A-LevelBiologyTransport in plants9700/23 · May/June 2020 · Q110 Marks

Water and mineral ions are transported up the stem of a plant to the leaves within xylem vessels. Some water and mineral ions can pass out of xylem vessel elements to supply parenchyma tissue in the stem.

(a)Fig. 1.1 is a plan diagram of a section through a stem. [Figure 1.1] Identify one location where xylem tissue occurs in the stem by drawing a label line and the letter X on Fig. 1.1.[1]
(b)Explain how hydrogen bonding between water molecules contributes to the movement of water within xylem vessels up the stem to the leaves.[3]
(c)(i)Fig. 1.2 is a diagram of a photomicrograph showing three adjacent parenchyma cells in the stem. These parenchyma cells can be described as typical plant cells. The arrows show the direction of movement of water between the cells. [Figure 1.2] Describe and explain the movement of water shown in Fig. 1.2.[3]
(c)(ii)Only some of the structures visible using the light microscope have been included in Fig. 1.2. List the features that can be seen using the high power of a light microscope that help identify a parenchyma cell as a plant cell and not as an animal cell.[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 (May/June 2020). The mark or mark type is shown in brackets.

[1] any one correct location ; R if more than one label line and one is to an incorrect location allow the mark if more than one label line and all point to xylem locations
[3] I ref. to adhesion / H bonding between water molecules and cellulose lining
any three from
(hydrogen bonding causes) cohesion between water molecules ;
water molecules attracted to each other /AW;
water leaving xylem (at top), pulls water molecules below ;
transpiration pull;
idea that contributes to, an unbroken / a continuous, column of water (within xylem vessels);
AVP; e.g. allows movement of water against (pull of) gravity
[3] any three from
description max 2
(movement to adjacent cell) via plasmodesmata ;
(movement) through symplasm / cytoplasm / symplastic pathway / cytoplasmic pathway ;
(includes) movement through vacuole / vacuolar pathway ;
explanation
cell B has a lower water potential than A ora
or
cell C has a lower water potential than B ; ora
or
water moves down the water potential gradient / from high to low water potential / to lower water potential / from less negative to more negative water potential; R from high to low water potential gradient R to lower water potential gradient
water moves, into / out of, vacuole by osmosis
or
water crosses tonoplast by osmosis;
[3] any three from
cell wall;
large (permanent) vacuole ;
tonoplast;
plasmodesmata / cytoplasmic strands between adjacent cells ;
nucleus at, edge / periphery, of cell / AW;
AVP; many small Golgi bodies A dictyosomes
starch, granules / grains
I lack of centrioles
I chloroplasts