Human reproductive anatomy and fertilization (Biology HL)

The big idea: In sexual reproduction, two gametes (sex cells) join to make a new individual.

The male gamete is the sperm, made in the testes. The female gamete is the egg (ovum), made in the ovaries.

Both gametes are made by meiosis, so each is haploid — it carries half the normal number of chromosomes.

When a sperm and an egg fuse — an event called fertilization — their nuclei join to form a single diploid cell called a zygote, which grows into a new organism.

Gamete

A sex cell (sperm or egg) used in sexual reproduction. Gametes are haploid — they carry one set of chromosomes.

Haploid (n)

Having only one set of chromosomes. Human gametes are haploid (23 chromosomes).

Diploid (2n)

Having two sets of chromosomes — one from each parent. Most human body cells are diploid (46 chromosomes).

Meiosis

The type of cell division that makes gametes. It halves the chromosome number, so a diploid cell produces haploid cells.

Fertilization

The fusion of a sperm nucleus with an egg nucleus to form a diploid zygote.

Zygote

The single diploid cell formed when a sperm fertilizes an egg. It divides to form an embryo.

Why gametes must be haploid: If gametes were diploid, fertilization would double the chromosome number every generation.

Making gametes haploid by meiosis means that when two gametes fuse, the diploid number is restored — kept constant from one generation to the next.

The two reproductive systems are built for different jobs.

The male system makes and delivers huge numbers of small, swimming sperm. The female system makes a few large eggs, and provides a place — the uterus — where a fertilized egg can develop.

The key structures: In the male system, the testes make sperm; the sperm duct carries them to the urethra, which passes through the penis and releases them.

In the female system, the ovaries make eggs; an egg travels down the oviduct (fallopian tube) to the uterus. The oviduct is usually where fertilization takes place.

Sperm and egg are built differently: Both gametes are haploid and made by meiosis, but their structures suit their roles.

Sperm are tiny, made in huge numbers, and swim to the egg using a tail (flagellum), carrying almost no food store.

The egg is large — the biggest human cell — released in small numbers, cannot swim, and carries a large food store to nourish the early embryo.

Fertilization — step by step: Many sperm swim up the oviduct towards the egg, but only one fertilizes it.

That one sperm enters the egg; the egg membrane then changes to block any other sperm (so the zygote does not get too many chromosomes).

Finally the haploid sperm nucleus fuses with the haploid egg nucleus, combining the genetic material of both parents into a single diploid zygote.

A memory hook: Sperm = small + swims (many, with a tail). Egg = enormous + energy store (few, stays put).

And remember the equation of fertilization: haploid + haploid → diploid (n + n → 2n).

How this is tested: On Paper 1A a single-mark question often asks you to label a structure on a diagram of the male or female reproductive system, or to identify what happens during fertilization — the answer being that a sperm nucleus fuses with an egg nucleus to form a diploid zygote.

On Paper 2 a short Outline question commonly asks why diploid organisms make their gametes by meiosis — the answer centres on halving the chromosome number so that the diploid number is restored, not doubled, at fertilization.

Why this scores full marks: It links cause to effect: meiosis makes gametes haploid, and that is why fertilization restores — rather than doubles — the diploid number.

Answering only 'meiosis makes gametes' without the chromosome-number point would score just one mark.