IB Biology · Theme D: Continuity and change · SL and HL
D3.1 Reproduction
A one-page summary of D3.1 Reproduction, the key terms to know, and sample exam questions with answers. For the full lesson, open the illustrated revision slides or practise in the app.
Guiding questions
How does asexual or sexual reproduction exemplify themes of change or continuity?
What changes within organisms are required for reproduction?
What D3.1 covers
Asexual and sexual reproduction
- D3.1.1Asexual reproduction copies; sexual reproduction shuffles
- D3.1.2Meiosis breaks up allele combinations and fertilization makes new ones
- D3.1.3The male gamete travels, so it is small and the egg is large
The human reproductive system
- D3.1.4The female-typical system: where eggs are made, met and nurtured
- D3.1.4The male-typical system: making, storing and delivering sperm
- D3.1.5The menstrual cycle: ovarian and uterine events run together
- D3.1.5Positive feedback triggers ovulation; negative feedback steadies the cycle
- D3.1.6Fertilization ends in one joint mitosis
- D3.1.7IVF uses hormones to override the natural cycle
Reproduction in flowering plants
- D3.1.8Flowering plants reproduce sexually, even when one flower has both sexes
- D3.1.9An insect-pollinated flower is built to attract and dust its visitor
- D3.1.10Flowers have several ways to promote cross-pollination
- D3.1.11Self-incompatibility blocks a plant’s own pollen
- D3.1.12Seed dispersal moves seeds away; it is not pollination
- D3.1.12Germination mobilizes food reserves to grow the embryo
HL — puberty, gametes, pregnancy and evidence
- D3.1.13 · HLGnRH switches puberty on; sex hormones make the changes
- D3.1.14 · HLGametogenesis gives four small sperm but one large egg
- D3.1.15 · HLTwo reactions let one sperm in and keep the rest out
- D3.1.16 · HLThe embryo becomes a hollow ball that implants
- D3.1.17 · HLPregnancy tests detect hCG with monoclonal antibodies
- D3.1.18 · HLThe placenta exchanges materials across a huge surface
- D3.1.19 · HLProgesterone keeps pregnancy going: first the corpus luteum, then the placenta
- D3.1.19 · HLFalling progesterone lets oxytocin drive childbirth by positive feedback
- D3.1.20 · HLCorrelation is not causation: the HRT and heart disease story
D3.1 Reproduction: summary
Sexual and asexual
- Asexual: identical clones, one parent, suits a stable environment. Sexual: meiosis and fertilization make new allele combinations. The male gamete travels, so it is small; the egg is large.
Human system and cycle
- Draw and annotate both systems. FSH grows a follicle, an LH surge (positive feedback) triggers ovulation, the corpus luteum makes progesterone (negative feedback). IVF: suspend hormones, FSH for superovulation.
Fertilization
- Membranes fuse, nucleus enters, tail and mitochondria destroyed, nuclear membranes dissolve, one joint mitosis gives two diploid nuclei.
Flowering plants
- Gametes in pollen grains and ovules; pollination then fertilization: always sexual. Cross-pollination by timing, separate flowers or plants, animals, wind; self-incompatibility blocks own pollen. Dispersal is not pollination; germination mobilizes reserves.
HL · Gametes and puberty
- GnRH then LH and FSH then testosterone and oestradiol. Spermatogenesis: four equal sperm. Oogenesis: one large egg and polar bodies. Acrosome reaction lets one sperm in; cortical reaction blocks the rest.
HL · Pregnancy and evidence
- Blastocyst implants; hCG test uses monoclonal antibodies; placental villi exchange. Progesterone (corpus luteum then placenta) sustains pregnancy; its fall allows oxytocin positive feedback. HRT: confounding by status; RCTs show small CHD increase.
Key terms
- Asexual reproduction
- Reproduction by one parent that produces genetically identical offspring.
- Sexual reproduction
- Reproduction in which gametes fuse, producing offspring with new combinations of genes.
- Fertilization
- The fusion of a male gamete with a female gamete, restoring the diploid number.
- Zygote
- The diploid cell formed when two gametes fuse.
- Endometrium
- The lining of the uterus, where an embryo implants.
- Menstrual cycle
- The ovarian and uterine cycles together, regulated by FSH, LH, oestradiol and progesterone.
- Corpus luteum
- The structure formed from the ruptured follicle after ovulation, which secretes progesterone.
- Superovulation
- The maturation of several follicles at once, induced in IVF by artificial doses of hormones.
- Pollination
- The transfer of pollen from an anther to a stigma.
- Cross-pollination
- Pollination in which pollen is carried to a different plant.
- Hermaphroditic
- Having both male and female reproductive parts in the same flower.
- Self-incompatibility
- Genetic mechanisms that stop a plant’s own pollen from fertilizing its ovules.
- Inbreeding
- Reproduction between closely related individuals, which reduces genetic diversity and vigour.
- Seed dispersal
- The movement of seeds away from the parent plant.
- Germination
- The growth and development of the embryo in a seed, supported by mobilization of food reserves.
- GnRH
- Gonadotropin-releasing hormone, released by the hypothalamus; its increase triggers the rise in LH and FSH at puberty.
- Polar body
- A small cell formed by unequal division of cytoplasm in oogenesis, which degenerates.
- Acrosome reaction
- Release of enzymes from the sperm’s acrosome that digest a path through the zona pellucida.
- Cortical reaction
- Release of the egg’s cortical granules after fusion, which alters the zona pellucida to block other sperm.
- Polyspermy
- Fertilization of an egg by more than one sperm.
- Blastocyst
- A hollow ball of cells with a trophoblast and an inner cell mass, which implants in the endometrium.
- hCG
- Human chorionic gonadotropin, secreted by the embryo and placenta and detected by pregnancy tests.
- Oxytocin
- The hormone whose positive feedback loop drives the contractions of childbirth.
- Confounding variable
- A variable linked to both the supposed cause and the outcome, which can create a misleading correlation.
Sample exam questions
Three of the 80 multiple-choice questions for D3.1. Try each one before opening the answer.
Question 1. Which events in a sexual life cycle produce new combinations of alleles?
- Mitosis and fertilization
- Mitosis only
- Meiosis and fertilization
- Fertilization only
Show the answer
Answer: C. Meiosis breaks up the combinations of alleles each parent inherited, and fertilization brings together alleles from two different parents.
Question 2. A strawberry plant produces new plants on runners and also produces seeds from its flowers. Which statement compares the two kinds of offspring?
- Plants from seeds are clones; plants from runners vary.
- Plants from runners are clones; plants from seeds vary.
- Both kinds of offspring are clones of the parent.
- Both kinds of offspring differ from the parent.
Show the answer
Answer: B. Runners are asexual reproduction, so their plantlets are genetically identical to the parent. Seeds come from the fusion of gametes, so each has a new combination of genes.
Question 3. In many animal species, males compete for mates, while females are more selective. Which difference between the gametes is the basis of this?
- Females make more gametes than males do.
- Sperm are larger and much more costly to make than eggs.
- Eggs and sperm cost the same to make.
- Each egg is costly, but sperm are cheap and many.
Show the answer
Answer: D. Differences in the number and cost of gametes lead to different reproductive strategies. A female invests heavily in each egg, while a male can produce huge numbers of cheap sperm.
Linking questions
Questions that connect D3.1 to other parts of the course, the kind that come up in Paper 2.
- Meiosis is a reduction division and a source of variation (D2.1.9, D2.1.11). Explain how meiosis and the fusion of gametes together generate new combinations of alleles in a sexual life cycle, and why the chromosome number stays constant from one generation to the next. (see D2.1)
- The menstrual cycle is regulated by the hypothalamus, the pituitary gland and the ovary (D3.1.5, C3.1.13). Explain how positive feedback and negative feedback regulate the levels of LH and FSH during the cycle. (see C3.1)
- How can interspecific relationships assist in the reproductive strategies of living organisms? Using an insect-pollinated flower (D3.1.9, D3.1.10) as an example, discuss how the relationship between a flower and its pollinator can benefit both species (see mutualism, C4.1.12). (see C4.1)
- What are the roles of barriers in living systems? Explain how self-incompatibility in flowering plants (D3.1.11) acts as a barrier to self-fertilization, and outline why this increases genetic variation within a species, linking to evolution as change in the heritable characteristics of a population (A4.1.1).
Practise D3.1
80 quiz questions6 data questions5 exam questionsmarkschemes included
Study notes, every question and full markschemes for D3.1 are in the app with Pro. Two lessons are completely free to try: A1.1 Water and B1.1 Carbohydrates and lipids.