Protein processing: rER and GolgiEdexcel A-Level Biology A: Subtopic test

1. 1
   Acinar cells of the pancreas make and secrete large quantities of the digestive enzyme amylase into the pancreatic duct. Electron micrographs of these cells show extensive rough endoplasmic reticulum, a prominent Golgi apparatus and many vesicles close to the cell-surface membrane.

   (a)

   Where is the polypeptide chain of amylase assembled?

   [1 mark]

   • AIn the nucleolus
   • BInside the Golgi apparatus
   • CIn lysosomes
   • DOn ribosomes attached to the rough endoplasmic reticulum

   (b)

   How is amylase released from the cell?

   [1 mark]

   • AVesicles fuse with the cell-surface membrane and release it by exocytosis
   • BEndocytosis
   • CDiffusion through the phospholipid bilayer
   • DActive transport through carrier proteins

   (c)

   Explain why these cells have extensive rough endoplasmic reticulum.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Pancreatic cells were incubated for three minutes in a medium containing radioactively labelled amino acids and then transferred to a medium containing only unlabelled amino acids. Samples were taken at intervals and the position of the radioactivity in the cells was found. After 5 minutes most of the radioactivity was in the rough endoplasmic reticulum, after 20 minutes it was mostly in the Golgi apparatus, and after 60 minutes it was in vesicles near the cell-surface membrane and in the surrounding medium.

   (a)

   Why were the cells transferred to the unlabelled medium after three minutes?

   [1 mark]

   • ATo stop the cells making any more protein
   • BSo that only proteins made during the three minutes were labelled and their movement could be followed
   • CTo remove the radioactive amino acids from the cells so they could be counted
   • DTo allow the cells to divide

   (b)

   Which is the correct order in which the labelled protein moved?

   [1 mark]

   • AGolgi apparatus, rough endoplasmic reticulum, vesicles, cell-surface membrane
   • BRough endoplasmic reticulum, lysosomes, Golgi apparatus, cell-surface membrane
   • CRough endoplasmic reticulum, Golgi apparatus, vesicles, cell-surface membrane
   • DVesicles, Golgi apparatus, rough endoplasmic reticulum, cell-surface membrane

   (c)

   Explain why radioactivity was found first in the rough endoplasmic reticulum.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A researcher treats pancreatic cells with a toxin that prevents vesicles from the rough endoplasmic reticulum from fusing with the Golgi apparatus. The untreated cells normally secrete digestive enzymes.

   (a)

   Predict and explain the effect of the toxin on the position of enzyme proteins in the cell and on the secretion of enzymes.

   [3 marks]

   (b)

   Describe the role of the Golgi apparatus in the formation of extracellular enzymes.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A company uses genetically modified yeast cells to make a human glycoprotein hormone. The gene for the hormone has been inserted into the yeast DNA, and the hormone is secreted into the growth medium, from which it is purified. Bacteria have been used to make other human proteins but are not used for this hormone.

   (a)

   Describe the pathway taken by the hormone, from the gene to its release into the growth medium.

   [6 marks]

   (b)

   Evaluate the use of yeast rather than bacteria to produce this hormone.

   [6 marks]

   Total for question 4: 12 marks

End of questions