1. 1
   Pancreatic acinar cells secrete digestive enzymes. In a pulse-chase experiment, slices of pancreas were given a radioactive amino acid for 3 minutes and then transferred to a solution of non-radioactive amino acids. At 3 minutes, most of the radioactivity was in the rough endoplasmic reticulum. At 20 minutes it was mainly in the Golgi apparatus. At 120 minutes it was in large secretory vesicles close to the plasma membrane facing the gland's duct, and some had been released into the duct. After secretion, patches of plasma membrane are taken back into the cell in small vesicles coated with the protein clathrin.

   (a)

   Why did the radioactivity appear first in the rough endoplasmic reticulum rather than in proteins made by free ribosomes?

   [1 mark]

   • AFree ribosomes cannot use radioactive amino acids
   • BProteins for secretion are synthesized by ribosomes bound to the rough endoplasmic reticulum
   • CThe rough endoplasmic reticulum makes amino acids for export
   • DFree ribosomes only synthesize proteins during mitosis

   (b)

   What is the role of the Golgi apparatus in these cells?

   [1 mark]

   • AIt synthesizes the polypeptides of the digestive enzymes
   • BIt digests damaged enzymes before secretion
   • CIt pumps enzymes directly across the plasma membrane
   • DIt processes the proteins and packages them into vesicles for secretion

   (c)

   Outline the role of clathrin in the formation of the vesicles that take membrane back into the cell.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A human cell nucleus has about 3000 nuclear pores, each of which can transfer up to about 1000 molecules per second between the nucleus and cytoplasm. Proteins of the nuclear envelope were labelled with a fluorescent marker in cells dividing by mitosis. At the end of prophase the fluorescence became scattered through the cytoplasm as small vesicles. During telophase the fluorescent vesicles gathered around each group of chromosomes and fused.

   (a)

   Why are nuclear pores needed in the double membrane of the nucleus?

   [1 mark]

   • ATo allow ribosomes to enter the nucleus for translation
   • BTo allow DNA to leave the nucleus for replication in the cytoplasm
   • CTo allow mRNA to leave the nucleus and proteins such as enzymes to enter
   • DTo allow the chromosomes to leave during interphase

   (b)

   Which process can occur in this human cell before translation begins, but not in a prokaryote?

   [1 mark]

   • ARemoval of introns from pre-mRNA
   • BTranscription of DNA
   • CBinding of mRNA to a ribosome
   • DFormation of peptide bonds

   (c)

   Explain why the nuclear membrane breaks into vesicles during mitosis.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   The inner mitochondrial membrane of heart muscle mitochondria has about three times more cristae membrane area per unit volume of mitochondrion than that of liver mitochondria. In another experiment, isolated mitochondria were supplied with oxygen and respiratory substrate. A mild detergent was then added that makes the outer mitochondrial membrane leaky but leaves the inner membrane intact. Oxygen consumption continued at the same rate, but the rate of ATP production fell by about 90%.

   (a)

   Explain the difference in cristae between heart muscle and liver mitochondria.

   [3 marks]

   (b)

   Explain why making the outer membrane leaky reduced ATP production even though oxygen consumption continued.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A single palisade mesophyll cell of a leaf contains about 50 chloroplasts and many mitochondria, so it can photosynthesize and carry out aerobic respiration at the same time. In each chloroplast the thylakoid membranes are arranged in stacks called grana, joined by single thylakoids, all surrounded by stroma. The cell's large vacuole contains hydrolytic enzymes and is kept at about pH 5.5, while the cytoplasm is kept at about pH 7.5.

   (a)

   Explain how the structure of the chloroplast is adapted for photosynthesis.

   [6 marks]

   (b)

   Discuss the advantages of compartmentalization within this palisade mesophyll cell.

   [6 marks]

   Total for question 4: 12 marks

End of questions