Topic 3 - Lesson 7

Question 1

Ovum

Answer 1

Large and incapable of independent movement
Wafted along the oviducts from the ovary to the uterus by the tubes ciliated cells and muscular contractions

Question 2

Sperm Cell

Answer 2

Much smaller than the ovum and motile
Continuously produced by males following maturity
Ejaculated into the vagina, swim to the uterus. Assisted by muscular contractions of uterus walls.
May meet an ovum in the oviduct
Attracted to the ovum by the chemicals released from it

Question 3

Problems with sperm cells

Answer 3

These could be medical conditions such as teratozoospermia or exposure to chemicals such as nicotine or anandamide
If motility is affected, chances of fertilisation are reduced as the sperm is unable to reach and penetrate the egg
If ATP production is affected, motility is reduced as flagellum requires ATP
Abnormal structures can also affect motility

Question 4

Adaptations of the ovum

Answer 4

Egg cells are specialised for fertilisation in the following ways:

Cytoplasm contains nutrients for growth of the developing embryo.

There is an outer layer called the zona pellucida - this changes after fertilisation so that no further sperm can penetrate.

Follicle cells outside the zona pellucida form a protective coating around the egg cell.

Question 5

Adaptations of the sperm

Answer 5

Overall streamlined shape for faster swimming.

Tail (flagellum) enables the sperm to swim.

Contain lots of mitochondria to provide ATP for movement.

The head of the sperm contains the acrosome - this is filled with digestive enzymes which break down the egg’s zona pellucida and allow the sperm to reach the egg cell membrane.

Question 6

Acrosome Reaction

Answer 6

When a sperm reaches the egg cell, it binds to its outer layer (the zone pellucida) through attachment to sperm-binding proteins. Once bound, it releases the digestive enzymes that are contained in the acrosome. These enzymes digest a tunnel through the zona pellucida so that the sperm can reach the plasma membrane of the egg cell. The plasma membranes of the sperm cell and the egg cell fuse and the sperm releases its nucleus into the egg cell cytoplasm. The nuclei of the sperm and egg fuse - fertilisation has taken place and a zygote is formed.

Question 7

The cortical reaction

Answer 7

The fusion of the sperm cell and egg cell membranes triggers the release of calcium ions which stimulates vesicles containing cortical granules to move and fuse with the egg cell membrane. The cortical granules are released into the zona pellucida, which continue breaking down the zona pellucida, removing the remaining sperm-binding proteins so that no further sperm can bind. Other substances within the cortical granules produce a new outer layer which is thick and impenetrable to sperm cells. This process prevents multiple sperm cells from fertilising the egg, which would result in the zygote having an abnormal number of chromosomes.