Xylem and phloem contain structures that are adapted for transport. Outline the differences between these structures in xylem and phloem.

Explain how the properties of water allow it to move through xylem vessels.

Outline how the structure of cellulose makes it suitable as a component of cell walls.

a. plasma membrane in phloem/sieve tubes but not in xylem/vessels
OR
xylem/vessels dead/acellular and phloem/sieve tubes alive ✔

b. xylem vessels have thicker walls «than phloem» ✔

c. xylem «vessel» walls are lignified «but phloem walls are not» ✔

d. phloem vessels have sieve plates «whereas xylem vessels have no cross walls» ✔

e. xylem/vessels are wider/larger than phloem/sieve tubes ✔

f. companion cells in phloem «but not in xylem» ✔

a. water is polar/a dipole/oxygen slightly negative and hydrogen slightly positive ✔

b. polarity results in hydrogen bonds/attraction between water molecules ✔

c. hydrogen bonding/polarity causes cohesion of water «molecules» ✔

d. cohesion/hydrogen bonding allows water to withstand tension/withstand low pressure/be pulled «upwards»/moved against gravity ✔

e. cohesion/hydrogen bonding prevents column of water «in xylem» from breaking/column of water is maintained ✔

f. adhesion of water to xylem/vessel walls «due to hydrogen bonds» ✔

a. chains of glucose/1-4 glycosidic linkages/covalent bonding between glucose ✔

b. beta glucose so alternating orientation of glucose units
OR
beta glucose forms straight chains ✔

c. forms microfibrils/long and thin/thin fibres/parallel bundles of cellulose molecules
OR
hydrogen bonding/cross linkage between cellulose molecules holds them together ✔

d. high tensile strength/rigid/doesn’t stretch so provides support/allows turgidity ✔

Many candidates knew nothing about the structure of the conducting tissues of plants and instead gave some rudimentary information on their functions. A common misconception is that phloem sieve tubes are wider than xylem vessels. This may have come from a failure to identify correctly the xylem and phloem tissue correctly in the micrograph. The most commonly known feature of xylem was the lignified wall and if phloem was the presence of companion cells.

This was better known, but many candidates still confuse adhesion with cohesion, or do not distinguish between them. Cohesion is far more significant in xylem transport because it explains how tensions can be resisted. Adhesion of water to the wall of xylem vessels is only significant when air-filled xylem vessels are refilling with aqueous sap under positive pressures. If this happens at all in a plant, it will generally only be once per year and yet adhesion to the xylem was the most common answer given. In some answers cause and effect had been confused so cohesion was given as the cause of hydrogen bonding rather than the effect.

Many candidates found this question the hardest in Section A. There were a few knowledgeable accounts that easily scored the two marks, but far more revealed little or no understanding of the structure of cellulose. It might be thought to be an obscure topic, but cellulose may be the most abundant biochemical on Earth. It is the key to how plants support themselves and grow. The paper that candidates wrote their answers on was mostly cellulose and nearly all candidates will have been wearing clothes composed largely of it. One misunderstanding that was often evident was that cellulose is freely permeable. The cell wall as a whole has this property, but it is due to the gaps between cellulose microfibrils, not to the cellulose itself. Another common mistake was to state that cellulose is made of starch, so contrasts between these two polysaccharides should be emphasised more widely.