IB DP Biology: Reproduction: sexual and asexual, human reproductive systems and flowering plants (DP Biology) – Revision Notes

These notes condense statements D3.1.1 to D3.1.12 of the IB Biology guide, first assessment 2025, for DP Biology (SL and HL). For full explanations read the study guide, then test yourself with the practice questions. Links: IB DP Biology hub, printable checklist, free 10-minute diagnostics.

Sexual v asexual (D3.1.1)

	Asexual	Sexual
Parents	1	Usually 2
Division	Mitosis	Meiosis + fusion of gametes
Offspring	Clones	Varied
Advantage	Fast; keeps a successful genotype in a stable environment	New allele combinations; some may suit a changed environment
Cost	No variation	Slower; needs a mate or pollinator

Examples of asexual reproduction: binary fission, budding, runners.

Life cycle (D3.1.2)

• Meiosis: 2n to n, makes gametes, breaks up parental allele combinations.
• Fusion of gametes (fertilization): n + n to 2n zygote, makes new combinations.
• Zygote then grows by mitosis.
• Human numbers: 46 in body cells, 23 in gametes.

Male v female (D3.1.3)

• Male gamete travels, so it is small, motile and has few food reserves.
• Female gamete does not travel, so it is large with food reserves.
• Result: males make very many gametes; females make few and often invest more in offspring.
• Magnification = image size ÷ actual size. Convert units first (1 mm = 1000 µm).

Anatomy (D3.1.4)

Male	Function
Testis	Sperm and testosterone
Scrotum	Keeps testes slightly cooler
Epididymis	Stores maturing sperm
Sperm duct	Carries sperm
Seminal vesicle, prostate	Add fluid to sperm
Urethra, penis	Deliver semen

Female	Function
Ovary	Eggs, oestradiol, progesterone
Oviduct	Egg transport; fertilization site
Uterus, endometrium	Embryo implants and develops
Cervix	Opening of uterus
Vagina	Receives semen; birth canal

Menstrual cycle (D3.1.5)

Hormone	Source	Main effect
FSH	Pituitary	Follicle growth
LH	Pituitary	Surge triggers ovulation; corpus luteum forms
Oestradiol	Follicle (and corpus luteum)	Thickens endometrium; feedback on FSH and LH
Progesterone	Corpus luteum	Maintains endometrium; inhibits FSH and LH

Sequence in steps:

1. Low progesterone: menstruation, FSH rises.
2. FSH: follicles grow and secrete oestradiol.
3. Oestradiol, low to moderate: negative feedback on FSH and LH.
4. Oestradiol high for long enough: positive feedback, LH surge.
5. Ovulation.
6. Corpus luteum: progesterone, negative feedback, endometrium maintained.
7. No pregnancy: corpus luteum breaks down, progesterone falls, menstruation.

Ovarian cycle plus uterine cycle = menstrual cycle. Ovulation is about the middle of a 28-day cycle.

Fertilization (D3.1.6)

1. Sperm and egg cell membranes fuse.
2. Sperm nucleus enters; tail and mitochondria are destroyed.
3. Nuclear membranes dissolve.
4. All condensed chromosomes join in one mitosis.
5. Two diploid nuclei form (2n = 46).

IVF (D3.1.7)

• Natural hormone secretion suspended.
• Artificial hormone doses induce superovulation.
• Eggs collected, fertilized in a dish, embryo(s) transferred to the uterus.
• Benefit: several eggs, so a better chance of an embryo. Risks: over-stimulation of ovaries, multiple pregnancy.

Flowering plant reproduction (D3.1.8)

• Gametes form in pollen grains (anther) and ovules (ovary).
• Pollination: pollen anther to stigma.
• Pollen tube grows down the style to the ovule.
• Fertilization gives a zygote; mitosis gives an embryo inside the seed.
• Hermaphrodite flowers still reproduce sexually.

Insect-pollinated flower (D3.1.9)

Part	Function
Petals	Attract insects
Nectary	Nectar reward
Anther and filament	Make and present pollen
Stigma	Catches pollen
Style	Pollen tube route
Ovary, ovule	Becomes fruit, seed
Sepals	Protect the bud

Cross-pollination (D3.1.10)

• Anthers and stigma ripen at different times.
• Separate male and female flowers.
• Separate male and female plants.
• Animals or wind carry pollen between plants.

Wind-pollinated flowers: small dull petals, exposed anthers, feathery stigmas.

Self-incompatibility (D3.1.11)

• Self-pollination causes inbreeding, which reduces diversity and vigour.
• Genetic mechanisms stop pollen from the same plant from fertilizing.
• Example: a stigma with S1S2 blocks pollen carrying S1 or S2.

Seeds (D3.1.12)

• Pollination: pollen moves. Dispersal: seed moves away from the parent.
• Germination: water absorbed, gibberellin released, amylase digests starch, sugars feed respiration and growth.
• Seed dry mass falls at first, then rises once leaves photosynthesise.

Must-know distinctions

Pair	Difference
Meiosis and fertilization	Meiosis halves the chromosome number to make gametes; fertilization joins gametes and restores 2n
Ovarian and uterine cycle	Ovary: follicle, ovulation, corpus luteum. Uterus: endometrium thickens, then is shed
Negative and positive feedback	Negative keeps FSH and LH low or steady; positive builds the LH surge
Pollination and fertilization	Pollination moves pollen to a stigma; fertilization is fusion of the gametes, later, in the ovule
Pollination and dispersal	Pollen moves, or a seed moves away from the parent
Cross- and self-pollination	Different plants, or the same plant
Oestradiol and progesterone	Oestradiol: repairs and thickens the lining, drives the surge. Progesterone: maintains the lining, inhibits FSH and LH
Wind and insect flowers	Wind: small, dull, exposed anthers, feathery stigma. Insect: coloured petals, nectar, sticky pollen

Reading a menstrual cycle graph

Work through a hormone graph in this order:

1. Identify each line from its shape. LH has one tall, narrow peak. FSH has a smaller peak at the same time and a rise at the start. Oestradiol rises gradually, peaks just before the LH surge, then rises again in the second half. Progesterone is low until after ovulation, then high.
2. Mark ovulation just after the LH peak, near the middle of the cycle.
3. Mark the corpus luteum phase wherever progesterone is high.
4. Link the endometrium to the hormones: it thickens as oestradiol rises, and is maintained while progesterone is high. It is shed when both fall.
5. Quote data: give values with units and the day numbers, and use a calculation such as percentage change if the question asks for one.

Percentage change = (new value − old value) ÷ old value × 100.

Worked reminders

Magnification. Image 63 mm, magnification ×1400. Actual size = 63 000 µm ÷ 1400 = 45 µm.

Fertilization rate. 12 eggs collected, 9 fertilized: 9 ÷ 12 × 100 = 75%.

Germination rate. 38 of 50 seeds germinate: 38 ÷ 50 × 100 = 76%.

Seed dry mass. 52 mg to 38 mg in 6 days. Rate of loss = 14 ÷ 6 = 2.3 mg per day, which is a fall of 26.9%. Explain the fall by respiration of stored food.

Self-incompatibility ratio. Stigma S1S2, pollen donor S1S3: half the pollen (S3) can fertilize, so 50%.

Command term reminders

• Distinguish: give a difference, with both sides stated.
• Outline: a brief account of the main points.
• Explain: give reasons, with a clear link between cause and effect.
• Describe: give a detailed account of what happens, in order.
• Annotate: add notes that say what each structure does, not only its name.
• Suggest: apply your knowledge to a new situation.

Linking ideas

• Variation: meiosis, fusion of gametes, cross-pollination and self-incompatibility all increase variation. Asexual reproduction and self-pollination reduce it.
• Feedback: the menstrual cycle uses both negative and positive feedback, so it is a good example for any question on control systems.
• Cost and benefit: sperm and pollen are made in very large numbers because few reach an egg. Eggs are few because each carries a store of food.
• Energy: germination uses stored food in respiration before the seedling can photosynthesise.

Quick self-test

1. State two ways asexual offspring differ from sexual offspring.
2. State the chromosome number in a human egg cell.
3. A sperm cell has a tail 50 µm long; a drawing shows it as 40 mm. Calculate the magnification.
4. Name the hormone whose surge triggers ovulation.
5. State the source of progesterone in the second half of the cycle.
6. Endometrium thickness rises from 3 mm to 12 mm. Calculate the percentage change.
7. Outline what happens to the sperm tail after it enters an egg.
8. State why IVF clinics give FSH injections.
9. Distinguish pollination from seed dispersal.
10. State why a hermaphrodite flower is still sexual reproduction.
11. Explain why a plant with S1S2 cannot be fertilized by pollen from its own flower.

Answers

1. Asexual: genetically identical, one parent, mitosis. Sexual: varied, two gametes, meiosis (any two).
3. 40 mm = 40 000 µm; 40 000 ÷ 50 = ×800.
4. LH.
5. The corpus luteum.
6. (12 − 3) ÷ 3 × 100 = +300%.
7. The tail and mitochondria are destroyed.
8. To stimulate several follicles to grow (superovulation).
9. Pollination moves pollen to a stigma; dispersal moves seeds away from the parent plant.
10. Gametes are made by meiosis and fuse.
11. Its own pollen carries S1 or S2, which matches the stigma, so pollen growth is blocked.

Where marks are usually lost

• Saying “hormone” without naming it: write FSH, LH, oestradiol or progesterone.
• Failing to name the feedback type; positive and negative feedback do different jobs.
• Mixing up the ovarian cycle (follicle, corpus luteum) and the uterine cycle (endometrium).
• Using “fertilization” for pollination or implantation.
• Forgetting units conversion in magnification.
• Stating that IVF uses hormones to “help fertilization” instead of inducing superovulation.
• Describing a labelled flower part without its function.
• Saying that self-incompatibility prevents pollination, when it blocks fertilization after pollen lands.

Official syllabus

IB Biology guide, first assessment 2025, published by the International Baccalaureate Organization, statements D3.1.1 to D3.1.12.