State the surfactant that contains the least amount of phospholipids.

Compare the composition of natural human surfactant with synthetic surfactants.

Phospholipids found in the surfactants form a surface film on the moist lining of the alveoli. Outline how the hydrophilic and hydrophobic parts of the phospholipids in the surfactants are aligned on the alveolar surface.

Identify the effect of increasing the concentration of synthetic surfactant A on the growth of GBS.

Compare the effect of the three surfactants, synthetic surfactants A and B and the modified human surfactant, on the growth of the different bacteria at a concentration of 20 mg ml^–1.

Using all the data provided, evaluate the hypothesis that the presence of proteins in surfactants can decrease bacterial growth.

natural human (surfactant)

main component of all surfactants is phospholipids;
(natural human surfactant ) has less phospholipids (than synthetic surfactants);
(natural human surfactant) has more cholesterol (than (synthetic surfactant) A);
(natural human surfactant) has more free fatty acids than (synthetic surfactant) A and less than (synthetic surfactant) B; (comparison with both synthetic surfactants required)
(natural human surfactant) has more proteins (than synthetic surfactants);

hydrophilic groups facing the surface/are in the moist lining/water and hydrophobic tails facing outwards/are in the air
Award [0] for a description of a phospholipid bilayer. The orientation of both hydrophilic and hydrophobic parts must be included.

growth reduced (by increases in concentration)/negative correlation

The question asks to compare how each surfactant affects each bacterium. However, some responses will instead compare how each bacterium is affected by each surfactant. Accept both types of answer.

(synthetic surfactant) A decreases growth of GBS most and S. aureus and E. coli much less/slightly;
(synthetic surfactant) B decreases the growth of GBS (and of S. aureus slightly) but increases the growth of E. coli;
modified human surfactant decreases growth of GBS (and S. aureus) but no (significant) effect on E. coli;
GBS greatly inhibited by (synthetic surfactant) A but less/slightly by (synthetic surfactant) B and modified human surfactant;
S. aureus (slightly) inhibited by all three surfactants;
E. coli increased by (synthetic surfactant) B but (synthetic surfactant) A and modified human surfactant have no significant effect;

(hypothesis supported as)
(synthetic surfactant) A has proteins and decreases bacterial growth;

(hypothesis not supported as)
modified human surfactant has no proteins and decreases bacterial growth;
(synthetic surfactant) B has proteins and enhances growth (of E. coli);
GBS inhibited more by modified human surfactant which has no protein than (synthetic surfactant) B which has protein;
S. aureus inhibited more by modified human surfactant which has no protein than by the other (surfactants) which have protein;
Do not accept answer without reference to proteins.

Part (a) was an easy first question to give candidates confidence. Almost all answered it correctly.

Most candidates also answered part (b) successfully. There were plenty of possible comparisons to make and only two acceptable ones were needed for full marks.  

Part (c) was the hardest part of Question 1. Most candidates described or drew a diagram of a phospholipid bilayer. This was not accepted as the question stated that the phospholipids formed a film on the surface of the moist lining of the alveoli. The phospholipids will therefore be in contact with the aqueous solution on one side and air in the alveolus on the other side. The expected answer was a phospholipid monolayer with the hydrophilic heads facing the water and the hydrophobic tails facing the air. Even the strongest candidates struggled with this.

In part (d) there were some very clear and accurate answers, but also many that showed either imperfect understanding of the data or ambiguous phrasing of the answer. The data showed that increases in the concentration of surfactant A caused greater and greater decreases in the growth of GBS. The ambiguous answers included statements such as “Surfactant A increased negative growth”.

The problem in part (e) was to cope with the large amount of data: the effects of three concentrations of three surfactants on the growth of three types of bacteria, though candidates should have only considered the highest of the three concentrations. The best answers worked systematically through the data by comparing either the effects of each surfactant in turn or the effects of the surfactants on each bacterium in turn. A fault in some answers was failure to make genuine comparisons and instead to describe only a single part of the data at once. Another common fault was to ignore the sizes of the effects on the bacteria and thus whether they were significant or not. Given that the y-axis scales were logarithmic, small bars above or below the zero line were not significant.

Part (f) was quite challenging. Some candidates failed at the first hurdle, which was to look at the table of data at the start of Question 1 to find out the protein content of each of the three surfactants expected in the answer here. Having done this, it was not too difficult to see that there was some evidence for the hypothesis from surfactant A. It contained the most protein and inhibited the growth of each species of bacterium, albeit only to a small extent with Staphylococcus aureus and Escherichia coli. The remainder of the data did not fit the hypothesis.