Structure, specialisation, and the fusion of nuclei in mammals and flowering plants.
The gametes that make sexual reproduction possible are formed in a process called gametogenesisThe formation of gametes by meiosis in the sex organs.. Meiosis is just one stage — it causes the reduction of genetic material, producing haploid cells. Mitosis provides the precursor cells. In human males, gametogenesis involving meiotic and mitotic divisions happens constantly from puberty onwards. In females, mitotic divisions occur before birth to form diploid primary oocytesA cell in an ovary which may form an ovum if it undergoes meiotic division., which remain inactive until after puberty.
The male gametes of most mammalian species are around 50 μm long. A "queue" of 200 sperm placed end to end would stretch just 1 cm. They have several critical tasks: carry genetic information, remain motile in fluid, and penetrate the protective barrier around the ovum using enzymes contained in the acrosomeThe region at the head of the sperm that contains enzymes to break down the protective layers around the ovum..
Spermatozoa of most animals are very similar in size. Ova vary tremendously — the human ovum is about 0.1 mm across, while an ostrich egg is around 6 mm in diameter. Eggs do not move on their own, so they don't need contractile proteins. They usually contain food for the developing embryo and have a protective layer of jelly known as the zona pellucidaA layer of protective jelly around the unfertilised ovum..
Use the drawing pad below to sketch a sperm cell from memory. On the right, list the key structures and their functions. This evidence-based technique combines visual and verbal channels to boost retention.
For sexual reproduction to be successful, the gametes must meet and fuse. The ovum is fully viable and able to receive the male gamete for only a few hours. The sperm will survive a day or two in the female reproductive tract. There is little evidence that sperm are attracted to the egg — their meeting seems entirely a matter of chance.
As sperm move through the female reproductive tract, the acrosomeThe region at the head of the sperm containing digestive enzymes. region matures so it can release enzymes and penetrate the ovum. Many sperm cluster around the ovum, and as the heads touch the surface, the acrosome reaction is triggered. Enzymes are released that digest the follicle cells and the zona pellucida. One sperm alone does not produce enough enzyme — this is one reason for the very large number of sperm released in ejaculation.
Eventually, one sperm wriggles through and touches the oocyte membrane. This triggers the second meiotic division, providing a haploid ovum nucleus. It is essential that no other sperm enter now, as polyspermyThe fertilisation of an egg by more than one sperm, producing a nucleus with too many chromosomes. would produce a nucleus with too many chromosomes.
First, ion channels change the electrical charge of the ovum membrane (from negative to positive inside), providing a fast, temporary block. Then the cortical reactionThe release of cortical granules from the ovum cytoplasm that harden the zona pellucida into a fertilisation membrane. takes place: cortical granules release enzymes into the zona pellucida, destroying sperm-binding sites and forming a tough fertilisation membrane — the permanent block.
Scientists tested the effect of an enzyme inhibitor (which blocks acrosomal enzymes) on fertilisation success. Study the data below:
| Inhibitor concentration (μmol dm⁻³) | % eggs fertilised |
|---|---|
| 0 (control) | 87 |
| 5 | 64 |
| 10 | 38 |
| 20 | 12 |
| 50 | 2 |
The formation of gametes in flowering plants is more complex because plants have two phases to their life cycles. The sporophyte generationThe diploid generation in plants that produces spores by meiosis. is diploid and produces spores by meiosis. The resulting gametophyte generationThe haploid generation in plants that gives rise to the gametes by mitosis. is haploid and produces gametes by mitosis. In flowering plants, the two phases have been combined into one plant — the main body is the diploid sporophyte.
The anthers of flowering plants are equivalent to the testes of animals. Meiosis occurs here, producing vast numbers of pollen grains that carry the male gametes. These are known as microgametesThe male gametes produced in plants, carried within the pollen grains.. Each pollen grain contains two haploid nuclei: the tube nucleus (controls pollen tube growth) and the generative nucleus (fuses with the female nucleus).
The ovary of the plant is equivalent to the animal ovary. Meiosis results in a relatively small number of ova contained within ovules inside the ovary. The ovule is attached to the ovary wall by a pad of special tissue called the placenta. Inside the ovule, the embryo sac forms the gametophyte generation. The mature embryo sac (the megagameteThe female gamete — the egg cell in plants.) contains: three antipodal cells, two polar nuclei, the female gamete (egg cell), and two synergids.
The male gamete is contained within the pollen grain. The female gamete is embedded deep in the tissue of the ovary. The pollen grain lands on the surface of the stigma during pollinationThe transfer of pollen from the anther to the stigma, often from one flower to another.. The molecules on the pollen surface and the stigma interact — if they "recognise" each other as the same species, the pollen grain begins to grow or germinate. This recognition mechanism also helps prevent self-fertilisation.
A pollen tube begins to grow out from the tube cell, down through the style. The tip produces hydrolytic enzymes to digest the tissue of the style, using the digested tissue as a nutrient source. As it grows, the tube nucleus and generative nucleus travel down it. The generative nucleus divides by mitosis to form two male nuclei. Eventually the tip of the tube passes through the micropyle of the ovule.
This is the unique feature of flowering plants. Once the pollen tube enters the ovule via the micropyle, the two male nuclei are released. Double fertilisationThe process in plants where one male nucleus fuses with the egg cell (→ diploid zygote) and the other fuses with the two polar nuclei (→ triploid endosperm nucleus). occurs:
Fusion 1: One male nucleus fuses with the egg cell → produces the diploid zygote (2n) → develops into the embryo.
Fusion 2: The other male nucleus fuses with the two polar nuclei → produces the triploid endosperm nucleus (3n) → the endosperm provides food when the seed germinates.
One of the highest-value exam skills is making comparative statements — not just describing each process separately, but writing sentences that explicitly link similarities and differences.
| Feature | Mammals | Flowering Plants |
|---|---|---|
| Male gamete delivery | Sperm swim using a flagellum (motile) | Pollen tube grows through the style (non-motile gamete) |
| Role of enzymes | Acrosomal enzymes digest zona pellucida | Pollen tube tip secretes hydrolytic enzymes to digest style tissue |
| Number of fusions | One: sperm nucleus + ovum nucleus → diploid zygote | Two (double fertilisation): zygote (2n) + endosperm (3n) |
| Polyspermy prevention | Electrical change + cortical reaction → fertilisation membrane | Pollen-stigma recognition prevents incompatible gamete fusion |
| Where nuclei fuse | In the oviduct (fallopian tube) | Inside the embryo sac within the ovule |
| Nutrient supply | Initially from ovum cytoplasm; then placenta (uterus) | Triploid endosperm provides food for germination |
Think of two supporting points and two counterpoints before revealing the model answer.
Supporting points:
1. There is little evidence that sperm are chemically attracted to the ovum — their meeting appears largely random. 2. The ovum is viable for only a few hours, and sperm survive only 1–2 days, so timing must coincide by chance.
Counterpoints:
1. The huge number of sperm released (~200–300 million per ejaculation) significantly increases the probability of at least one reaching the ovum. 2. Biological mechanisms improve success: sperm capacitation in the reproductive tract, the acrosome reaction being triggered specifically at the zona pellucida, and internal fertilisation placing gametes close together — these are not "chance" but evolved adaptations.
"Explain fertilisation in flowering plants from pollination to formation of the zygote and endosperm."
Use this checklist to plan your answer. Tick off each point as you include it:
Before you leave, answer these three quick questions to check your understanding:
1. List three ways a sperm cell is adapted for its function.
2. What is the key difference between the acrosome reaction and the cortical reaction?
3. Explain why double fertilisation is called "double" — what are the two products?