(a) Explain how rice is adapted to grow with its roots submerged in water. (b) Explain how the leaves of maize or sorghum are able to maximise carbon dioxide fixation at high temperatures.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2017). The mark or mark type is shown in brackets.

1. [6] 9(a) six from: 1 aerenchyma ; 2 in stem and roots ; 3 help oxygen to, move / diffuse, to roots ; 4 shallow roots ; 5 air (film) trapped on underwater leaves ; 6 fast internode growth ; 7 (modified) growth regulated by, gibberellin / ethene ; 8 anaerobic respiration, underwater / when submerged ; 9 tolerant to high ethanol concentration / high tolerance to ethanol ; 10 ethanol dehydrogenase (switched on in anaerobic conditions) ; 11 AVP ; e.g. growth stops / carbohydrates conserved / quiescence, in short-term (flash) floods
2. [9] 9(b) nine from: 1 RuBP / rubisco, in bundle sheath (cells) ; 2 away from, oxygen / air ; 3 to avoid photorespiration ; 4 carbon dioxide combines with PEP ; 5 (catalysed by) PEP carboxylase ; 6 in mesophyll (cells) ; 7 forms oxaloacetate ; 8 converted to malate ; 9 malate passes to bundle sheath (cells) ; 10 (malate) releases (high concentration of) carbon dioxide ; 11 RuBP, carboxylated / reacts with carbon dioxide ; 12 PEP carboxylase / enzyme(s), has high optimum temperature / tolerate high temperatures ;