A-LevelBiologyTransport in plants9700/22 · Oct/Nov 2017 · Q58 Marks

Descriptions A, B and C relate to the movement of water from the soil to the xylem in roots.

(a)State the correct term to match each of the descriptions A, B and C. A The specialised root epidermal cell that provides a large surface area for the uptake of water from the soil. B The band of suberin in the cell walls of the endodermis that prevents the movement of water by the apoplastic pathway. C The cell structures that allow water to pass from one cell to the next as part of the symplastic pathway.[3]
(b)Explain, with reference to the structure of xylem vessel elements, why water does not take a symplastic pathway in the xylem to the leaves.[2]
(c)Explain why the rate of movement of water in the xylem may slow down at night.[3]

Mark scheme

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

[3] A = root hair (cell) ;
B = Casparian (strip) ;
C = plasmodesmata / plasmodesma ;
[2] xylem has no cytoplasm / symplast pathway is cytoplasmic (and vacuolar) ; A empty / hollow / no contents A cytosol for cytoplasm
xylem (vessel elements) are dead cells / symplastic through living cells ;
[3] three from
stomata close; I stomatal pore smaller / stomata partially open
only cuticular transpiration;
no photosynthesis / carbon dioxide needed ; I less photosynthesis
transpiration (rate) decreases ; A less, transpiration / transpiration pull, A described in terms of loss of water vapour from leaves
evaporation (rate) (from cell walls of spongy mesophyll cells) decreases ; R evaporation, from leaf surface / through stomata
water potential gradient between, soil / root, and leaf becomes less steep ;

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