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.

Using the graph, estimate isotonic sucrose solutions for potato tissue and carrot tissue.

Potato:

Carrot:

Suggest a reason for the difference in the isotonic points for the potato and the carrot tissues.

From the evidence provided by the graph, evaluate the reliability of these data.

Explain one reason for calculating the percentage change in mass.

potato: 0.26 mol dm^–3 ✔ Allow a range of 0.22–0.32
carrot: 0.50 mol dm^–3 ✔ Allow a range of 0.45–0.55 (Allow 1 max if no units or either unit is omitted)

different dissolved solutes/sugars/sucrose/salts/molarities ✔
OR
may have been grown in different soils giving their tissues different contents ✔
OR
may have been stored under different conditions ✔
OR
may be more dehydrated / different water content ✔
OR
different types of tissue / different age ✔

Do not accept starch.

a. the data show clear trends
OR
a trend line could be drawn through these data
OR
there are no outliers ✔

b. the error bars/standard deviations shown
OR
the error bars/standard deviations vary with concentration ✔

c. sample size is unknown ✔

a. the change in mass indicates whether the tissue has gained/lost water ✔

b. the pieces of tissue will not be the same mass «at the beginning of the experiment» ✔

c. to compare the relative changes in mass ✔

Deduce, with a reason, which stage of mitosis is shown.

The cells visible in the onion root tip were classified and counted.

Calculate the mitotic index.

a. telophase ✔

b. chromatids/chromosomes have been pulled to the poles of the cell
OR
«2» new/daughter nuclei forming ✔

Do not accept cytokinesis

0.3 or 30 % ✔

Using the scale bar, calculate the magnification of the image.

Explain how electron micrographs such as this helped to falsify the Davson–Danielli model of membrane structure.

Explain how the amphipathic nature of phospholipids allows them to form bilayers.

$\frac{\text{scale bar length}}{\text{image size}}=\times 660$ ✔

a. the Davson–Danielli model proposed two layers of protein on either side of a lipid bilayer ✔ OWTTE

b. micrographs illustrate proteins in and/or crossing the membrane ✔

a. amphipathic means that they are both hydrophilic and hydrophobic ✔ 

b. the outside hydrophilic parts are exposed to water ✔ 

c. hydrophobic parts are away from water in the inside ✔

Water or lack of needs to be mentioned for mpb 

Two marks were awarded for an answer within the given range, since many candidates did not show working, and the question did not request it. Many candidates measured the whole diagram for the numerator instead of the scale bar, and others inverted the formula. A formula in words that appeared correct was not credited if the numbers then negated this. Errors in unit conversion were frequent, but credit was given for working.

Descriptions of protein layers either side of the phospholipid bilayer in the Davson-Danielli model were unclear. Embedded or integral proteins were frequently stated for the second marking point.

The terms hydrophilic and hydrophobic were commonly stated, although sometimes assigned to heads and tails incorrectly; however this was not penalised. Orientation towards or away from water was less often explained.

State from which organ the section was taken.

Identify the layer of tissue found at X.

The actual length of the structure labelled Y is 0.8 mm between the two black lines. Calculate the magnification of the micrograph. Working should be shown.

small intestine

Do not accept villi/villus or intestine alone.

epithelium

Do not accept microvilli/brush border.

a. calculation shown with accurate measurement of length of villus

OR

$\frac{53}{0.8}$ or $\frac{54}{0.8}$ or $\frac{55}{0.8}$ «mm»

For the first marking point to be awarded, the measurement must be between 53 and 55 mm.

b. 67 or 68 or 68

Allow any value between 67 and 69 inclusive.
Accept decimals e.g. 68.75.
Allow ECF from first marking point.

Using the scale bar, determine the diameter of the nucleus of the cell on the left, giving the units.

Deduce the magnification of the image.

Identify the structure labelled A. 

answer between 1350 − 1560 nm (accept between 1.35 and 1.56 μm)

Correct answer and units for 2 marks.

allow 27 500 − 32 500 (×/times)

Golgi apparatus

Accept the first answer only.

Identify the vein by labelling it with the letter V.

Distinguish between the vein and the artery with reference to structures visible in the micrograph.

label pointing to the upper of the two blood vessels in the micrograph

Note: check the answer carefully as the scan of the diagram is not always clear for candidates writing in pencil

a. vein has larger lumen

b. vein has less elastic tissue

c. vein has less muscular/thinner walls/tunica media

    OR

    ratio of wall thickness to lumen is less in the vein

d. vein less rounded/squashed more easily

Accept inverse for artery

Do not accept non-visible differences such as valves

No ECF

[Max 2 Marks]