Xylem and phloem are the transport tissues of plants. Both tissues have more than one cell type. The conducting cells of xylem contain xylem sap and those of phloem contain phloem sap. The composition of xylem sap differs from the composition of phloem sap. (a) The main component of xylem sap and phloem sap is water. Explain why water is the main component of xylem sap and phloem sap.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2022). The mark or mark type is shown in brackets.

1. [1] (water is a) solvent, for, assimilates / minerals / ions / polar molecules OR A hydrophilic OR A named substances, in xylem and phloem sap / that are transported, dissolve in water / are in solution ; A (water is a) good / universal / versatile, solvent
2. [1] feature for xylem ; e.g. water can move up, as part of transpiration stream ; ref. cohesion between water molecules ; adhesion between water and, cellulose / wall of xylem / xylem lining ; hydrogen bonds for cohesion and adhesion ; I for cohesion-tension
3. [1] feature for phloem ; e.g. water used to build up, hydrostatic pressure / pressure gradient / pressure differences, for, mass flow / translocation / transport / AW ; if not gained for mp2 allow similar ref. to cohesion / adhesion, for, phloem sap movement / transport
4. [1] AVP; e.g. (xylem sap) water moving down a water potential gradient contributes to transpiration stream
5. [1] A phloem sieve tube element
6. [1] B companion cell
7. [1] C companion cell