IGCSE Biology Reproduction notes

Asexual reproduction is a process resulting in the production of genetically identical offspring from one parent — no gametes, no fusion. Examples to recognise: binary fission in bacteria, budding in yeast, runners in strawberry plants, tubers in potatoes, and many growers’ cuttings. Sexual reproduction involves the fusion of the nuclei of two gametes to form a zygote, and produces offspring that are genetically different from each other and from the parents.

Supplement — gamete numbers: the nuclei of gametes are haploid (one set of chromosomes); when they fuse, the zygote is diploid (two sets — one from each parent). This is why fertilisation is defined as the fusion of the nuclei of gametes: each parent contributes half the chromosomes.

A labelled diagram to picture an insect-pollinated flower in section: green sepals protect the bud; bright petals attract insects; stamens — the male parts, each a filament topped by an anther that makes and releases pollen; and the carpel — the female part, a stigma (sticky pollen-receiving surface) on a style, above the ovary containing ovules.

Pollination is the transfer of pollen grains from an anther to a stigma. Fertilisation occurs when a pollen nucleus fuses with a nucleus in an ovule. Supplement: after landing on a compatible stigma, the pollen grain grows a pollen tube down through the style; the pollen nucleus travels down the tube, the tube enters the ovule, and fertilisation follows.

Supplement — self vs cross-pollination: self-pollination transfers pollen from an anther to the stigma of the same flower or another flower on the same plant; cross-pollination transfers pollen to the stigma of a flower on a different plant of the same species. Self-pollination needs no pollinator and guarantees a crop, but produces little variation and can pass on harmful recessive traits — the population is less able to respond to environmental change. Cross-pollination produces variation, aiding adaptation, but relies on pollinators or wind and is less reliable.

Seeds germinate when three environmental conditions are met: water (to activate enzymes and swell the seed), oxygen (for respiration to power growth) and a suitable temperature (so enzymes work fast enough). A labelled diagram to picture the classic test: four jars of cress seeds on cotton wool — jar 1 with water at room temperature (all conditions: germinates); jar 2 dry, room temperature (no water: no germination); jar 3 with water in a fridge (too cold: no germination); jar 4 with water at room temperature but sealed with a lid limiting oxygen (no oxygen: no germination). Only jar 1 has all three, so only jar 1 grows — each other jar differs by one condition, isolating its effect.

Fertilisation in humans is the fusion of the nuclei from a male gamete (sperm) and a female gamete (egg cell), usually in an oviduct. In early development, the zygote divides and forms an embryo — a ball of cells — which implants into the lining of the uterus.

Supplement — gamete adaptations: a sperm is built for delivery — a flagellum swims it, mitochondria power the swimming, and enzymes in the acrosome digest a path through the egg’s jelly coat. An egg cell is built for support — large energy stores for the early embryo, and a jelly coat that changes at fertilisation so no second sperm can enter. Compared side by side: sperm are small, motile and produced in huge numbers; eggs are large, non-motile and produced in small numbers.

A labelled diagram to picture the pregnant uterus: the fetus floats in amniotic fluid, inside the tough amniotic sac; the umbilical cord connects fetus to placenta, the organ embedded in the uterus lining. The functions at Core: the placenta allows exchange between maternal and fetal blood; the umbilical cord carries blood between fetus and placenta; the amniotic sac holds the fluid that protects and supports the fetus, cushioning it against bumps.

Supplement — the placenta exchange: the mother’s blood and the fetus’s blood flow close together but do not mix, separated by thin placental tissues. Across this barrier, dissolved nutrients (glucose, amino acids), oxygen and antibodies pass from mother to fetus, while carbon dioxide and urea (excretory products) pass from fetus to mother — each down its concentration gradient.

Supplement: the placenta is not a perfect shield — some pathogens and toxins can pass across it and affect the fetus (for example, viruses such as rubella, or toxins such as alcohol and nicotine). This is why maternal health and diet matter throughout pregnancy.

At puberty, testosterone (from the testes) drives the development and regulation of secondary sexual characteristics in males — such as a deeper voice, facial hair and broader shoulders — while oestrogen (from the ovaries) drives them in females — such as breast development and widened hips.

The menstrual cycle (about 28 days) has two linked stories. In the ovaries: an egg cell matures and is released (ovulation) at about the middle of the cycle. In the uterus: the lining builds up (thickening with blood vessels) ready for a possible embryo; if fertilisation does not happen, the lining breaks down — menstruation — and the cycle begins again.

Supplement — the hormone control: FSH (from the pituitary) causes an egg to mature in the ovary; oestrogen (from the ovaries) repairs and thickens the uterus lining and, with LH, triggers ovulation; after ovulation the empty follicle produces progesterone, which maintains the thick lining. In pregnancy, progesterone (and oestrogen) continue to be produced, keeping the lining intact. Falling progesterone (and oestrogen) at the end of a barren cycle triggers menstruation.

A sexually transmitted infection (STI) is an infection transmitted through sexual contact. HIV (human immunodeficiency virus) is a pathogen that causes an STI — and HIV infection may lead to AIDS, a condition in which the immune system is so damaged that other infections become life-threatening.

HIV is transmitted in body fluids: unprotected sexual contact, sharing needles, transfusion with infected blood (rare where blood is screened) and from mother to baby across the placenta or in birth/breast-feeding. It is not spread by casual contact, shared cups, or insect bites — an exam favourite.

The spread of STIs is controlled by cutting the transmission routes: limiting the number of sexual partners, using barrier protection (condoms), screening blood and organs before transfusion or transplant, not sharing needles, treating infected individuals and tracing their partners, and educating people about how STIs pass between hosts. HIV itself can be managed with antiretroviral drugs, which also reduce infectiousness.