A Level Biology Revision "The Structure and Functions of Xylem"

Xylem vessels originate as plant cells that form a continuous column from roots to leaves. Lignin, an impermeable carbohydrate, forms within their cell walls, causing the living contents to die and end walls to break down. This creates non-living, hollow tubes. Regions free of lignin, called pits, allow water and dissolved substances to pass between vessels and to surrounding cells. The primary function of xylem vessels is to transport water and dissolved minerals upwards through the plant.

Lignin in xylem vessels can be arranged in spirals or rings, or be nearly continuous with pits. It provides structural support and prevents the vessels from collapsing when water is pulled up, which can cause a drop in pressure.

Xylem fibers are long, narrow cells with large amounts of lignin. Like xylem vessels, their internal contents die. However, xylem fibers do not transport water; instead, their main function is to provide mechanical support to the plant.

While xylem vessels and fibers are non-living, xylem also contains living parenchyma cells. These cells store starch and can contain tannins, bitter compounds that deter herbivores from consuming the plant.

Xylem is composed of two main tissue types: xylem vessels and xylem fibers. This video focuses on their structure and functions.