1. 1
   A student examines a sample of onion epidermis cells and a sample of human cheek cells under a light microscope. Both samples show a nucleus and a cell membrane, but only the onion cells show a rigid outer layer surrounding the membrane and a large fluid-filled space inside the cytoplasm.

   (a)

   Which structure is responsible for the rigid outer layer seen only in the onion cells?

   [1 mark]

   • ACell wall
   • BMitochondrion
   • CRibosome
   • DPlasmid

   (b)

   The student notices that the onion cells also contain a large, permanent fluid-filled space that the cheek cells do not have. Which structure is this?

   [1 mark]

   • ANucleus
   • BChloroplast
   • CVacuole
   • DFlagellum

   (c)

   The cheek cells the student observed did not contain a cell wall or a large vacuole, yet they still had a fixed, rounded shape when unstressed. Suggest two other sub-cellular structures found in both onion and cheek cells and state one function of each.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A microbiologist stains a sample of bacteria and observes, under high magnification, a single circular loop of DNA in the centre of each cell, several smaller separate rings of DNA, and thread-like structures projecting from the cell surface that allow the bacteria to swim through liquid.

   (a)

   What is the name of the DNA structure found floating separately from the main circular loop?

   [1 mark]

   • ANucleus
   • BChromosome
   • CRibosome
   • DPlasmid

   (b)

   What is the name of the thread-like structures projecting from the bacterial cell surface that allow the bacteria to swim through liquid?

   [1 mark]

   • ACilia
   • BFlagella
   • CVilli
   • DAcrosomes

   (c)

   Explain why bacterial cells are described as prokaryotic rather than eukaryotic, using evidence from the microbiologist's observations.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In 1665, early microscopes could magnify specimens up to about 270 times, which allowed scientists to see the outlines of cells but not the structures inside them. Modern electron microscopes can magnify specimens up to two million times and have a much higher resolution.

   (a)

   Explain the difference between magnification and resolution, and explain why an increase in resolution, not just magnification, was needed to see sub-cellular structures such as ribosomes.

   [3 marks]

   (b)

   A typical animal cell has a diameter of approximately 20 micrometres. A typical ribosome has a diameter of approximately 20 nanometres. Calculate how many times larger the diameter of the cell is than the diameter of the ribosome, showing your working. State your answer as an ordinary number.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A fertility clinic is investigating why a couple has struggled to conceive naturally. Tests show that the man's sperm cells have a normal haploid nucleus and acrosome, but a much shorter and less mobile tail than average, and their mitochondria appear structurally abnormal. The woman's egg cells are unusually small, with far fewer nutrient stores in the cytoplasm than typical egg cells, although the membrane's normal post-fertilisation changes still occur correctly.

   (a)

   Analyse how each of these three features (nucleus, acrosome, mitochondria) normally contributes to the sperm cell's function, and evaluate how the abnormalities described would be likely to affect fertilisation.

   [6 marks]

   (b)

   Evaluate how the woman's reduced egg nutrient store might affect early embryo development, and suggest what additional investigation the clinic could carry out to determine whether the sperm or egg abnormalities are the more significant cause of the couple's infertility.

   [6 marks]

   Total for question 4: 12 marks

End of questions