1. 1
   In mouse skin, a region of each hair follicle called the bulge contains stem cells. At the start of each hair growth cycle, some bulge cells divide and their daughter cells differentiate into the cells of the new hair and its surrounding sheath. After a skin wound, bulge cells also divide and move out to help form new epidermis. When researchers destroyed the bulge cells of some follicles with a laser, those follicles stopped producing hair, but skin wounds in the same area still healed, although more slowly than in untreated skin.

   (a)

   How should the bulge stem cells be classified?

   [1 mark]

   • AMultipotent
   • BTotipotent
   • CPluripotent
   • DFully differentiated

   (b)

   What is meant by the stem cell niche of the bulge?

   [1 mark]

   • AThe hair shaft produced by the stem cells
   • BThe microenvironment that maintains the stem cells and can promote their proliferation and differentiation
   • CA group of fully differentiated cells that replaces dead skin
   • DThe layer of dead cells on the skin surface

   (c)

   Deduce what the laser experiment shows about the roles of the bulge stem cells.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   The epithelium lining human alveoli contains two cell types. Type I pneumocytes make up about 40% of the epithelial cells but cover about 95% of the alveolar surface; over much of their area the cytoplasm is only about 0.2 μm thick. Type II pneumocytes make up about 60% of the epithelial cells but cover only about 5% of the surface; they are cuboidal and contain many lamellar bodies. Babies born before about 28 weeks of pregnancy often have very few mature type II pneumocytes and suffer from alveoli that collapse when they breathe out.

   (a)

   Which feature of type I pneumocytes most directly increases the rate of gas exchange?

   [1 mark]

   • AMany lamellar bodies in the cytoplasm
   • BA cuboidal shape with a large nucleus
   • CExtreme thinness, giving a short diffusion distance
   • DSecretion of surfactant into the alveolus

   (b)

   What is the function of the lamellar bodies in type II pneumocytes?

   [1 mark]

   • AThey are mitochondria that supply ATP for gas exchange
   • BThey store oxygen for release when breathing stops
   • CThey digest dust particles that enter the alveoli
   • DThey are secretory vesicles that discharge surfactant into the alveolar lumen

   (c)

   Using the data, explain why the alveolar epithelium needs two cell types.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A human red blood cell has a volume of about 90 μm³ and a surface area of about 136 μm². A sphere with the same volume would have a radius of 2.78 μm. Cells of the proximal convoluted tubule of the nephron reabsorb glucose, amino acids and ions from the filtrate. On the side facing the filtrate, each cell has about 4000 microvilli on 100 μm² of cell surface; each microvillus can be treated as a cylinder 1.0 μm long and 0.10 μm in diameter. The opposite side of the cell has deep invaginations of the plasma membrane with many mitochondria between them.

   (a)

   Calculate the surface area-to-volume ratio of the red blood cell and of a sphere of the same volume, and explain the significance of the difference.

   [3 marks]

   (b)

   Calculate how many times the microvilli increase the area of the cell surface facing the filtrate, and explain how the structure of these cells is adapted for reabsorption.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A striated muscle fibre from a human thigh muscle is about 60 μm in diameter and several centimetres long, is unbranched and contains several hundred nuclei located just under its plasma membrane. It forms during development by the fusion of many precursor cells. A cardiac muscle cell is about 100 μm long and 15 μm in diameter, is branched, usually has one nucleus near its centre and is joined end to end to other cardiac cells by intercalated discs. Both contain myofibrils with a striped (striated) pattern.

   (a)

   Compare and contrast cardiac muscle cells and striated muscle fibres, suggesting reasons for the differences.

   [6 marks]

   (b)

   Discuss whether a striated muscle fibre should be regarded as a cell.

   [6 marks]

   Total for question 4: 12 marks

End of questions