IB DP Biology: Human reproduction in detail: puberty, gametogenesis, pregnancy and HRT (HL) (DP Biology) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – the IB holds copyright in its own papers. Use these alongside the official past papers available through your school or the IB store.

These questions are aligned to the IB Biology guide, first assessment 2025, and cover statements D3.1.13 to D3.1.20 of sub-topic D3.1 (Reproduction). All of the content is Additional higher level, so every question is HL only. Data in the questions are invented for practice. Read the study guide first if needed, or use the revision notes. Other links: course hub, printable checklist and all free 10-minute diagnostics.

Questions

1. Outline how an increase in the release of gonadotropin-releasing hormone in childhood leads to the changes of puberty. [3]

2. A researcher follows 75 primary spermatocytes and 75 primary oocytes through gametogenesis.

(a) State the number of sperm produced. [1] (b) State the number of eggs produced. [1] (c) State the chromosome number of a primary spermatocyte and of a spermatid. [2]

3. Compare spermatogenesis and oogenesis. [4]

4. A micrograph of a human sperm has a total length of 62 mm. The magnification is ×1240.

(a) Calculate the actual length of the sperm in micrometres. [2] (b) Explain why the egg is much larger than the sperm. [2]

5. Explain how the acrosome reaction and the cortical reaction ensure that only one sperm fertilizes an egg. [5]

6. Describe the development of a blastocyst and its implantation. [4]

7. The table shows the hCG concentration in a woman’s blood.

Day after ovulation	10	12	14	16	18
hCG (mIU/mL)	8	17	36	75	150

(a) Calculate the percentage increase in hCG from day 12 to day 14. [2] (b) A home test detects hCG at 40 mIU/mL or above. State the first day on which it would be positive. [1] (c) Explain how monoclonal antibodies make a pregnancy test specific. [2] (d) State the source of hCG. [1]

8. The placenta supports the foetus.

(a) State two substances that pass to the foetus and one that passes to the mother. [3] (b) Explain the importance of the placental villi. [2] (c) Suggest why placental mammals can keep a foetus in the uterus to a later stage than mammals without a placenta. [2]

9. Hormones control pregnancy and childbirth.

(a) State the source of progesterone in early pregnancy and in late pregnancy. [2] (b) Explain how oxytocin release during labour is an example of positive feedback. [4]

10. A cohort study followed women aged 50 to 60. Of 3000 women using hormone replacement therapy (HRT), 45 developed coronary heart disease (CHD). Of 5000 women not using HRT, 120 developed CHD. A later randomized controlled trial gave HRT to 5000 women and a placebo to 5000 women. CHD developed in 70 women on HRT and 60 on placebo.

(a) Calculate the percentage of women who developed CHD in each group of the cohort study and the relative risk for HRT users. [3] (b) Calculate the relative risk for the HRT group in the trial, to two significant figures. [2] (c) Explain why the two studies reached different conclusions. [3] (d) State what the trial suggests about the effect of HRT on CHD. [1]

11. The table gives hormone concentrations in a girl.

	Age 9	Age 13
FSH (IU/L)	1.5	6.0
Oestradiol (pmol/L)	20	150

(a) Calculate how many times greater the FSH concentration is at age 13. [1] (b) Explain the link between the rise in FSH and the rise in oestradiol. [3] (c) State two changes caused by oestradiol at puberty. [2] (d) Explain what happens to a secondary oocyte if it is not fertilized, and what happens if it is. [2]

Answers

1. The hypothalamus releases more GnRH [1]. This stimulates the anterior pituitary to release more LH and FSH [1]. These act on the gonads, which make more sex hormones (testosterone, oestradiol), and these bring about the changes of puberty [1] [3] Examiner insight: A full-mark outline names the gland that releases each hormone; “the brain makes sex hormones” gives too little to score.

2. (a) 75 × 4 = 300 sperm [1] (b) 75 × 1 = 75 eggs [1] (c) Primary spermatocyte: 46 [1]; spermatid: 23 [1] Examiner insight: Show the multiplication even for one-mark counts, and watch that polar bodies are never counted as eggs.

3. Any four from: both involve mitosis, growth, meiosis and differentiation; one primary spermatocyte gives four sperm whereas one primary oocyte gives one egg; cytoplasm is divided equally in spermatogenesis but unequally in oogenesis; polar bodies are made in oogenesis only; sperm production starts at puberty and continues, whereas oogenesis starts in the foetus; sperm lose most cytoplasm, whereas the egg keeps food reserves [4] [4] Examiner insight: “Compare” answers read better when each point mentions both processes; a list about one process alone will not show the comparison.

4. (a) Actual size = image size / magnification [1] = 62 mm / 1240 = 0.05 mm = 50 micrometres [1] (b) The egg carries food reserves for the early embryo [1], whereas the sperm travels to the egg and is small and light, so needs little cytoplasm [1] [4] Examiner insight: Convert units explicitly (1 mm = 1000 micrometres); an answer of 0.05 with no unit would leave the reader guessing.

5. Acrosome reaction: the sperm binds to the zona pellucida and the acrosome releases enzymes by exocytosis [1]; these digest a path through the zona pellucida [1]. The sperm membrane fuses with the egg membrane [1]. Cortical reaction: cortical granules release their contents by exocytosis [1]; the contents change the zona pellucida so other sperm cannot pass [1] [5] Examiner insight: Keep clear which reaction belongs to the sperm and which to the egg; swapping them is a typical way to lose the later points.

6. The zygote divides repeatedly by mitosis as it moves along the oviduct [1]. It forms a hollow ball of cells, the blastocyst, with an outer layer and an inner cell mass [1]. About a week after fertilization it reaches the uterus and attaches to the endometrium [1]. Enzymes digest some of the endometrium and the blastocyst embeds in it (implantation) [1] [4] Examiner insight: Use “mitosis” rather than “meiosis” for the cleavage divisions, and do not add other embryo stage names, as they are not needed.

7. (a) (36 − 17) / 17 × 100 [1] = 112% (111.8%) [1] (b) Day 16 (75 is the first value at or above 40) [1] (c) Monoclonal antibodies are all identical and bind to one specific site on hCG [1], so they capture hCG and not other hormones such as LH [1] (d) The embryo and the developing placenta [1] [6] Examiner insight: In a percentage question, divide by the starting value and show the working; dividing by the final value is a common slip.

8. (a) Any two to the foetus from oxygen, glucose, amino acids, antibodies, water [2]; any one to the mother from carbon dioxide or urea [1] (b) The villi give a large surface area [1] so that more diffusion and transport of substances can occur between maternal and foetal blood [1] (c) The placenta supplies the foetus with nutrients and oxygen over a long period [1] and removes its wastes, so the foetus can keep developing inside the uterus [1] [7] Examiner insight: Name actual substances; “food” and “waste” alone are too vague to count.

9. (a) Early: the corpus luteum [1]; late: the placenta [1] (b) Progesterone falls, allowing oxytocin to stimulate contractions [1]; contractions push the baby’s head against the cervix [1]; this stimulates release of more oxytocin from the posterior pituitary [1]; so contractions become stronger, which is positive feedback because the output increases the stimulus [1] [6] Examiner insight: To earn the positive feedback point, say that the response increases the stimulus; just stating “feedback” does not.

10. (a) HRT: 45 / 3000 × 100 = 1.5% [1]; non-users: 120 / 5000 × 100 = 2.4% [1]; relative risk = 1.5 / 2.4 = 0.63 [1] (b) HRT 70 / 5000 = 1.4%; placebo 60 / 5000 = 1.2% [1]; relative risk = 1.4 / 1.2 = 1.2 [1] (c) The cohort study was observational: women chose whether to use HRT [1]; HRT users had a higher socioeconomic status, which lowers CHD risk (a confounding variable) [1]; in the trial, random assignment spread confounding variables evenly between groups [1] (d) HRT slightly increases the risk of CHD [1] [9] Examiner insight: Evaluation answers gain credit for naming a specific confounding variable and linking it to the study design.

11. (a) 6.0 / 1.5 = 4 times [1] (b) The hypothalamus releases GnRH, which stimulates the pituitary to make FSH [1]; FSH acts on the ovaries [1]; the ovaries secrete more oestradiol [1] (c) Any two from: breast development, widening of hips, start of the uterine cycle, growth spurt [2] (d) If not fertilized the secondary oocyte stays arrested in meiosis II and degenerates [1]; if fertilized it completes meiosis II, forming an ovum and a second polar body [1] [8] Examiner insight: Give the target organ for each hormone in a chain; skipping a link in the chain loses a point.

Where marks are usually lost

• Saying the pituitary makes testosterone or oestradiol.
• Counting polar bodies as eggs.
• Swapping acrosome and cortical reactions.
• Calling the cleavage divisions meiosis.
• Calculating percentage change against the wrong starting value.
• Leaving out units, or failing to convert millimetres to micrometres.
• Naming oxytocin feedback as negative.
• Saying HRT “prevents” or “causes” CHD without reference to study design.
• Mentioning “food and waste” without naming substances.

Next steps

• Review the revision notes and the study guide.
• Go to the IB DP Biology hub and the printable checklist.
• Take all free 10-minute diagnostics.
• Book a free trial class.

Official syllabus

Source: Biology guide, first assessment 2025, International Baccalaureate Organization (Diploma Programme), sub-topic D3.1, statements D3.1.13 to D3.1.20 (Additional higher level).