Outline how the properties of water make it an ideal transport medium in plants.

Distinguish between the xylem and phloem of plants.

Explain how the light-independent reactions of photosynthesis rely on the light-dependent reactions.

a. polarity of water;
b. hydrogen bonds between water molecules;
c. cohesion between water molecules/water molecules stick together;
d. cohesion allows tensions/low pressures/transpiration pull/movement upward/against gravity;
e. adhesion to cellulose/cell walls generates tensions/pull (in xylem)
OR
adhesion to xylem walls/vessel walls causes capillary rise/upward movement;
f. solvent for many substances / many substances dissolve;
g. liquid at most temperatures experienced by plants / liquid so can flow;

Polarity of water and/or hydrogen bonding can be shown in an annotated diagram.

a. light-dependent reactions produce ATP/reduced NADP;
b. ATP generated by chemiosmosis/by photophosphorylation/by ATP synthase;
c. reduced NADP produced by/using electrons from Photosystem I;
d. RuBP + CO₂ to glycerate 3-phosphate (in light independent reactions);
e. glycerate 3-phosphate reduced to triose phosphate (in light independent reactions);
f. ATP/reduced NADP used in the light-independent reactions;
g. reduced NADP provides electrons/hydrogen / to reduce (glycerate 3-phosphate)
OR
reduced NADP used to convert glycerate 3-phosphate to triose phosphate;
h. ATP provides energy (for reduction of glycerate 3-phosphate);
i. ATP needed to regenerate RuBP
j. ATP/reduced NADP run out in darkness
k. Calvin cycle only possible with light/in the day/is indirectly dependent on light;

Most candidates knew at least some properties of water that make it useful as a medium for transport in plants. As in previous papers, cohesion and adhesion are often treated as the same process or were confused. Another common error is to refer to hydrogen bonds as strong – their effects are strong because so many hydrogen bonds are formed in water but, thinking of them individually, they are weak interactions.

The average mark was 2/4 for the differences between xylem and phloem. As in 6(c) marks were often lost because only one side of a distinction was given in the answer. A common misconception is that transport in xylem is unidirectional whereas in phloem it is bidirectional. Simultaneous bidirectional transport in individual sieve tubes was hypothesized at one time but this has been falsified. Sap can move in either direction at different times in both phloem sieve tubes and xylem vessels and recent research shows that xylem sap drops back down to the roots as often as every night in some herbaceous plants, making the ascent of sap in air-filled vessels a daily task. Adhesion to cellulose (not lignin) in xylem walls and capillary action is therefore more important for water transport in plants than previously realized.

This was a fair but challenging question and it yielded the highest correlation coefficient on the paper. Answers covered the whole gamut from the thoroughly confused to the masterly. Full names of intermediates in the Calvin cycle are preferred because abbreviations such as GP are often ambiguous. Weak points in some answers were the need for reduced NADP in the reduction of glycerate 3-phosphate to triose phosphate and the need also for energy from ATP both for this reduction reaction and for phosphorylation reactions in the regeneration of ribulose bisphosphate.