Fertilisation

Question 1

Describe structure of a sperm cell

Answer 1

‘Head’ contains lysosome modified into acrosome, flagellum attached containing mitochondria

Question 2

How is a sperm cell adapted for its function?

Answer 2

• Flagellum = allows motility so sperm can reach egg
• Acrosome = contains hydrologic enzymes to break through zona pellucid a
• Mitochondria = provides energy for flagellar rotation
• Very little cytoplasm or non essential organelles = increases sperm motility

Question 3

Describe structure of the ovum

Answer 3

Large cell containing cortical granules and lipid droplets, surrounded by zona pellicuda which is surrounded by follicle cells

Question 4

Describe adaptations the ovum has for its function

Answer 4

• Large cell with lots of cytoplasm = ready to divide immediately after fertilisation
• Zona pellicuda = releases chemicals to attract sperm
• Cortical granules = contain enzymes to harden zona pellicuda after fertilisation, preventing polyspermy
• Lipid droplets = provide energy store for cell division

Question 5

Name the cell produced from fertilisation

Answer 5

Diploid zygote

Question 6

Define fertilisation

Answer 6

When nuclei of haploid gametes fuse

Question 7

Describe the first stage of fertilisation

Answer 7

Acrosome reaction
1. Sperm reaches ovum
2. Chemicals are released from cells surrounding ovum, triggering acrosome reaction
3. Acrosome swells, fusing with sperm cell surface membrane
4. Digestive enzymes in the acrosome are released
5. Enzymes digest through follicle cells + zona pellicuda surrounding ovum
6. Sperm fuses with ovum membrane and nucleus enters ovum

Question 8

Describe the second stage of fertilisation

Answer 8

1. Enzymes released from lysosomes (cortical granules) in the ovum thicken the jelly like layer, preventing entry of other sperm
2. Nuclei of the ovum and sperm fuse