A-LevelBiologyTransport in plants9700/23 · May/June 2023 · Q510 Marks

Phloem and xylem are specialised tissues involved in the transport of water, ions and assimilates in plants.

(a)A photomicrograph of a transverse section of a dicotyledonous stem is shown in Fig. 5.1. Use label lines and labels to identify the phloem tissue and xylem tissue in Fig. 5.1.[2]
(b)(i)Describe how sucrose is transported in phloem sieve tubes from photosynthesising leaves to other parts of the plant.[4]
(b)(ii)Cyanide ions (CN¯) inhibit the activity of an enzyme involved in respiration. Suggest why the treatment of photosynthesising leaves with CN¯ results in less sucrose being transported into phloem sieve tubes.[2]
(c)A student was asked to carry out semi-quantitative Benedict's tests on two solutions. • Solution A was extracted from the cytoplasm of cells in the mesophyll tissue of photosynthesising leaves. • Solution B was extracted from the phloem sap in phloem sieve tubes. The solutions were taken from the same plant, and other variables were standardised. For each solution, the student measured the time taken for the first colour change to appear. Suggest which of the two solutions, A or B, would change colour in the shortest time. Explain your answer.[2]

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 2023). The mark or mark type is shown in brackets.

[2] phloem (identified); xylem (identified); e.g. two marks for: [diagram shows phloem and xylem labelled]
[4] any four from: 1 (transport is by) mass flow; 2 entry of sucrose into phloem sieve tube, from companion cell / through plasmodesmata ; 3 sucrose, is dissolved in water / is part of phloem sap / lowers water potential; 4 (transport is) down a hydrostatic pressure gradient / from high hydrostatic pressure to low hydrostatic pressure ; high hydrostatic pressure 5 water moves into phloem sieve tube, by osmosis / down water potential gradient / from xylem (at source); 6 increased volume increases hydrostatic pressure ; hydrostatic needed once only low hydrostatic pressure 7 (at sink / in other parts of plant) sucrose unloaded, water follows osmotically ; AW
[2] any two from: respiration 1 less/no, ATP produced from aerobic respiration; A ETC / oxidative phosphorylation 2 less ATP/energy, for sucrose synthesis (in mesophyll cell); in context of companion cell 3 less/no, ATP / energy, for, proton gradient / active transport of protons (out of companion cell) / AW ; A hydrogen ions / H+ for protons 4 less sucrose cotransported (with, protons / hydrogen ions / H+) into companion cells ; 5 (less sucrose in companion cells means) decreased diffusion of sucrose (via plasmodesmata) into phloem sieve tubes; if no marks gained, allow 1 mark for less ATP for, active loading/active transport of protons
[2] Benedict's solution does not react with, non-reducing sugar/sucrose or Benedict's solution only reacts with, reducing sugar/glucose ; AW A Benedict's is, a test for reducing sugars / not a test for non-reducing sugars solution A because sugar present in, cytoplasm / solution A, (mainly) reducing sugars / glucose and or sugar in, phloem sap / solution B, is mainly, sucrose / non-reducing sugar solution B because phloem sap / solution B, contains, a higher concentration of / AW, reducing sugar / glucose, (than, cytoplasm / solution A) ; A in cytoplasm glucose stored as, polysaccharide / starch