A-LevelBiologyTransport in plants9700/22 · Oct/Nov 2020 · Q110 Marks

Fig. 1.1 is a diagram drawn from a photomicrograph of a transverse section through part of a leaf. The arrows in Fig. 1.1 show the movement of water through the cells of the leaf after it has left the xylem. [Figure 1.1]

(a)Water from the xylem can enter cell A and then moves to cells B and C without crossing their cell walls. The cell structures through which water passes from cell A to cell B are not visible in Fig. 1.1.
(a)(i)Name the cell structures through which water passes from cell A to cell B without crossing their cell walls.[1]
(a)(ii)Explain what causes water to move from cell B to cell C.[1]
(a)(iii)Name the pathway taken by water between cell A and cell C.[1]
(b)Most of the water that arrives at the leaf passes to the external atmosphere. With reference to Fig. 1.1, describe and explain the sequence of events occurring between point P and point Q.[4]
(c)The actual diameter of cell D in Fig. 1.1 along the length X-Y is 25 μm. Calculate the magnification of the image. Write down the formula used to make your calculation. Show your working. formula answer = x[3]

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 (Oct/Nov 2020). The mark or mark type is shown in brackets.

[1] plasmodesmata ;
I cytoplasmic strands / cytoplasm
R if other structures also given
[1] (existence of / down, a) water potential gradient ;
I ref. to solute / osmotic, potential
A lower/more negative, water potential of (cell) C (than cell B)
A higher/less negative, water potential of (cell) B (than cell C)
A Y for water potential
[1] symplastic (pathway) ; I vacuolar / vacuole, pathway
A symplast
[4] max 3 if no ref. to evaporation or water vapour and ref. only to water
any four from:
1 evaporation of water / water to water vapour ; must be in context of P
2 P described as, surface / cell wall, of (spongy) mesophyll (cell);
R palisade mesophyll cell
A film/layer, of moisture / water (for surface of cell)
3 water vapour enters (substomatal) air space ;
R intracellular air space
A other correct refs. to air space in sequence of events
4 diffusion of water vapour (out) through stoma / stomatal pore (to Q) ;
R through guard cells R diffusion by osmosis
5 (towards Q because) down water potential gradient / from high to low water potential / to lower water potential ;
I potential gradient alone
A down (water) vapour pressure gradient
A Y for water potential
context is movement out to atmosphere, not movement from P to air space
6 AVP; e.g. stoma stated as open
heat energy required for evaporation / ref. to latent heat of vapourisation
air space described as, intercellular / saturated / substomatal
[3] 1 correct formula ;
(magnification) = image length + actual length ;
A rearranged formula A magnification triangle
2 correct working;
3 correct answer for magnification; R if units given
(x) 1440 (36 mm) / 1460 (36.5mm) /1480 (37mm) / 1500 (37.5 mm)
1520 (38 mm) / 1540 (38.5 mm) / 1560 (39 mm);
if answer given to 2 sig. figs, look for the correct answer at the end of working
or allow if stated by answer that this is to 2 sig. figs.
allow ecf for incorrect conversions used in working or incorrect measured value of X-Y