chose HRT tended to have a higher socioeconomic status (1), and that status itself lowers the risk of CHD, so the link was not cause and effect (1). (c) Random allocation spreads other variables evenly between the groups, so a difference in CHD can be attributed to the treatment (1).

Final answer

(a) 38% lower; 18% higher. (b) HRT users had higher socioeconomic status, which lowers CHD risk (confounding variable). (c) Random allocation spreads other variables evenly between groups.

Question S3

HL[5 marks]

A pregnancy test strip has a test line of fixed monoclonal antibodies against hCG and a control line of fixed antibodies that bind the free labelled antibodies. A woman tests her urine and only the control line appears. (a) Suggest two reasons for this result. [2] (b) Explain why the control line is needed. [1] (c) Explain why a rise in LH would not produce a test line. [2]

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(a) Any two: she is not pregnant, so no hCG is present (1); implantation has only just happened, so the hCG concentration is too low to detect (1); the urine was too dilute (1). (b) The control line shows that urine flowed along the strip and the labelled antibody was carried to the end, so the strip worked (1). (c) Monoclonal antibodies bind only to one site on hCG (1). LH has a different shape, so it does not bind and cannot form a line (1).

Final answer

(a) hCG absent (not pregnant) or too early / too dilute. (b) Shows the labelled antibody reached the end, so the strip worked. (c) Antibody binds only hCG (specific shape); LH has a different shape.

Question S4

[6 marks]

The relative concentration of three hormones, X, Y and Z, is given for selected days of a 28-day menstrual cycle. Day: 2, 6, 10, 12, 13, 14, 18, 21, 25. X: 0.2, 0.3, 1.0, 2.8, 3.0, 2.1, 1.5, 1.8, 0.8. Y: 0.1, 0.1, 0.1, 0.1, 0.2, 0.6, 2.0, 2.6, 1.1. Z: 4, 4, 5, 9, 30, 12, 5, 4, 4. (a) Identify X, Y and Z. [3] (b) Deduce the day on which ovulation most probably occurred. [1] (c) Explain the rise in the concentration of Z between day 12 and day 13. [2]

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(a) X is oestradiol, Y is progesterone and Z is LH, one mark each. (b) Ovulation is about day 14, just after the LH peak on day 13 (1). (c) Oestradiol rises to its peak by day 12 to 13 (1). A high oestradiol level stimulates the pituitary to release more LH by positive feedback, which produces the surge (1). Maximum 3 marks for (a), 1 for (b) and 2 for (c).

Final answer

(a) X oestradiol, Y progesterone, Z LH. (b) About day 14. (c) Oestradiol above a threshold stimulates LH by positive feedback.

Question S5

[5 marks]

A strawberry grower plants 500 runners from a single plant. A fungus then spreads through the field and kills 90\% of these plants. In a neighbouring field grown from seed, containing about 40 different genotypes, the fungus kills 35\%. (a) Explain why the fungus killed a higher percentage of the plants in the first field. [2] (b) State one advantage to the grower of using runners. [1] (c) Explain why sexual reproduction would be better for the species if the environment changed. [2]

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(a) The runners are clones with identical alleles, so they share the same susceptibility to the fungus (1). The seed-grown plants differ in their alleles, so some are resistant and survive (1). (b) Asexual reproduction copies a genotype with the desired fruit quality exactly and quickly (1). (c) Sexual reproduction gives new allele combinations (1), so if the environment changes some plants are likely to be adapted to it and natural selection can act on them (1). Maximum 5.

Final answer

(a) Clones share one genotype, so all are equally susceptible. Seed-grown plants vary; some resistant. (b) Offspring keep the high-quality genotype. (c) Variation lets some survive a changed environment.

Connected questions

Question X1

Connected questions · HL[6 marks]

D3.1 + D2.1. In one primary oocyte, the two chromosomes numbered 21 fail to separate during meiosis I. (a) State the number of chromosomes in this secondary oocyte and in the zygote if a normal sperm fertilises it. [2] (b) State the stage at which the secondary oocyte is paused at ovulation and what makes it continue. [2] (c) Explain why the first divisions of the zygote preserve the error. [2]

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(a) The secondary oocyte has 24 chromosomes instead of 23 (1). After fertilisation by a normal sperm (23) the zygote has 47 (1). (b) The oocyte pauses in metaphase II at ovulation (1) and completes meiosis II only if a sperm enters (1). (c) The zygote divides by mitosis, which produces genetically identical diploid cells (1), so each daughter cell also has 47 chromosomes (1).

Final answer

(a) 24; 47. (b) Metaphase II; sperm entry. (c) Mitosis; 47 in each cell.

Question X2

Connected questions · HL[6 marks]

D3.1 + D3.3. Blood glucose is kept steady by negative feedback. In the menstrual cycle and at birth, hormones also use positive feedback. (a) Compare the effect of negative and positive feedback on a variable. [2] (b) Explain how oestradiol shows both types of feedback in the first half of the menstrual cycle. [2] (c) Explain why the positive feedback of childbirth stops. [2]

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(a) Negative feedback reduces the change that started the response and returns the variable to its set point (1). Positive feedback increases the change, so the variable moves further from where it started (1). (b) Rising oestradiol first inhibits FSH (1), but above a threshold it stimulates the pituitary to release LH, producing the surge that causes ovulation (1). (c) Positive feedback in birth stops only when the baby is born (1), because the pressure on the cervix that stimulated oxytocin release is then removed (1).

Final answer

(a) Negative feedback reverses a change; positive feedback increases it. (b) Oestradiol above threshold stimulates LH by positive feedback, causing the surge and ovulation. (c) Birth ends the stimulus (pressure on the cervix).

Question X3

Connected questions[6 marks]

D3.1 + D4.1. In summer, aphids on a plant reproduce without fertilisation and each female gives birth to genetically identical young. In autumn, males and females appear and mate, and the fertilised eggs survive the winter. (a) Explain why asexual reproduction suits the aphids in summer. [2] (b) Explain why sexual reproduction in autumn gives an advantage. [2] (c) Suggest why it is the products of sexual reproduction that survive the winter. [2]

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(a) Asexual reproduction needs no mate and no fertilisation, so each female can produce young quickly (1). The young carry a genotype that is already adapted to the summer plant, so the population grows rapidly (1). (b) Meiosis and the fusion of gametes make new allele combinations (1), which gives variation on which natural selection can act when conditions change (1). (c) The eggs form a new generation with varied genotypes, so some are likely to be able to survive the cold of winter, which is a changed environment (1); the shell and food store of the egg protect them (1). Accept any two valid reasons.

Final answer

(a) Fast population growth, no mate, copy a successful genotype. (b) New combinations give variation for selection. (c) The varied eggs mean some survive the changed (winter) conditions.

From the book

Photon Biology: Study Guide for the IB Diploma (SL & HL)

Reproduction from the core ideas to full exam questions, with question types, worked examples and graded practice.

About the book