Gametes and fertilisation in mammalsEdexcel A-Level Biology A: Subtopic test

1. 1
   A biologist examines human gametes using an electron microscope. The sperm cells have a head, a mid-piece and a tail. The egg is a large cell surrounded by a jelly-like layer called the zona pellucida, with small vesicles lying just beneath its cell surface membrane.

   (a)

   What does the acrosome at the front of the sperm head contain?

   [1 mark]

   • AThe haploid nucleus
   • BMitochondria in a spiral arrangement
   • CCortical granules
   • DHydrolytic enzymes that can digest the zona pellucida

   (b)

   Why does the mid-piece of a sperm contain many mitochondria?

   [1 mark]

   • ATo digest the zona pellucida
   • BTo release ATP for the movement of the tail (flagellum)
   • CTo carry the genetic material of the sperm
   • DTo release the contents of the cortical granules

   (c)

   Describe two ways in which the egg is specialised for its function.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In a fertility laboratory, sperm are added to a dish containing a human egg. The scientists watch as sperm reach the zona pellucida, and they study the events that follow the contact between one sperm and the egg.

   (a)

   Which event happens first when a sperm head makes contact with the zona pellucida?

   [1 mark]

   • AThe cortical reaction
   • BFusion of the two haploid nuclei
   • CThe acrosome reaction
   • DThe first division of the zygote by mitosis

   (b)

   What is the function of the cortical reaction?

   [1 mark]

   • AIt changes the zona pellucida so that no other sperm can penetrate the egg
   • BIt digests a path through the zona pellucida for the sperm
   • CIt provides energy for the sperm tail
   • DIt causes the two haploid nuclei to fuse

   (c)

   Explain why it would be harmful if more than one sperm fused with the egg.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A fertility clinic investigates two men whose sperm fail to fertilise eggs. In man 1 the sperm have normal tails and mitochondria but have no acrosome. In man 2 the sperm have a normal acrosome but have very few mitochondria in the mid-piece, and their tails barely move.

   (a)

   Explain why the sperm from man 1 are unable to fertilise an egg.

   [3 marks]

   (b)

   Explain why the sperm from man 2 are unlikely to fertilise an egg in the female reproductive tract, and suggest why fertilisation is more likely if these sperm are placed in a dish directly beside an egg.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student is writing a report on fertilisation in mammals and on how the gametes are suited to their roles. In humans, an ejaculate contains several hundred million sperm, but only one egg is normally released from the ovary in each menstrual cycle, and only one sperm fertilises it.

   (a)

   Describe the events of fertilisation, from the moment a sperm makes contact with the zona pellucida to the formation of the zygote, explaining the importance of each stage.

   [6 marks]

   (b)

   Explain why so many more sperm than eggs are produced, and evaluate how well the structure of each gamete suits its function. Include a justified conclusion.

   [6 marks]

   Total for question 4: 12 marks

End of questions