A-LevelBiologyEnzymes9700/22 · Feb/Mar 2020 · Q210 Marks

Phosphatidate phosphatase (PAP) enzymes have an important role in lipid metabolism. The reaction catalysed by PAP is shown in Fig. 2.1. phosphatidate + H2O PAP diglyceride + inorganic phosphate (Pi) Fig. 2.1 Experiments were carried out to investigate the activity of PAP extracted from the cotyledons (seed leaves) of bitter gourd, Momordica charantia.

(a)There are two types of PAP enzymes: • PAP1 enzymes need magnesium ions (Mg2+) in the active site to function • PAP2 enzymes do not need Mg2+. The effect of different concentrations of Mg2+ on the activity of PAP extracted from M. charantia was investigated. The results are shown in Fig. 2.2. [Figure 2.2] Explain, with reference to Fig. 2.2, whether the PAP extracted from M. charantia is a PAP1 enzyme or a PAP2 enzyme.[2]
(b)Fig. 2.3 shows the effect of increasing phosphatidate concentration on the activity of PAP extracted from M. charantia. [Figure 2.3] With reference to Fig. 2.3, describe and explain the effect of increasing phosphatidate concentration on the activity of PAP.[4]
(c)(i)The diglycerides formed as a result of the action of PAP can be used to synthesise triglycerides and membrane phospholipids. Explain how the structure of a triglyceride is suited to its function as an energy storage molecule.[2]
(c)(ii)Explain why phospholipids are able to form a bilayer in cell membranes.[2]

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Mark scheme

Marking points from the official mark scheme (Feb/Mar 2020). The mark or mark type is shown in brackets.

[2] 2(a) any two from:
PAP2, because
activity is shown at 0.0 mmol dm-³ / does not require Mg2+ for activity ;
(at 0.0 mmol dm³) 30 arbitrary units activity ;
no / very little, change at all concentrations of Mg2+ ;
[4] 2(b) A substrate for phosphatidate throughout
A enzyme for PAP throughout
any four from:
1 increasing concentration of phosphatidate increases PAP activity;
2 at higher phosphatidate concentrations the increase in PAP activity is less steep;
3 data to support marking point 1 or 2;
4 phosphatidate (concentration) is limiting factor / enzyme concentration begins to be limiting at higher concentrations (of phosphatidate) ;
5 at low phosphatidate concentrations, not all (enzyme) active sites are occupied / active sites are available
or
more active sites occupied at higher concentrations / AW ;
R all active sites saturated (at higher concentrations)
[2] 2(c)(i) any two from:
(fatty acid tails are long) hydrocarbon chains / many C-H bonds ;
ref. to dense packing / large mass per unit volume ;
AVP; e.g. qualified ref. to role of hydrogens (in oxidative phosphorylation)
bond energy released when bonds are broken ;
[2] 2(c)(ii) any two from:
hydrophilic / polar, (phosphate) head and, hydrophobic / non-polar, (fatty acid) tails;
(so) heads face, watery environment / tissue fluid / cytoplasm / cytosol / aqueous environment ;
(so) tails, form hydrophobic core / form area away from water / face each other;
A ref. to tails and hydrophobic interactions

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