A-LevelBiologyTransport in plants9700/22 · Oct/Nov 2023 · Q66 Marks

Water that is absorbed from the soil solution by the roots of a plant enters xylem vessels and is transported to the leaves and buds. Fig. 6.1 shows four important requirements for the efficient transport of water from the roots to the leaves of a plant.

(a)Name the specialised cells that are arranged end to end to form xylem vessels.[1]
(b)Explain how tension is created in the xylem vessels.[2]
(c)(i)The walls of the cells forming the xylem vessel walls become lignified during development. Explain how this feature is important for the efficient transport of water.[2]
(c)(ii)During the development of xylem vessels, the end walls of the cells forming the vessels break down. This contributes to minimising resistance to the flow of water. Describe one other main change that needs to occur to these cells so that their structure becomes suited to their function.[1]

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

[1] xylem vessel elements; A singular A vessel elements A xylem elements
[2] any two from: transpiration pull; A water, drawn up / pulled up
[2] owing to / AW, water evaporating from spongy mesophyll surfaces / water vapour lost from, leaf / plant, during transpiration ; ref. to transpiration not required if transpiration pull stated
[2] (overall) water potential gradient ; in context of creates pulling force low water pressure at top and high water p at bottom A pressure gradient A hydrostatic pressure gradient
[2] cohesion and adhesion (of water molecules) / cohesive and adhesive forces (of water) ; A cohesion between water molecules or adhesion between water molecules and (xylem vessel) walls / cellulose / hydrophilic parts of lignin
[2] any two from: 1 (lignified wall / lignin) prevents, (inward) collapse / compression / AW (from tensions created); R bursting
[2] 2 (lignified wall / lignin) rigid / strong / secondary thickening, for support (of xylem vessels) ; R support, stem / plant
[2] 3 adhesion of water to hydrophilic groups in lignin, to maintain column of water / for water pathway / for transpiration stream/AW;
[2] 4 waterproofing properties (of lignin) / AW / water prevented from leaving xylem vessels (supports idea of continuous columns);
[2] 5 lignification, associated with cell death / allows formation of hollow cells / allows formation of hollow tubes (for minimum resistance to flow) ;
[2] 6 AVP; cell walls rigid to allow tension to develop
[1] any one from: cell death / apoptosis;
[1] loss of cell contents / become hollow ; A loss of (all) organelles R idea that some organelles remain
[1] loss of cytoplasm; A loss of protoplasm
[1] loss of nucleus ;
[1] AVP; e.g. formation of pits