Question 19M.2.SL.TZ1.4b
| Date | May 2019 | Marks available | [Maximum mark: 2] | Reference code | 19M.2.SL.TZ1.4b |
| Level | SL | Paper | 2 | Time zone | TZ1 |
| Command term | Explain | Question number | b | Adapted from | N/A |
Explain how the properties of water allow it to move through xylem vessels.
[2]
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» ✔
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.
