IB Biology HL D3.1 Reproduction Notes

Asexual reproduction uses mitosis without gametes or fertilization, so offspring are clones of the parent. Sexual reproduction uses meiosis and fertilization, so alleles are reshuffled and offspring show genetic variation. High-scoring answers link the method to clone production or variation, not just to the number of parents.

Meiosis makes haploid gametes, which prevents chromosome number doubling every generation. Fertilization then fuses two haploid gametes to restore the diploid number. Because different gametes meet at random, each zygote receives a unique combination of maternal and paternal chromosomes.

Meiosis halves chromosome number; fertilization restores it and creates a unique zygote.

In biology, male and female are defined by gamete type, not by body size or behaviour. Male gametes are small, numerous, and usually motile. Female gametes are larger, fewer, and contain resources for early development. That resource difference explains why eggs are bigger and sperm are produced in high numbers.

Learn reproductive anatomy as routes and functions. In males, testes make sperm, the epididymis stores sperm, sperm ducts move sperm, glands add fluid, the urethra carries semen, and the penis delivers it. In females, ovaries release eggs, oviducts are where fertilization normally occurs, the uterus and endometrium support pregnancy, and the cervix, vagina, and vulva form the lower reproductive tract.

Structure labels are strongest when tied to the route taken by gametes or embryos.

The ovarian and uterine cycles are one coordinated story. FSH stimulates follicle growth. Rising oestradiol rebuilds the endometrium and, when high enough, triggers the LH surge. LH causes ovulation. The corpus luteum secretes progesterone, which maintains the endometrium. If pregnancy does not occur, progesterone and oestradiol fall, so the endometrium breaks down in menstruation.

Ovarian hormone changes explain both ovulation and endometrium changes.

Human fertilization normally occurs in the oviduct after sperm reaches the egg. The sperm and egg membranes fuse, the sperm nucleus enters, and the paternal and maternal nuclei combine. The result is a diploid zygote whose genome contains chromosomes from both parents.

IVF is a controlled sequence, not just fertilization in a dish. Hormones stimulate superovulation and control egg maturation so several eggs can be collected. Eggs are fertilized outside the body, embryos are cultured briefly, and one or more embryos are transferred to the uterus for possible implantation.

IVF uses hormones to obtain multiple eggs before fertilization outside the body.

Flowering plants reproduce sexually when male gametes from pollen reach female gametes in ovules. Pollination places pollen on the stigma. A pollen tube grows down toward the ovule, carrying male gametes. Fertilization forms an embryo inside a seed.

Pollination transfers pollen; fertilization produces the embryo inside the seed.

Insect-pollinated flowers are built to attract an animal and make pollen stick. Petals, scent, and nectar attract insects. Sticky pollen attaches to the insect, and a sticky stigma receives pollen. The positions of anthers and stigma matter because they put the insect in contact with the parts that transfer pollen.

Flower structures improve the chance that insects move pollen from anther to stigma.

Cross-pollination means pollen moves between different plants, which increases genetic variation. Plants promote it by separating male and female functions. Dioecy puts male and female flowers on different plants. Self-incompatibility blocks self-pollen. Different maturation times prevent a flower from using its own pollen when it is receptive.

Self-incompatibility is a recognition system. The stigma or style identifies pollen from the same plant or same compatibility type and blocks pollen-tube growth. This prevents self-fertilization, reduces inbreeding, and supports genetic variation through outcrossing.

Seed dispersal and germination solve different problems. Dispersal moves offspring away from the parent, reducing competition and spreading offspring into new places. Germination begins when a seed takes up water; enzymes become active and mobilize stored food reserves so the embryo can grow.

Dispersal spreads seeds; germination restarts embryo growth.

At puberty, the control pathway starts in the brain. GnRH from the hypothalamus stimulates the pituitary to release FSH and LH. These hormones act on the gonads, increasing oestradiol or testosterone. Those sex hormones drive primary sexual development and the secondary sexual characteristics associated with puberty.

Both spermatogenesis and oogenesis use meiosis, but they distribute cytoplasm differently. Spermatogenesis uses equal divisions and differentiation to produce four sperm. Oogenesis uses unequal cytokinesis, so one large ovum receives most cytoplasm and the remaining products become polar bodies.

Same meiotic framework, different cytoplasm distribution and gamete output.

The egg must allow one sperm in but block the rest. First, acrosome enzymes help a sperm penetrate the zona pellucida. After sperm-egg membrane fusion, cortical granules are released from the egg. They change the zona pellucida, so additional sperm cannot enter. That block prevents polyspermy and keeps the zygote genome viable.

Acrosome reaction permits entry; cortical reaction prevents extra sperm entry.

After fertilization, cleavage divisions increase cell number without major growth. The embryo becomes a morula, then a blastocyst with an inner cell mass. The blastocyst implants in the endometrium, which allows pregnancy to continue and placental development to begin.

Cleavage forms the blastocyst; implantation anchors it in the endometrium.

Early pregnancy changes the hormone signal. The early embryo and developing placenta secrete hCG, which maintains the corpus luteum so progesterone remains high. Pregnancy tests detect hCG in urine using monoclonal antibodies that bind specifically to hCG.

The placenta is an exchange organ and an endocrine organ. Placental villi give a large surface area and short diffusion distance while maternal and fetal blood remain separate. Oxygen, nutrients, antibodies, and some hormones move to the fetus; carbon dioxide and other wastes move to the mother.

Placental villi exchange materials across a barrier without mixing maternal and fetal blood.

During pregnancy, progesterone and oestradiol maintain the endometrium and help inhibit further ovulation. Near birth, oxytocin and prostaglandins stimulate uterine contractions. Contractions can trigger more oxytocin release, creating positive feedback that strengthens contractions until childbirth occurs.

HRT replaces declining oestradiol and/or progesterone after menopause to reduce symptoms. The focus is evaluation, not promotion. Benefits must be weighed against risks and side effects, including evidence about coronary heart disease. Observational correlations can be misleading, so randomized-trial evidence carries more weight for causal claims.

Core D3.1 route: reproduction creates offspring, gametes or pollen move, fertilization or germination follows, and the consequence is variation, embryo formation, seed production, or successful early growth.

HL D3.1 is about ordered mechanisms and evidence judgment: endocrine control, gametogenesis contrast, one-sperm fertilization, early embryo stages, hCG detection, placental exchange, birth feedback, and HRT evaluation.

Core D3.1 exam questions usually combine reproduction strategy with gamete formation, fertilization, human cycles, IVF, plant pollination, or seed germination. Treat each answer as a route: name the process, say what moves or changes, then give the biological consequence.

Explain how reproductive processes increase the chance of successful offspring production in animals or flowering plants.

HL D3.1 moves from reproductive events into control and evidence. The strongest answers keep sequences in order: endocrine axis at puberty, gametogenesis outcomes, fertilization blocks, blastocyst implantation, hCG testing, placental exchange, childbirth feedback, and HRT evaluation.

Explain how hormonal control and early-development mechanisms support human reproduction, and evaluate one reproductive-health intervention.