Question
The micrograph shows a vascular bundle from the stem of a barley plant.
a. Xylem and phloem contain structures that are adapted for transport. Outline the differences between these structures in xylem and phloem. [2]
b. Explain how the properties of water allow it to move through xylem vessels. [2]
c. Outline how the structure of cellulose makes it suitable as a component of cell walls. [2]
▶️Answer/Explanation
Markscheme
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 ✔