A-LevelBiologyTransport in plants9700/23 · May/June 2024 · Q49 Marks

Lignin and suberin are polymers that are present in plant tissues.

(a)Describe and explain the roles of lignin and suberin in the transport of water through the roots and stem of a plant.[4]
(b)The enzyme laccase catalyses the formation of lignin in plants. Fig. 4.1 is a diagram of the mode of action of laccase. [Figure 4.1] Describe and explain the mode of action of laccase when catalysing the formation of lignin.[5]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (May/June 2024). The mark or mark type is shown in brackets.

[4] Max 3 for suberin, max 3 for lignin
any four from:
1 lignin / suberin, is, hydrophobic / (partially) impermeable to water / AW ; e.g. waterproof/ barrier to water
suberin
2 suberin found in, Casparian strip / endodermal cells / endodermis ;
3 water must move from apoplast (pathway) to, symplast (pathway) / cytoplasm (at endodermis);
4 allows control of solutes / AW, before entering, endodermal cells / xylem ; A helps to prevent, toxins / bacteria / AW, entering xylem in context of mp3
lignin
5 lignin found in, cell walls of xylem vessel elements / walls of xylem vessels ; A xylem / vessel, elements
6 idea of lignin / lignified wall, prevents water leaving xylem / loss of water from xylem;
7 lignified walls, prevents, (inward) collapse of/ compression of / AW, xylem ; R bursting
8 provides strength / secondary thickening, for (structural) support (of xylem) ;
9 AVP; e.g. ref. to lignin as part of endodermis adhesion of water molecules to hydrophilic parts of lignin prevents collapse of water column / AW lignification causes death of xylem vessel elements context of producing empty xylem vessels for transport
[5] max 4 if no reference to laccase/monolignols allow substrate(s) for monolignols
any five from:
1 laccase uses induced fit;
2 idea that active site shape not fully complementary to (shape of) monolignols ;
3 active site changes shape to, fit / bind to, monolignols; AW
4 becomes fully complementary ;
5 forming enzyme-substrate complex ;
6 lowers, activation energy / energy of activation;
7 suggestion of how activation energy is lowered ;
8 laccase returns to original shape, after product leaves active site / for re-use/AW;
9 AVP; e.g. suggestion that, copper ions are, cofactors / necessary for enzyme action suggestion that monolignols are held in place by (temporary) H bonds