Question 20N.2.SL.TZ0.7b
| Date | November 2020 | Marks available | [Maximum mark: 4] | Reference code | 20N.2.SL.TZ0.7b |
| Level | SL | Paper | 2 | Time zone | TZ0 |
| Command term | Describe | Question number | b | Adapted from | N/A |
Describe the processes that cause water to move from the roots of plants to their leaves.
[4]
a. water moved/transported in xylem vessels;
b. transported under tension/suction/pulled up (in xylem vessels);
c. transpiration/loss of water (vapour) generates pulling forces/low pressure/tension;
d. tension/pull generated when water evaporates from cell walls (in mesophyll);
e. transpiration is loss of water vapour from leaf (surface)/stomata;
f. cohesivity/cohesion in water due to hydrogen bonding/attractions between water molecules;
g. cohesion/WTTE so chain/column of water (molecules) doesn’t break/remains continuous;
h. transpiration stream is a column of/flow of water in xylem from roots to leaves;
Do not award marks for absorption of water by roots.
This was answered quite well. There were some traditional areas of confusion, with cohesion and adhesion either muddled up or treated as the same thing. Some candidates thought high pressure caused by roots and low pressure caused by leaves could exist at the same time in xylem. Few responses referred to water being transported in the vessels of xylem. In some answers water was said to evaporate from stomata, instead of the moist, blotting-paper-like cells walls of the mesophyll. Some answers included details of how water is absorbed into roots, which was outside the scope of the question. Despite these common faults, many candidates described clearly how tension is generated in xylem and how cohesive columns of water can be pulled up to leaves.
