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
a. Differences between structures in xylem and phloem [2]
- Xylem vessels are made of dead, hollow cells (vessel elements) with lignified walls to provide strength and support; they form continuous tubes for the one-way upward transport of water and minerals.
- Phloem is made of living cells, including sieve tube elements (which have sieve plates and no nuclei) and companion cells (with nuclei and mitochondria) for the two-way transport of sugars and other organic molecules.
b. How water moves through xylem [2]
- Cohesion: Water molecules stick together due to hydrogen bonding, forming a continuous column in xylem.
- Adhesion: Water molecules also stick to the walls of xylem vessels, helping water move upwards against gravity (capillary action).
These properties allow water to be pulled up the plant in a transpiration stream.
c. Structure of cellulose for cell wall function [2]
- Cellulose is made of β-glucose units linked by β-1,4-glycosidic bonds, forming long, unbranched chains.
- These chains form microfibrils through hydrogen bonding, giving the cell wall high tensile strength, rigidity, and resistance to pressure ideal for supporting plant cells.