Reproduction — IB Diploma Biology HL
IB Biology D3.1: sexual and asexual reproduction, human reproductive systems, the menstrual cycle, IVF, flowering plant reproduction and HL pregnancy.
Revision notes
11 short notes, in the order of the specification. Each one in short:
Asexual reproduction produces genetically identical offspring from one parent, while sexual reproduction produces offspring with new gene combinations by meiosis and fusion of gametes. Asexual reproduction suits individuals already adapted to an existing environment. Sexual reproduction creates the variation needed for adaptation to a changed environment, because meiosis breaks up parental allele combinations and fertilization makes new ones.
The prime difference between the sexes is that the male gamete travels to the female gamete, so it is smaller with less food reserves than the egg. Males therefore produce far more gametes. The human male system makes and delivers sperm; the female system makes eggs and supports a developing embryo and foetus.
The menstrual cycle is the ovarian and uterine cycles together, controlled by FSH and LH from the pituitary and oestradiol and progesterone from the ovary. FSH stimulates follicle growth; rising oestradiol thickens the endometrium and, by positive feedback, causes the LH surge that triggers ovulation. Progesterone from the corpus luteum maintains the endometrium and inhibits FSH and LH.
Fertilization in humans is the fusion of a sperm's cell membrane with the egg cell membrane, after which the sperm nucleus enters the egg and the tail and mitochondria are destroyed. Both nuclear membranes dissolve and all the chromosomes join one mitosis, producing two diploid nuclei. In IVF, normal hormone secretion is suspended and artificial hormone doses induce superovulation.
Reproduction in flowering plants is sexual: male gametes are produced inside pollen grains and female gametes inside ovules. Pollination transfers pollen from an anther to a stigma; a pollen tube then grows down the style to an ovule, where a male gamete fuses with the egg cell to produce a zygote that develops into an embryo.
Cross-pollination is the transfer of pollen between flowers on different plants. Plants promote it by different maturation times for pollen and stigma, separate male and female flowers or plants, and transfer by animals or wind. Self-incompatibility mechanisms make gametes that fuse come from different plants, preventing inbreeding, which decreases genetic diversity and vigour.
Seed dispersal is the movement of seeds away from the parent plant, whereas pollination is the transfer of pollen from anther to stigma before fertilization. Dispersal reduces competition and colonises new areas. During germination the seed absorbs water, the embryo grows, and stored starch, proteins and lipids are mobilized by enzymes to supply the growing embryo.
Puberty is triggered when the hypothalamus increases its release of gonadotropin-releasing hormone (GnRH) in late childhood, causing more LH and FSH release and so more sex hormone production. Spermatogenesis and oogenesis both involve mitosis, cell growth, two divisions of meiosis and differentiation, but give many small sperm and few eggs with much cytoplasm.
Polyspermy, fertilization of an egg by more than one sperm, is prevented by the cortical reaction. The acrosome reaction lets a sperm digest a path through the zona pellucida; when it fuses with the egg, cortical granules release enzymes that change the zona pellucida so other sperm cannot pass. The embryo then forms a blastocyst, which implants in the endometrium.
A pregnancy test detects human chorionic gonadotropin (hCG), a hormone produced by the embryo and developing placenta, using monoclonal antibodies that bind to hCG in urine. The placenta is the exchange organ between maternal and foetal blood; its villi give a large surface area, and it lets the foetus stay in the uterus to a later stage of development.
Pregnancy is maintained by progesterone, secreted first by the corpus luteum and then by the placenta. Childbirth is triggered when progesterone levels fall, allowing oxytocin secretion to increase; oxytocin causes uterine contractions, which stimulate more oxytocin release by positive feedback. Hormone replacement therapy studies show that a correlation with lower heart disease was not cause and effect.
Exam questions
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Quick check questions
State one advantage of asexual reproduction.
Show answer
Offspring are genetically identical to a parent that is already adapted to the existing environment.
Which hormone surge triggers ovulation, and what causes it?
Show answer
An LH surge, caused by positive feedback from high oestradiol levels.
What happens to the sperm's tail and mitochondria at fertilization?
Show answer
They are destroyed; only the sperm nucleus contributes.
What is the function of the stigma?
Show answer
A sticky surface that receives pollen.
HL only Which reaction prevents polyspermy?
Show answer
The cortical reaction, which alters the zona pellucida so other sperm cannot pass through.
Frequently asked questions
What is the difference between sexual and asexual reproduction?
Asexual reproduction uses one parent and produces genetically identical offspring, while sexual reproduction fuses gametes to give offspring new gene combinations. Asexual reproduction suits a parent already adapted to an existing environment. Sexual reproduction produces the variation needed for a population to adapt when the environment changes, because meiosis and fertilization reshuffle alleles.
How do hormones control the menstrual cycle?
FSH from the pituitary stimulates a follicle to grow and secrete oestradiol, which thickens the endometrium. High oestradiol causes an LH surge by positive feedback, triggering ovulation. The corpus luteum then secretes progesterone, which maintains the endometrium and inhibits FSH and LH by negative feedback. When progesterone falls, menstruation begins.
Why are sperm small and eggs large?
Sperm are small because the male gamete has to travel to the female gamete, so it carries very little cytoplasm or food reserves. The egg does not move, so it can be large and store food for the early embryo. As a result, males produce far more gametes than females.
How do plants prevent self-pollination?
Plants promote cross-pollination by releasing pollen and making the stigma receptive at different times, having separate male and female flowers or separate male and female plants, and relying on animals or wind to carry pollen. Many species also have self-incompatibility genes that stop pollen from the same plant growing a pollen tube.
How does a pregnancy test work?
HL only A pregnancy test detects hCG, a hormone made by the embryo and developing placenta, in urine. Mobile monoclonal antibodies with a coloured marker bind hCG; fixed antibodies in the test line then trap this complex, producing a coloured line. A control line of fixed antibodies binds the mobile antibodies to show the test worked.
Written and checked against the IB Biology HL specification · Updated October 2026