TOPIC 3: Fertilisation

Question 1

Describe 3 structural differences between an egg cell and a sperm cell

Answer 1

-Sperm is smaller than egg cell
-Sperm has flagellum but egg doesnt
-Sperm have an acrosome

Question 2

Outline 4 features of the egg cell

Answer 2

ZONA PELLUCIDA
-Protective layer sperm have to penetrate in order to fertile the egg
-It then hardens after the sperm has entered to prevent polyspermy
-Glycoprotein layer

HAPLOID NUCLEUS
-A full set of chromosomes is restored at fertilisation

CORTICAL GRANULES
-Release substances which cause the zona pellucida to harden

FOLLICILE CELLS
-Form a protective coating around the egg

Question 3

Outline features of the sperm cell

Answer 3

MITOCHONDRIA
-Provide energy for sperm to swim to egg

ACROSOMES
-Contain digestive enzymes which break down zona pellucida and allow sperm to penetrate egg

FLAGELLA
-Allows sperm to swim to egg cell

Question 4

Outline fertilisation

Answer 4

-Sperm head fuses with zona pellucida and acrosome reaction occurs
-Sperm head fuses with cell membrane of the egg cell thus allowing the sperm nucleus to enter the egg
-The cortical reaction occurs which causes the zona pellucida to harden and prevent polyspermy
-Nuclei fuse and a full set of chromosomes is restored, thus forming a diploid zygote

Question 5

What is fertilisation?

Answer 5

Fertilisation is the fusion of the nuclei from a male gamete and a female gamete

Question 6

Outline the acrosome reaction

Answer 6

During fertilisation, the head of a sperm cell releases enzymes that digest a path through the protective outer layer of the egg cell (the zona pellucida), allowing the sperm to pass through the egg cell membrane

Question 7

Outline the cortical reaction

Answer 7

The egg cell releases cortical granules into the space between the egg cell membrane and the zona pellucida

The chemicals contained within the cortical granules cause the zona pellucida to rapidly thicken and harden, preventing any more sperm cells from entering, ensuring only one sperm cell can fertilise the egg cell

Question 8

Compare and contrast the acrosome reaction and the cortical reaction.

Answer 8

Acrosome reaction – the release of hydrolytic enzymes which soften the zona pellucida (jelly coat)
Cortical reaction – hardening of the jelly coat to prevent polyspermy

Both involve vesicles
Both release enzymes by exocytosis
AR causes digestion of Zona pellucida, CR thickens Zona pellucida

Question 9

Explain why a second sperm cell cannot fertilise the egg cell.

Answer 9

After the acrosome reaction, the zona pellucida hardens due to cortical reaction
This prevents another sperm from entering the egg and fertilising it, preventing polyspermy