Gametes and fertilisationEdexcel International A Level Biology: Revision notes
Section 1
Specialisation of mammalian gametes
Gametes are haploid cells that fuse at fertilisation. Each is specialised for its role.
The sperm is small and motile. Its head contains the haploid nucleus and the acrosome, a membrane-bound vesicle at the tip that holds hydrolytic (digestive) enzymes. The midpiece is packed with mitochondria, which make the ATP that powers the flagellum (tail) as it beats.
The secondary oocyte is large and non-motile, with a food store in its cytoplasm. It is surrounded by the zona pellucida, a glycoprotein layer that the sperm must penetrate. Just beneath the oocyte plasma membrane lie cortical granules, vesicles that are used after a sperm enters.
Link every feature to its function: mitochondria give ATP for the tail; the acrosome gives enzymes to digest the zona pellucida.
Section 2
Mammalian fertilisation: the acrosome reaction
When a sperm reaches the secondary oocyte it binds to the zona pellucida. This triggers the acrosome reaction: the acrosome membrane fuses with the sperm plasma membrane and releases its hydrolytic enzymes by exocytosis.
The enzymes digest a path through the zona pellucida. The sperm head then reaches the oocyte, and the plasma membranes of the sperm and the oocyte fuse, so the sperm nucleus enters the oocyte cytoplasm.
Section 3
The cortical reaction and nuclear fusion
Sperm entry triggers the cortical reaction. The cortical granules fuse with the oocyte plasma membrane and release their contents. These harden the zona pellucida, so no further sperm can bind or enter. This prevents polyspermy, which would give a zygote with too many chromosomes.
The secondary oocyte then completes meiosis II, forming the ovum and a second polar body. The haploid sperm nucleus and the haploid ovum nucleus fuse (nuclear fusion) to form the diploid zygote nucleus. In humans: 23 + 23 = 46 chromosomes.
The acrosome reaction is a sperm event; the cortical reaction is an oocyte event. Do not swap them.
Section 4
Fertilisation in flowering plants
A pollen grain lands on the stigma and germinates. A pollen tube grows down through the style, controlled by the tube nucleus, digesting a path through the tissue with enzymes. It grows towards the ovule and enters through the micropyle.
The tube reaches the embryo sac and releases the two male nuclei (gametes) formed when the generative nucleus divided by mitosis.
Section 5
Double fertilisation
Two fusions take place in the embryo sac:
- one male nucleus fuses with the nucleus of the egg cell to form the diploid zygote
- the other male nucleus fuses with the two polar nuclei of the central cell to form the triploid endosperm nucleus, which grows into a food store for the embryo
Example: in a plant with 2n = 14, male and female gametes have 7 chromosomes, so the zygote has 14 and the endosperm nucleus has 7 + 7 + 7 = 21.
Section 6
Practical: investigating pollen tube growth
Pollen grains are placed on slides in a range of sucrose concentrations, incubated at a fixed temperature, and examined under a microscope. Either the percentage of grains with a tube or the tube length (measured with an eyepiece graticule) is recorded.
Sucrose supplies energy and sets the water potential. If the concentration is too low, grains may burst or lack fuel; too high and water leaves by osmosis. For reliability, count many grains on several slides and calculate a mean; control temperature, time and pollen source.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Gametes and fertilisation
- A fertility clinic scientist is examining a sample of human sperm under the electron microscope. Each sperm cell has a head containing the nucleus and a membrane-bound vesicle at its tip, a midpiece packed with mitochondria, and a long tail (flagellum).The sperm midpiece contains many mitochondria. Explain how this adaptation helps the sperm to reach and fertilise an oocyte.2 marks
- In humans, once a sperm cell has made contact with a secondary oocyte, a series of reactions follows. The sperm releases enzymes, and the oocyte then releases the contents of vesicles lying just beneath its plasma membrane. A laboratory study uses mouse gametes. Mouse body cells contain 40 chromosomes.Explain why it is important that only one sperm nucleus fuses with the oocyte nucleus.2 marks
- A plant breeder studies a diploid flowering plant with 14 chromosomes in each body cell. Pollen grains land on the stigma of a flower. Each pollen grain contains a tube nucleus and a generative nucleus, and the generative nucleus divides to give two male nuclei. The ovule contains an embryo sac with an egg cell and a central cell that has two polar nuclei.Describe how the male nuclei reach the embryo sac after a pollen grain lands on the stigma.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).