Gametes and fertilisationEdexcel International A Level Biology: Subtopic test

1. 1
   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).

   (a)

   What is the acrosome of a sperm cell?

   [1 mark]

   • AA region of the tail that contains many mitochondria
   • BThe structure that contains the haploid set of chromosomes
   • CA vesicle at the tip of the head that contains hydrolytic (digestive) enzymes
   • DA glycoprotein layer that surrounds the oocyte

   (b)

   Which statement about the zona pellucida is correct?

   [1 mark]

   • AIt is a glycoprotein layer around the secondary oocyte that the sperm must penetrate
   • BIt is a layer of cells forming the lining of the uterus
   • CIt is the outer membrane of the sperm head
   • DIt is a vesicle produced by the sperm to carry digestive enzymes

   (c)

   The sperm midpiece contains many mitochondria. Explain how this adaptation helps the sperm to reach and fertilise an oocyte.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   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.

   (a)

   What is the main function of the cortical reaction?

   [1 mark]

   • ATo release enzymes that digest a path through the zona pellucida
   • BTo start meiosis I in the primary oocyte
   • CTo allow the sperm nucleus to fuse with a polar body
   • DTo make the zona pellucida unable to bind further sperm, so that polyspermy is prevented

   (b)

   Fertilisation involves fusion of the haploid sperm nucleus with the haploid nucleus of the oocyte. How many chromosomes are in the mouse zygote nucleus immediately after nuclear fusion?

   [1 mark]

   • A20
   • B40
   • C60
   • D80

   (c)

   Explain why it is important that only one sperm nucleus fuses with the oocyte nucleus.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   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.

   (a)

   Describe how the male nuclei reach the embryo sac after a pollen grain lands on the stigma.

   [3 marks]

   (b)

   Describe the nuclear fusions that then take place in the embryo sac, and give the number of chromosomes in each nucleus that results.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student is preparing a report comparing fertilisation in a mammal and in a flowering plant. To support it, she investigated the effect of sucrose concentration on the germination of pollen. She placed pollen from one flower on microscope slides in sucrose solutions of five concentrations, kept the slides at 25 °C for 30 minutes, then counted the percentage of grains that had produced a pollen tube. The results were: 0 g sucrose per 100 cm³, 12% germinated; 5 g, 41%; 10 g, 78%; 15 g, 83%; 20 g, 35%.

   (a)

   Describe the events that occur from the moment a mammalian sperm makes contact with the zona pellucida of a secondary oocyte until the zygote is formed.

   [6 marks]

   (b)

   Describe and explain the results of the investigation, and suggest how the student could improve the reliability of her conclusion about the best sucrose concentration.

   [6 marks]

   Total for question 4: 12 marks

End of questions