Question 19M.3.SL.TZ2.1
| Date | May 2019 | Marks available | [Maximum mark: 7] | Reference code | 19M.3.SL.TZ2.1 |
| Level | SL | Paper | 3 | Time zone | TZ2 |
| Command term | Deduce, Label, State, Suggest | Question number | 1 | Adapted from | N/A |
Two models of plasma membrane structure are shown.
[Source: diagram from article published in The American Journal of Pathology, 65, J Singer and G Nicolson,
The structure and chemistry of mammalian cell membranes, 427–437, Copyright Elsevier (1971)]
State the scientists who proposed model A.
[1]
Davson–Danielli ✔
There were a variety of suggestions for the names of the scientists with Singer and Nicolson, as given in the question paper, being a common incorrect answer.
Label the model A diagram to show a region of protein.
[1]
[Source: diagram from article published in The American Journal of Pathology, 65, J Singer and G Nicolson, The structure and chemistry of mammalian cell membranes, 427–437, Copyright Elsevier (1971)]
Accept label to top protein.
Most students could label the areas with protein and phospholipid.
Label the model B diagram to show a phospholipid.
[1]
[Source: diagram from article published in The American Journal of Pathology, 65, J Singer and G Nicolson, The structure and chemistry of mammalian cell membranes, 427–437, Copyright Elsevier (1971)]
Accept a label to any part of any phospholipid
Accept different form of labelling that clearly indicate the phospholipids.
Most students could label the areas with protein and phospholipid.
Phospholipase C is an enzyme that digests the polar heads of phospholipids. Scientists used phospholipase C to test these models of membrane structure. They found that the enzyme could digest the heads of phospholipids in the plasma membranes of red blood cells.
Deduce one conclusion about the structure of the plasma membrane reached by the scientists from their results.
[1]
a. phospholipids on outside/exposed ✔
b. impossibility of continuous protein layer «of Davson–Danielli/model A» ✔
c. supports idea of mosaic pattern of membrane
OR
supports model B ✔
Award any other valid conclusion.
Do not accept "membrane is made of phospholipids".
Many candidates answered that the enzyme acted on the phospholipids but failed to make a deduction about the structure of the membrane.
Suggest one reason for maintaining a pH of 7.5 throughout the experiment.
[2]
a. pH values away from optimum pH affect enzyme
OR
so enzyme can function properly
OR
pH 7.5 is the optimum pH for the enzyme ✔
b. sketch of enzyme activity versus pH ✔ Sketch of enzyme activity needs labels.
c. change in pH affects 3D structure of protein/active site
OR
change in pH denatures the enzyme / protein ✔
d. substrate does not fit in active site
OR
interaction of substrate and active site affected ✔
Most candidates answered correctly that a specific pH was necessary for the enzyme to function properly and that deviation from this pH could denature the enzyme.
State one technological improvement, other than enzymatic digestion, that led to the falsification of previous models to determine the current model of membrane structure.
[1]
a. scanning electronmicrography / SEM ✔
b. freeze fracture/etching ✔
c. X-ray diffraction
OR
crystallography ✔
d. fluorescent antibody / marker tagging ✔
Do not accept electron microscope
Accept description of process
Stating a technological improvement that led to the currently accepted model of the cell membrane proved more challenging and only the better candidates answered correctly.