Prokaryotic cellsEdexcel International A Level Biology: Subtopic test

1. 1
   Penicillin-type antibiotics work by preventing the formation of cross-links in the cell wall of growing bacteria. A culture of bacteria growing in a very dilute nutrient solution is treated with such an antibiotic and the cells are seen to burst.

   (a)

   What is the main component of the bacterial cell wall?

   [1 mark]

   • APeptidoglycan
   • BCellulose
   • CChitin
   • DCollagen

   (b)

   Which statement best explains why the treated bacteria burst?

   [1 mark]

   • AWater leaves the cells by osmosis and the cytoplasm shrinks until the cell breaks
   • BThe antibiotic dissolves the cell surface membrane directly
   • CWater enters the cells by osmosis and the weakened wall cannot withstand the increase in pressure
   • DIons diffuse out through the cell wall until the cell collapses

   (c)

   Explain why an antibiotic that prevents cell wall formation does not damage human cells.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Two strains of a bacterium were compared. Strain S has a capsule and pili; strain R has neither. Both strains have circular DNA in the cytoplasm. When mice were injected with strain S they developed pneumonia and died. When mice were injected with strain R they survived and no bacteria were found in their lungs after a few days. Strain S bacteria were found attached to the lung epithelium of the dead mice.

   (a)

   Which structure would be present in both strain S and strain R?

   [1 mark]

   • AA nucleus containing linear chromosomes
   • BMitochondria with a folded inner membrane
   • CA Golgi apparatus made of flattened sacs
   • DCircular DNA that is not enclosed in a nuclear envelope

   (b)

   What is the most likely function of the pili of strain S?

   [1 mark]

   • ATo rotate and propel the bacterium through liquid
   • BTo attach the bacterium to the surface of host cells
   • CTo carry genes for antibiotic resistance
   • DTo synthesise protein for the cell

   (c)

   Explain why the mice injected with strain R survived.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A hospital laboratory isolates a bacterium that is resistant to the antibiotic ampicillin. The gene for resistance is on a small circular DNA molecule that is separate from the main bacterial chromosome. When this strain was mixed with a non-resistant strain, a thin tube formed between pairs of cells. After several hours some of the previously non-resistant cells were able to grow on agar containing ampicillin.

   (a)

   Explain how the non-resistant bacteria became resistant to ampicillin.

   [3 marks]

   (b)

   Describe two ways in which this small circular DNA molecule differs from the main bacterial chromosome, and suggest why resistance spreads faster through a population when the gene is on a plasmid rather than on the main chromosome.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A bacterium that causes a gut infection is a rod-shaped cell about 2 µm long. It has a cell wall, a capsule, several flagella and pili on its surface, and circular DNA and small plasmids in its cytoplasm. A student compares this bacterium with a human epithelial cell from the lining of the gut.

   (a)

   Compare the structure of this bacterium with that of the human epithelial cell.

   [6 marks]

   (b)

   Explain how the structures of the bacterium help it to cause a gut infection, and justify which you consider to be the most important.

   [6 marks]

   Total for question 4: 12 marks

End of questions