In the mesophyll tissue of leaves, products of photosynthesis can be used to synthesise organic compounds, such as the polysaccharide cellulose and some amino acids. A source of nitrogen for amino acid synthesis can be provided by nitrate ions that have been taken up in the roots and transported to the leaves.

Mark scheme

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

1. [1] (monomers of) beta-glucose / ẞ-glucose ; R a-glucose / two ẞ-glucoses
2. [1] (joined by) 1, 4 glycosidic bonds ; A 1,4 glucosidic bonds R 1,4 and 1,6 bonds
3. [1] each (ẞ-) glucose rotated by 180° compared to adjacent (ẞ-)glucose ; AW
4. [1] linear / straight, chain / molecule;
5. [1] unbranched;
6. [1] increases rate of translation of, enzyme / nitrate reductase ;
7. [1] increases, enzyme / nitrate reductase, synthesis / concentration ; A more nitrate reductase produced
8. [1] idea of more metabolically efficient ; e.g. enzyme only produced when needed
9. [1] AVP; e.g. more nitrate is reduced to nitrite / more nitrite is formed leads to increased supply of nitrite for amino acids / amino acid synthesis
10. [1] improves accuracy of results / results not by eye / results not subjective;
11. [1] provides quantitative results / obtain numerical values ; A described e.g. absorbance values / can measure absorbance/gives percentage transmission / absorbance readings
12. [1] idea that different, intensities / shades of, colour / magenta / red-purple, relate to different, quantities / concentrations, of nitrites; A relate to (rates of) nitrate uptake
13. [1] can produce calibration curve (for quantitative results);
14. [1] AVP; e.g. can detect, (very) low concentrations / faintly coloured samples
15. [1] protons, moved by active transport/pumped, out of (companion) cell ; A using, energy / ATP for active transport
16. [1] protons, moved into / enter / AW, cell wall / apoplast;
17. [1] proton gradient builds up in, apoplast / cell wall ; A higher concentration A high concentration if mp4 gained
18. [1] protons move back (into companion cell), by facilitated diffusion / AW; e.g. down, electrochemical / concentration, gradient from high(er) to low(er) concentration A diffuse if movement is described through a transport protein in mp5
19. [1] protons cotransport amino acids or amino acids move with protons through, a cotransporter / a cotransport protein / (amino acid) transporter ;
20. [1] amino acids, transported / AW, against their concentration gradient ;
21. [1] plasmodesmata present;
22. [1] A movement is down the, concentration / diffusion, gradient
23. [1] A because there is a high(er) concentration in the companion cell ora for phloem sieve tube (sap)
24. [1] 0.2 mmol dm³ constant rate v 5.0 mmol dm³, rate not constant / described ;
25. [1] 0.2 mmol dm³ lower rate than 5.0 mmol dm-³; ora 5.0 mmol dm³ higher rate A faster/slower, rate for higher / lower, rate A (overall) steeper rate / less steep rate
26. [1] calculated rates to support ;
27. [1] (experiments suggest mainly) active transport/active uptake; A uptake, is an active process / needs energy / needs ATP if mp8 given, allow ‘active transport and facilitated diffusion'
28. [1] comparison of experiments with control, any one ; expt discussion point 1 oxygen required for (complete) uptake A aeration required or almost no /AW, uptake, with no oxygen / in anaerobic conditions
29. [1] 2 almost no / AW, uptake, in, low temperature / at 3°C or uptake, higher / AW, at 30°C/ in control
30. [1] 3 proteins / enzymes, are, required for / involved in, uptake or (much) lower / AW, uptake when, proteins not synthesised / protein synthesis inhibitor present
31. [1] 4 slightly lower / very similar, uptake / AW, without any bacteria or slightly higher / very similar, uptake / AW, with bacteria (control)
32. [1] one comparative result / manipulated data, with units, to support mp2;
33. [1] aerobic respiration needs oxygen ;
34. [1] lower temperature means lower activity of enzymes involved in respiration;
35. [1] ref. to ATP (for active transport); e.g. (aerobic) respiration produces ATP less respiration means less ATP produced no ATP for, conformational change / AW (of carrier protein)
36. [1] some (facilitated) diffusion because some uptake without oxygen ;
37. [1] protein synthesis needed for transport proteins; A membrane / transport/carrier / channel, proteins not being synthesised A no protein synthesis so less ATP synthase
38. [1] effect of lower temperature on transport; ora for control e.g. reduces rate of (facilitated) diffusion affects, ATPase / carrier protein, for active transport transport proteins less efficient at low temperature
39. [1] bacteria will have, little / no, effect on uptake or (normally) bacteria (associated with roots), increase / help, nitrate uptake ;
40. [1] AVP; e.g. idea that nitrate uptake involves metabolic processes