Phosphate ions are absorbed from the soil solution by roots and are needed for cellular processes throughout plants. Scientists investigated the movement of phosphate ions in flowering plants. The scientists discovered that phosphate ions in the leaves are transported from the roots in the xylem. Only a small proportion of the phosphate ions that are absorbed are transported to the growing points of the roots and shoots. Gossypium hirsutum is the most common species of plant grown for the production of cotton across the world. Scientists carried out an investigation to trace the pathway taken by phosphate ions from the leaves of cotton plants into the stems. The scientists used a radioactive isotope of phosphorus (32P) to trace the pathway of phosphate ions. Some cotton plants were divided into two groups: A and B. In group A, the scientists: • inserted impermeable waxed paper between the xylem and phloem in the stem below a leaf of each plant • injected a solution containing phosphate ions labelled with 32P (labelled phosphate ions) into a vein of each leaf, as shown in Fig. 5.1. The procedure was repeated on the plants in group B but without inserting the waxed paper. After one hour, the scientists determined the percentage of labelled phosphate ions in the four sections of the stem, S1 to S4, shown in Fig. 5.1. The results are shown in Table 5.1. [Figure 5.1] [Table 5.1]

Mark scheme

Marking points from the official mark scheme (May/June 2025). The mark or mark type is shown in brackets.

1. [1] (a) 1 most water (in xylem) goes to the leaves taking (dissolved) phosphate ions with it ;
2. [1] (a) 2 xylem tissue does not extend into, growing points / shoot tips / root tips;
3. [1] (a) 3 higher demand for phosphate ions in leaves (for photosynthesis) ;
4. [1] (a) 4 water and phosphate ions are transported upwards in the xylem (not down to root tips);
5. [1] (a) 5 phosphate ions are absorbed in area above root tips (so don't go down) ; A transport to root tips involves movement in phloem
6. [1] (a) 6, 7 AVP ;; e.g. correct ref to blockage by, endodermis / Casparian strip / suberin (apoplast pathway) not all phosphate ions cross cell (surface) membrane of endodermal cells (symplast pathway) phosphates may be stored in (vacuoles of cells in), roots / stems phosphate ions are used in the root for making, ATP / nucleotides / AW
7. [1] (b) 1 phosphate ions transported in the phloem move, out of the leaf / into stem ;
8. [1] (b) 2 (group B data suggests that) phloem transports (some of) phosphate ions downwards ;
9. [1] (b) 3 phosphate ions can move from phloem to xylem ;
10. [1] (b) 4 waxed paper prevents movement (between phloem and xylem) ; ora
11. [1] (b) 5 data in support of difference between waxed and unwaxed in the same sample area ; e.g. in S1 1% in xylem in A as opposed to 5% in xylem in B / percentage in xylem always higher in group B
12. [1] (b) 6 some phosphate ions may transfer to surrounding cells in the stem ;
13. [1] (b) 7 much of the phosphate may not have left the leaf / much of the phosphate has been transported away from the leaf in the hour ;
14. [1] (b) 8 AVP; e.g. some may travel upwards in the phloem but no data not clear that results show phosphate ions transported upwards in xylem ref. to anomalous results / results not conclusive not all phosphate ions injected are accounted for