State the scientists who proposed model A.

Label the model A diagram to show a region of protein.

Label the model B diagram to show a phospholipid.

Deduce one conclusion about the structure of the plasma membrane reached by the scientists from their results.

Suggest one reason for maintaining a pH of 7.5 throughout the experiment.

State one technological improvement, other than enzymatic digestion, that led to the falsification of previous models to determine the current model of membrane structure.

Davson–Danielli ✔

[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.

[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.

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".

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 ✔

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

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.

Most students could label the areas with protein and phospholipid.

Most students could label the areas with protein and phospholipid.

Many candidates answered that the enzyme acted on the phospholipids but failed to make a deduction about the structure of the membrane.

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.

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.