1. 1
   Rat liver tissue is chopped and homogenised in an ice-cold buffer solution that is isotonic with the cells. The homogenate is filtered and then centrifuged at increasing speeds, and the pellet formed at each stage is removed before the next spin. A spin at 1000 g for 10 minutes gives a pellet rich in nuclei. A spin at 10 000 g for 20 minutes gives a pellet rich in mitochondria and lysosomes. A spin at 100 000 g for 60 minutes gives a pellet of small membrane vesicles and ribosomes. Ultracentrifuges capable of the highest speeds were developed in the 1930s and 1940s.

   (a)

   Why is the buffer used to homogenise the tissue isotonic with the cells?

   [1 mark]

   • ATo make the organelles denser so that they sediment faster
   • BTo denature enzymes that would digest the organelles
   • CTo prevent organelles from bursting or shrinking by osmosis
   • DTo supply energy so that the organelles keep functioning

   (b)

   Which of these cell components would NOT be classed as an organelle?

   [1 mark]

   • ANucleus
   • BRibosome
   • CVesicle
   • DCytoskeleton

   (c)

   Outline how the development of the ultracentrifuge contributed to understanding the functions of organelles.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A human gene is transcribed in the nucleus to give a pre-mRNA molecule 8400 nucleotides long. Before it leaves the nucleus, introns are removed from this molecule, and the mRNA that passes into the cytoplasm is 2100 nucleotides long. In the bacterium Escherichia coli, researchers observed that ribosomes attach to an mRNA molecule and begin translating it within seconds, while the far end of the same mRNA is still being transcribed.

   (a)

   Which feature of E. coli makes the observation in the bacterium possible?

   [1 mark]

   • AIts DNA is not separated from its ribosomes by a nuclear membrane
   • BIts ribosomes are located inside the nucleus
   • CIts mRNA contains introns that ribosomes remove
   • DIts cell wall holds the mRNA close to the ribosomes

   (b)

   What percentage of the pre-mRNA is removed before the mRNA leaves the nucleus?

   [1 mark]

   • A25%
   • B75%
   • C80%
   • D400%

   (c)

   Using the data, explain an advantage of separating the nucleus from the cytoplasm.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Macrophages were cultured with yeast cells, which they engulf into phagocytic vacuoles. A pH-sensitive fluorescent dye showed that the pH inside a newly formed vacuole was 7.2, the same as the surrounding cytoplasm. Within 20 minutes, lysosomes had fused with the vacuole and its pH had fallen to 4.8. The main lysosomal protease has its highest activity at pH 5.0 and less than 10% of that activity at pH 7.2. When a drug that prevents lysosomes from fusing with vacuoles was added, the engulfed yeast cells were still intact after 2 hours.

   (a)

   Using the data, explain two advantages of enclosing digestive enzymes in lysosomes and phagocytic vacuoles.

   [3 marks]

   (b)

   Evaluate the conclusion that digestion of the yeast depends on the fusion of lysosomes with phagocytic vacuoles.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student lists the structures visible in electron micrographs of two cells. A palisade mesophyll cell from a leaf has a cell wall, plasma membrane, cytoplasm, nucleus, chloroplasts, mitochondria, ribosomes, a large vacuole, small vesicles and a network of cytoskeleton filaments. A cell of the bacterium Escherichia coli has a cell wall, plasma membrane, cytoplasm, ribosomes and a nucleoid region containing DNA, but no membrane-bound structures inside it.

   (a)

   Discuss which of the structures listed for the two cells should be classified as organelles.

   [6 marks]

   (b)

   Explain the advantages of compartmentalization to eukaryotic cells such as the palisade mesophyll cell, compared with E. coli.

   [6 marks]

   Total for question 4: 12 marks

End of questions