What you need to know
- 3.12Mammalian fertilisationThe fusion of the nuclei of a male and a female gamete to form a diploid zygote.: acrosomeA vesicle at the tip of the sperm head containing enzymes that digest the zona pellucida. reaction, cortical reactionAfter one sperm enters, cortical granules release their contents, which harden the zona pellucida and block entry of further sperm. and fusion of nuclei.
- 3.13Flowering-plant fertilisation from pollen-tube growth to nuclear fusion.
By the end, you should be able to…
- Sequence mammalian fertilisation.
- Explain the block to polyspermy.
- Explain pollen-tube growth and double fertilisation.
Build your understanding
Mammals: contact, entry and nuclear fusion
In the school-level sequence, sperm contact with the egg’s surrounding layers is associated with the acrosomeA vesicle at the tip of the sperm head containing enzymes that digest the zona pellucida. reaction: acrosomal enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction. are released, helping the sperm penetrate towards the egg cell surface membraneThe partially permeable membrane around a cell that controls which substances enter and leave.. Sperm and egg membranes fuse and the sperm nucleusThe organelle that contains the cell’s chromosomes, surrounded by a nuclear envelope with pores. enters the egg cytoplasmEverything inside the cell surface membrane except the nucleus: the fluid cytosol and the organelles..
Sperm entry triggers the cortical reactionAfter one sperm enters, cortical granules release their contents, which harden the zona pellucida and block entry of further sperm.: cortical granules release their contents by exocytosisReleasing material from a cell when a vesicle fuses with the cell surface membrane. It uses ATP.. Changes to the zona pellucidaThe glycoprotein layer around a mammalian egg cell. Sperm must digest a path through it. help prevent entry of additional sperm, reducing polyspermy. This preserves the normal chromosome-set number.
The male and female haploidHaving one set of chromosomes (n), as in gametes. nuclei form pronuclei and their genetic material combines to establish the diploidHaving two sets of chromosomes (2n), one from each parent. zygoteThe diploid cell formed when two gametes fuse at fertilisation. nucleus. This is described as fusion of nuclei at specification level. fertilisationThe fusion of the nuclei of a male and a female gamete to form a diploid zygote. restores diploidy; it does not double each gameteA haploid sex cell, such as a sperm or an egg cell.’s chromosomeA long DNA molecule wound around histone proteins. After replication it is two sister chromatids joined at a centromere. by DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code. replication.
Plants: pollen is not the sperm cell
Pollination transfers pollen to a stigma. A compatible pollen grain germinates and grows a pollen tube down the style towards an ovule in the ovary. The tube grows at its tip and carries the male gametes; they do not swim down the style.
The generative cell or nucleus divides by mitosisNuclear division that produces two genetically identical nuclei with the same number of chromosomes as the parent. to form two male gametes or nuclei; this can occur before or during pollen-tube growth depending on the speciesA group of organisms that can interbreed to produce fertile offspring.. The tube nucleus supports tube activity and does not fertilise the egg. The tube enters the ovule through the micropyle and releases its contents into the embryo sac.
Two fusions, two outcomes
One haploid male nucleus fuses with the haploid egg nucleus to produce a diploid zygote, which develops into an embryo. The other male nucleus fuses with the two haploid polarHaving an uneven distribution of charge, so the molecule has partially charged ends. Polar molecules dissolve in water. nuclei, producing a triploid primary endosperm nucleus in the standard flowering-plant model. Endosperm supports the developing embryo.
This is double fertilisation: the two male nuclei have different fusion partners. The ovule develops into a seed and the ovary develops into a fruit in the usual model. Pollination can occur without successful pollen-tube growth or fertilisation.
Apply the sequence to an investigation
For the specification’s recommended pollen-tube investigation, compare a defined factor such as sucroseA disaccharide of glucose and fructose. It is the sugar transported in phloem. concentration while controlling species, pollen age, temperature, pHA measure of how acidic or alkaline a solution is, based on its H⁺ ion concentration. Below 7 is acidic, above 7 is alkaline. and incubation time. Use teacher-approved culture conditions, replicate samples and a consistent criterion for germination.
Measure tube lengths with calibrated microscopy and report both germination proportion and tube length where useful: a treatment can affect these differently. This is an additional investigation, not a replacement for CP6.
NOT: fertilisation is not the flagellum moving. It includes fusion of nuclei.
- One sperm normally contributes its nucleus
- The block reduces polyspermy
- Diploid number is restored, not doubled again by replication
- A pollen tube is transport; fusion is fertilisation
Two traps this diagram blocks
- Do not call the acrosome reaction the cortical reaction. One is sperm, one is egg.
- Do not say the zygote is haploid.
From the diagram alone
Question
Describe the roles of the acrosome reaction and the cortical reaction in mammalian fertilisation. [4]
Show a model answer
In the acrosome reaction, enzymes from the sperm acrosome digest a path through the zona pellucida so one sperm can reach the egg membrane. After entry, the cortical reaction releases cortical granule contents and the zona changes, which helps stop further sperm entering. The nuclei can then fuse.
| Fusion partners | Product | Ploidy |
|---|---|---|
| Male nucleus + egg nucleus | Zygote → embryo | 2n |
| Male nucleus + two polar nuclei | Primary endosperm nucleus | 3n |
Make your reasoning visible
Name both plant products
Egg + male nucleusThe organelle that contains the cell’s chromosomes, surrounded by a nuclear envelope with pores. → diploidHaving two sets of chromosomes (2n), one from each parent. zygoteThe diploid cell formed when two gametes fuse at fertilisation.. Two polarHaving an uneven distribution of charge, so the molecule has partially charged ends. Polar molecules dissolve in water. nuclei + male nucleus → triploid endosperm nucleus.
Do not substitute pollination
Pollen reaching the stigma is transport; fertilisationThe fusion of the nuclei of a male and a female gamete to form a diploid zygote. requires fusion of male and female nuclei.
Use the correct reaction
The cortical reactionAfter one sperm enters, cortical granules release their contents, which harden the zona pellucida and block entry of further sperm. belongs to the egg and blocks further entry; it does not digest a route into the egg.
Avoid this misconception
The claim: Both male nuclei in a pollen tube fuse with the egg.
The correction: Only one fuses with the egg; the other joins the polar nuclei in double fertilisation.
Put fertilisation in order
Fertilisation is the fusion of two haploid nuclei. Order the events in a mammal, then in a flowering plant, where it happens twice.
Order the steps
What to do
- Pick a chain with Chain, just below these steps: Mammal or Flowering plant.
- Reorder the cards below: drag a card (on a touch screen, drag its number), tap a card then tap where it goes, or use the ↑ and ↓ arrows at its right.
- Press Check under the cards to mark each one green (right place) or red; the count shows how many are right.
- Stuck? Hint first outlines the first wrong place, then gives a clue, then places that step for you.
- Shuffle mixes the cards again; Start again returns them to the starting order. Keyboard: Space picks up, arrow keys move, Space drops.
A sperm reaches an egg cell. Put fertilisation in order.
0 of 6 in the right place
The two fusions in the ovule happen at about the same time; the order shown is conventional. Pollen-tube guidance by chemicals is left out.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
How does the cortical reaction help preserve the chromosome number of the zygote?
Show suggested answer
It changes the egg’s surrounding layer to reduce further sperm entry, preventing additional haploid sets being introduced.
Practice question
Retrieval and explanation · suggested answer
A flowering plant has 2n = 18. Give chromosome numbers in the male gamete, zygote and endosperm nucleus.
Show suggested answer
9, 18 and 27 respectively in the standard double-fertilisation model.
Practice question
Retrieval and explanation · suggested answer
Explain why successful pollination does not prove fertilisation occurred.
Show suggested answer
Compatible pollen must germinate, its tube must reach the ovule, and the male nuclei must be released and fuse with their partners.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A pollen tube delivers one functional male nucleus. Can you conclude that a normal seed will form?
Show a hint
Normal development requires two different fusion events.
Explore the reasoning
No. One nucleus cannot normally complete both fusions. Embryo formation alone does not establish normal endosperm formation, and successful delivery does not itself prove either fusion.
Recall this lesson
Lesson recall questions
Question 1 of 5: Compare the acrosome reaction with the cortical reaction.
Mammalian fertilisation sequence
Practice question
Retrieval and explanation · suggested answer
Compare the acrosome reaction with the cortical reaction.
Lesson: 3B 5: Fertilisation in Mammals and Plants
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Fertilisation outside the body
3B · Fertilisation in Mammals and PlantsConventional IVF brings eggs and sperm together in a laboratory. It separates the location of fertilisation from the biological need for successful gamete interaction and development. IVF is…Specification 3.12Read & use the scienceClose article
Why this matters
Conventional IVF brings eggs and sperm together in a laboratory. It separates the location of fertilisation from the biological need for successful gamete interaction and development. IVF is familiar from news and family stories, but it is not a different biological definition of fertilisation. The gamete nuclei still have to combine. In mammals, sperm entry involves the acrosome and changes at the egg surface that help prevent entry of further sperm. In flowering plants, a pollen tube delivers male nuclei to the ovule. Comparing these pathways helps separate shared ideas from organism-specific steps. A clinician can observe fertilisation in the laboratory while recognising that embryo quality and implantation require separate evidence. Public stories about IVF often compress these stages into one event; check what outcome a headline actually reports.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 3.12 know the process of fertilisation in mammals, including the acrosome reaction, the cortical reaction and the fusion of nuclei
Use the science
Why is fertilisation not equivalent to a successful pregnancy?
Explore a possible response
Fertilisation produces a zygote; subsequent embryo development and implantation are separate events. A result at one stage does not guarantee later outcomes.
Explore the source
- HFEA: in vitro fertilisationHuman Fertilisation and Embryology Authority
Two fusions inside a flower
3B · Fertilisation in Mammals and PlantsPlant researchers study how the pollen tube delivers two male gametes to the ovule. In flowering plants, one fuses with the egg and the other with the central…Specification 3.13Read & use the scienceClose article
Why this matters
Plant researchers study how the pollen tube delivers two male gametes to the ovule. In flowering plants, one fuses with the egg and the other with the central cell; these events begin embryo and endosperm development. A peer-reviewed review explains the cell communication involved. The specification begins at pollen-tube growth and ends at fusion of nuclei, so the article offers context rather than extra labels to memorise. The application matters for seed production and crop breeding, where failure at a particular stage can reduce yield. A student should still distinguish pollen transfer from actual fertilisation: arrival alone is not nuclear fusion.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 3.13 know the process of fertilisation in flowering plants, starting with the growth of a pollen tube and ending with the fusion of nuclei
Use the science
A pollen grain lands on a stigma but no seed forms. Which required step might still have failed?
Explore a possible response
The pollen tube may not have reached the ovule, or the male and female nuclei may not have fused. Pollination alone does not demonstrate fertilisation.