1. 1
   In the fruit fly Drosophila melanogaster, the mother deposits mRNA for a protein called Bicoid at the future head (anterior) end of the egg. After fertilization, Bicoid protein is made and diffuses away from this end, so its concentration is highest at the anterior end and falls towards the posterior end. Cells exposed to high Bicoid concentrations switch on genes for head structures. When Bicoid mRNA was injected into the posterior end of an embryo whose mother lacked the gene, head structures developed at the posterior end.

   (a)

   What is the name of the process by which the unspecialized cells of the embryo develop into specialized cells?

   [1 mark]

   • AFertilization
   • BDifferentiation
   • CMeiosis
   • DMutation

   (b)

   Which statement best explains how cells in the same embryo become different from each other?

   [1 mark]

   • ACells lose the genes they do not need as they divide
   • BCells at the anterior end receive more chromosomes
   • CDifferent concentrations of a signal molecule cause different genes to be expressed
   • DMutations occur in different genes in each cell

   (c)

   Explain the result of injecting Bicoid mRNA into the posterior end.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In mice, any single cell separated from an 8-cell embryo can develop into a complete mouse, including its placenta. About five days after fertilization, cells of the inner cell mass of the embryo can form every type of body cell but cannot form a placenta. In adult mice, stem cells in the bone marrow can produce red blood cells, platelets and every type of white blood cell, but have not been found to produce neurons or liver cells.

   (a)

   Which row correctly classifies the three types of stem cell described?

   [1 mark]

   • A8-cell embryo: pluripotent; inner cell mass: totipotent; bone marrow: multipotent
   • B8-cell embryo: multipotent; inner cell mass: pluripotent; bone marrow: totipotent
   • C8-cell embryo: totipotent; inner cell mass: multipotent; bone marrow: pluripotent
   • D8-cell embryo: totipotent; inner cell mass: pluripotent; bone marrow: multipotent

   (b)

   Which property is shared by all three types of stem cell?

   [1 mark]

   • AThey can divide repeatedly and differentiate along different pathways
   • BThey can develop into a complete organism
   • CThey are found only in embryos
   • DThey have already differentiated into a specific cell type

   (c)

   Outline the function of the stem cell niche in the bone marrow.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student models cells using cubes of agar jelly, dyed pink with an indicator that turns colourless in acid. Cubes with sides of 1 cm, 2 cm and 4 cm are placed in dilute hydrochloric acid for 10 minutes. The acid diffuses 0.3 cm into every cube from all faces. The percentage of each cube's volume that has turned colourless is 94% for the 1 cm cube, 66% for the 2 cm cube and 39% for the 4 cm cube.

   (a)

   Calculate the surface area-to-volume ratio of the 2 cm and 4 cm cubes, and state the relationship shown by the data.

   [3 marks]

   (b)

   Using the model, explain why cell size is limited, and evaluate the use of agar cubes as a model of cells.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Approximate sizes of some human cells: egg cell (secondary oocyte), 110 μm in diameter; sperm, head 5 μm long with a total length of about 55 μm; red blood cell, 7.5 μm in diameter; neutrophil (a white blood cell), 12 μm in diameter; motor neuron, cell body about 100 μm across with an axon that can be up to 1 m long; striated muscle fibre, 10–100 μm in diameter and up to several centimetres long.

   (a)

   Discuss how the size of each of these cells is related to its function.

   [6 marks]

   (b)

   The egg cell has a low surface area-to-volume ratio. Using this concept, discuss why the egg can be so large while most body cells are small.

   [6 marks]

   Total for question 4: 12 marks

End of questions