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

These revision notes are aligned to the IB Biology guide, first assessment 2025, and condense statements D3.1.13 to D3.1.20 of sub-topic D3.1 (Reproduction). Everything here is Additional higher level, so it is HL only. For full explanations go to the study guide. Then try the practice questions. Other links: course hub, printable checklist and all free 10-minute diagnostics.

Statement map

Statement	One-line summary
D3.1.13	More GnRH from hypothalamus, then more LH and FSH, then more sex steroids, then puberty
D3.1.14	Spermatogenesis and oogenesis: mitosis, growth, two meiotic divisions, differentiation
D3.1.15	Acrosome reaction lets one sperm in; cortical reaction keeps others out
D3.1.16	Blastocyst implants in the endometrium
D3.1.17	hCG from embryo and placenta, detected with monoclonal antibodies
D3.1.18	Placenta: exchange across villi, allows long development in the uterus
D3.1.19	Progesterone keeps pregnancy going; falling progesterone and oxytocin positive feedback drive birth
D3.1.20	HRT and CHD: correlation was not causation

Puberty (D3.1.13)

Chain to learn as a sentence: hypothalamus, GnRH, anterior pituitary, LH and FSH, gonads, steroid sex hormones, body changes.

Step	Where	What
1	Hypothalamus	GnRH release increases in childhood
2	Anterior pituitary	LH and FSH release rises
3	Testes	Testosterone rises; sperm production starts
3	Ovaries	Oestradiol and progesterone rise; cycles start
4	Body	Secondary sexual characteristics

Never say the pituitary makes sex steroids, or the gonads make GnRH.

Gametogenesis (D3.1.14)

Four stages in both: mitosis, cell growth, meiosis (two divisions), differentiation.

	Spermatogenesis	Oogenesis
Site	Seminiferous tubules of testis	Ovary (completion after fertilization)
Start	Puberty onwards, continuous	Foetal development, pauses in meiosis I
Cell after meiosis I	Two equal secondary spermatocytes	One large secondary oocyte, one small polar body
Cell after meiosis II	Four equal spermatids	One large ovum, polar bodies
Functional gametes from one cell	4	1
Cytoplasm	Mostly lost in differentiation	Retained in egg as food reserve

Chromosome counts in humans: primary spermatocyte or oocyte 46, secondary cell 23 (chromosomes with two chromatids each), mature gamete 23.

Method for counting questions. Multiply the starting cells by 4 for sperm. Multiply by 1 for eggs. Polar bodies are not gametes.

Reminder: 50 primary spermatocytes give 50 × 4 = 200 sperm. 50 primary oocytes give 50 eggs.

Polyspermy (D3.1.15)

• Acrosome reaction (sperm): enzymes from the acrosome digest a route through the zona pellucida; the sperm reaches the egg membrane.
• Cortical reaction (egg): cortical granules fuse with the egg membrane and release contents that change the zona pellucida so no more sperm can pass.
• Result of failure: a zygote with three sets of chromosomes (69), which cannot develop normally.

Blastocyst and implantation (D3.1.16)

• Mitosis forms a hollow ball of cells, the blastocyst, in the oviduct.
• Outer cells attach to the endometrium, secrete enzymes, and the blastocyst embeds.
• The progesterone-primed endometrium is thick and vascular.
• Do not learn other embryo stage names. They are not required.

hCG pregnancy tests (D3.1.17)

• hCG made by the embryo, then the developing placenta, and found in blood and urine.
• Test strip has monoclonal antibodies against hCG: one set mobile with a coloured label, one set fixed in a line.
• Positive result: coloured line from labelled hCG-antibody complexes caught at the fixed line.
• Specificity comes from monoclonal antibodies binding one site on hCG.

Placenta (D3.1.18)

• Large surface area of the placental villi.
• Foetus receives oxygen, glucose, amino acids, water, minerals, vitamins, antibodies.
• Foetus loses carbon dioxide, urea, heat.
• Blood of mother and foetus do not mix.
• Harmful substances (alcohol, some drugs, some viruses) can cross.
• Benefit: foetus stays in the uterus to a later stage than in mammals without a placenta.

Pregnancy and childbirth hormones (D3.1.19)

Stage	Hormone	Source	Effect
Early pregnancy	Progesterone	Corpus luteum	Keeps endometrium
Later pregnancy	Progesterone	Placenta	Keeps endometrium; suppresses contractions
Before birth	Progesterone falls		Uterus becomes responsive to oxytocin
Birth	Oxytocin	Posterior pituitary (made in hypothalamus)	Contractions

Positive feedback loop for birth: contractions, then head presses on cervix, then impulses to hypothalamus, then more oxytocin released, then stronger contractions. It stops at birth.

HRT and CHD (D3.1.20)

Study type	Finding	Reliability
Early epidemiological (observational)	HRT users had less CHD	Confounded: HRT users had higher socioeconomic status
Later randomized controlled trials	Small increase in CHD with HRT	Randomization spreads confounders evenly

Lesson: a correlation does not prove cause and effect. Look for a confounding variable.

Method in steps: evaluating HRT evidence

1. State what was measured and in whom.
2. Say whether groups were chosen by the women or randomly assigned.
3. Calculate the risk in each group and compare.
4. Name a possible confounding variable.
5. State a conclusion limited to what the design supports.

Worked reminders

Data skill: hormone rise. FSH rises from 1.5 to 6.0 IU/L across puberty. Fold change = 6.0 / 1.5 = 4. Percentage increase = (6.0 − 1.5) / 1.5 × 100 = 300%. Always give the working, the unit and what the change means (here, more stimulation of the gonads).

Data skill: hCG doubling. If hCG goes 8, 17, 36, 75, 150 mIU/mL on days 10, 12, 14, 16 and 18, each two-day step is close to a doubling. Over 8 days the rise is 150 / 8, about 19-fold. A test with a detection threshold of 20 mIU/mL would first be positive on day 14. A test with a threshold of 40 would first be positive on day 16. A more sensitive test therefore gives an earlier result. (The values are illustrative.)

Data skill: relative risk. Risk = cases / women in the group. Relative risk = risk in treated group / risk in comparison group. A value below 1 means lower risk in the treated group, above 1 means higher. Always say whether the difference came from a randomized trial or an observational comparison.

Calculation: magnification. Magnification = image size / actual size. Convert units first. A sperm that is 50 micrometres long and appears 62 mm long in a micrograph has a magnification of 62 000 / 50 = ×1240.

Must-know distinctions

Pair	Difference
Acrosome reaction and cortical reaction	Sperm enzymes digest a path in; egg granules change the zona pellucida to keep others out
Polar body and ovum	Polar bodies have a nucleus but almost no cytoplasm; the ovum keeps the cytoplasm
Secondary oocyte and ovum	The oocyte is arrested in meiosis II; the ovum forms when sperm entry allows meiosis II to finish
GnRH and LH or FSH	GnRH comes from the hypothalamus; LH and FSH come from the anterior pituitary
Corpus luteum and placenta	Both secrete progesterone; the corpus luteum is first, the placenta later
Progesterone and oxytocin	Progesterone maintains pregnancy and suppresses contractions; oxytocin drives contractions in labour
Observational study and randomized trial	Observational groups choose themselves; randomized groups are assigned by chance, which controls confounders
Blastocyst and embryo	The blastocyst is the hollow ball; the embryo forms from its inner cells

Command-word hints

• Outline: give the main points briefly, such as the four stages in gametogenesis.
• Explain: give reasons. For polyspermy, say what the reaction does and why that stops a second sperm.
• Compare: use both sides in each sentence, for instance spermatogenesis and oogenesis in cytoplasm and in number.
• Evaluate: weigh strengths and limitations of evidence, as for the HRT studies.
• Suggest: use reasoning from the data or from principles you know.

Quick self-test

1. Name the hormone released by the hypothalamus that starts the rise in LH and FSH at puberty.
2. A micrograph shows an oocyte 45 mm across. Its actual diameter is 0.15 mm. Calculate the magnification.
3. How many sperm come from 50 primary spermatocytes?
4. Why do polar bodies contain almost no cytoplasm?
5. Which reaction involves exocytosis of cortical granules?
6. How many chromosomes would a human zygote have if two sperm fused with the egg?
7. Name the layer of the blastocyst that attaches first to the endometrium.
8. Why are monoclonal antibodies used in pregnancy tests?
9. A hormone rises from 4 to 9 units. Calculate the percentage increase.
10. State the main source of progesterone in late pregnancy.
11. Why is oxytocin release in labour an example of positive feedback?
12. Give a confounding variable in early HRT studies.

Answers

1. Gonadotropin-releasing hormone (GnRH).
2. 45 / 0.15 = ×300.
3. 50 × 4 = 200.
4. The divisions are unequal, so nearly all cytoplasm stays in the egg as a food store.
5. The cortical reaction.
6. 23 × 3 = 69.
7. The outer cells (trophoblast).
8. They bind specifically to hCG, so the test responds to it and not to similar hormones.
9. (9 − 4) / 4 × 100 = 125%.
10. The placenta.
11. The effect (stronger contractions, more pressure on the cervix) causes more oxytocin release, which increases the original change.
12. Socioeconomic status.

Where marks are usually lost

• Mixing up the acrosome reaction and the cortical reaction.
• Giving the pituitary as the source of sex steroids.
• Saying four eggs come from one primary oocyte.
• Writing “the egg” for the secondary oocyte without noting meiosis II finishes only after sperm entry.
• Describing oxytocin feedback as negative.
• Stating that HRT “causes” or “prevents” heart disease from observational data alone.
• Forgetting units or the working in magnification and percentage questions.
• Writing “hCG is released by the ovary” instead of the embryo and placenta.

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).