Understanding Reproduction: IGCSE Biology Explained

Reproduction is one of the highest-mark topics on your IGCSE Biology paper. Get it wrong, and you're giving away easy points.

This topic covers everything from bacteria splitting in two to the hormonal cycle in humans. Examiners test it heavily across Papers 2, 4, and 6 because it crosses multiple syllabus sections and demands precise terminology. Here's what you need to know for IGCSE Biology, stripped down to exam essentials.

Asexual vs sexual reproduction: the core distinction

Every reproduction question starts with one divide: does the process involve one parent or two? Get this wrong and the rest of your answer falls apart.

Asexual reproduction produces genetically identical offspring (clones) from a single parent. There's no fusion of gametes. It's fast, energy-efficient, and works well when conditions are stable. Bacteria dividing by binary fission, strawberry plants sending out runners, and potato tubers all count as asexual reproduction in IGCSE Biology.

Sexual reproduction involves the fusion of two gametes (sex cells) to form a zygote. The offspring are genetically different from each other and from both parents. This genetic variation is the key advantage: it allows populations to adapt to changing environments through natural selection.

Sexual reproduction in plants

Flower structure

You will be asked to label a flower diagram in your IGCSE exam. Don't just memorise the names. Know what each part does and why it matters for reproduction.

The stamen (anther + filament) is the male reproductive organ. The carpel (stigma + style + ovary) is the female reproductive organ. Some flowers have both (bisexual flowers), while others have only one set (unisexual flowers).

Pollination: insect vs wind

Pollination is the transfer of pollen from an anther to a stigma. Self-pollination happens within the same flower or between flowers on the same plant. Cross-pollination transfers pollen between different plants of the same species, which produces greater genetic variation in the offspring.

Examiners love comparing insect-pollinated and wind-pollinated flowers. This table appears in IGCSE Biology past papers regularly.

Fertilisation, seed formation, and dispersal

After pollen lands on a compatible stigma, a pollen tube grows down through the style to reach the ovule. The male gamete nucleus travels through this tube and fuses with the female gamete nucleus inside the ovule. That fusion of nuclei is fertilisation.

Following fertilisation, the ovule develops into a seed (containing the embryo plant and a food store), and the ovary wall develops into the fruit. The fruit's job is to aid seed dispersal.

Seeds are dispersed by four main methods: wind (lightweight seeds with wings or parachute structures, such as dandelions and sycamore), animals (fleshy fruits eaten and seeds excreted, or hooks that attach to animal fur), water (waterproof seeds that float, such as coconuts), and self-dispersal (explosive pods that fling seeds outward when they dry out, such as pea pods). The purpose of dispersal is to reduce competition between parent and offspring for light, water, and soil minerals.

Germination requires three conditions: water (to activate enzymes and allow cell expansion), oxygen (for aerobic respiration to release energy for growth), and a suitable temperature (so enzymes can function at an adequate rate). Note that light is not required for germination itself.

Sexual reproduction in humans

Male and female reproductive systems

You need to know the structures and functions of both systems. Examiners may ask you to label diagrams, describe functions, or explain how structures relate to their roles in reproduction.

Male system: The testes produce sperm cells and the hormone testosterone. Sperm travel along the sperm duct (vas deferens) and mix with fluid from the prostate gland and seminal vesicles to form semen. The urethra carries semen through the penis during ejaculation. The urethra is shared with the urinary system, but the two functions never operate simultaneously.

Female system: The ovaries produce egg cells (ova) and the hormones oestrogen and progesterone. Each month, one ovary typically releases a mature egg into the oviduct (fallopian tube). Fertilisation normally occurs in the oviduct. If the egg is fertilised, the resulting embryo travels to the uterus and implants in the thickened uterus lining. The cervix is the narrow muscular opening between the uterus and the vagina.

Fertilisation, pregnancy, and birth

During sexual intercourse, sperm are deposited near the cervix. They swim through the uterus and into the oviducts. Of the millions released, only a few hundred reach the egg. If an egg is present, one sperm penetrates it and the two nuclei fuse, forming a zygote. This is fertilisation.

The zygote divides repeatedly by mitosis as it travels down the oviduct, forming a ball of cells (the embryo) that implants in the uterus lining approximately six days after fertilisation.

The placenta develops at the site of implantation and connects the embryo to the mother's blood supply. Its functions are critical and frequently examined:

• Oxygen and glucose pass from the mother's blood to the fetus by diffusion
• Carbon dioxide and urea pass from the fetus to the mother's blood for excretion
• Antibodies pass from mother to fetus, providing temporary passive immunity
• The placenta acts as a barrier, preventing most pathogens from crossing (though some, including rubella and HIV, can cross)
• The maternal and fetal blood supplies do not mix directly

The umbilical cord connects the fetus to the placenta, carrying blood between them in umbilical arteries and veins. The amniotic fluid surrounding the fetus acts as a shock absorber, protecting it from physical damage and temperature fluctuations.

Sex hormones and the menstrual cycle

This subtopic trips up students who memorise hormone names without understanding what each one triggers and when. The menstrual cycle is approximately 28 days, and the hormones work in a specific sequence.

Sexually transmitted infections

STIs are infections spread through sexual contact. You need to know about HIV specifically for IGCSE Biology.

HIV (Human Immunodeficiency Virus) attacks and destroys white blood cells, specifically helper T-lymphocytes. Over time, this weakens the immune system to the point where the body can no longer fight off infections. This condition is called AIDS (Acquired Immune Deficiency Syndrome). HIV is a virus and cannot be cured, but antiretroviral drugs can slow its progression significantly.

Gonorrhoea is a bacterial STI that causes pain during urination and abnormal discharge. Because it's caused by bacteria, it can be treated with antibiotics, though some strains are becoming antibiotic-resistant.

Prevention methods: Using condoms provides a physical barrier that prevents the transfer of pathogens during intercourse. Limiting the number of sexual partners reduces exposure risk. For HIV specifically, pre-exposure prophylaxis (PrEP) and antiretroviral treatment of infected individuals also reduce transmission rates.

Common exam mistakes

1. Confusing pollination with fertilisation. Pollination is the transfer of pollen to the stigma. Fertilisation is the fusion of the male and female gamete nuclei inside the ovule. They are separate events separated by the growth of the pollen tube.
2. Saying the blood supplies mix in the placenta. They don't. Substances diffuse across the thin placental wall between maternal and fetal blood. Always write "the blood supplies are kept separate" or "do not mix."
3. Listing light as a condition for germination. The three requirements are water, oxygen, and a suitable temperature. Light is needed later for photosynthesis, not for germination.
4. Forgetting to say "nuclei" in fertilisation definitions. Fertilisation is the fusion of the nuclei of the gametes, not the fusion of the gametes themselves. This distinction is marked precisely by Cambridge examiners.
5. Mixing up oestrogen and progesterone. Oestrogen repairs and thickens the uterus lining (days 5-14). Progesterone maintains it (days 14-28). Swapping their roles loses marks.
6. Writing "pollen" when you mean "male gamete." The pollen grain is the structure that carries the male gamete nucleus. The grain itself is not the gamete. Be precise with terminology.
7. Forgetting that cross-pollination increases variation. Self-pollination is faster and more reliable, but cross-pollination produces genetically varied offspring. Examiners want you to explain why variation matters for natural selection.

Self-check questions

1. State two advantages and two disadvantages of asexual reproduction for a wild plant population.
2. Describe three structural differences between wind-pollinated and insect-pollinated flowers, and explain how each difference relates to the method of pollination.
3. Explain the role of the placenta during pregnancy, including the direction of transfer for oxygen, glucose, carbon dioxide, and urea.
4. Name the hormone that triggers ovulation, state where it is produced, and explain what happens to the uterus lining if the released egg is not fertilised.
5. A student writes: "In fertilisation, the pollen fuses with the ovule." Identify two errors in this statement and write a corrected version.