How does water move through xylem vessels?

Water moves through xylem vessels via a process called transpiration, driven by the evaporation of water from the leaves.

Transpiration is a crucial process in the movement of water through xylem vessels. It begins in the leaves of the plant, where water evaporates from the cells into the air spaces within the leaf. This evaporation creates a water vapour deficit in the leaf air spaces, causing water to be drawn out from the neighbouring cells, creating a 'pull' effect. This pull is transmitted all the way down the plant, through the xylem vessels, to the roots.

The xylem vessels are essentially long, hollow tubes that run from the roots to the leaves of the plant. They are made up of dead cells, which have lost their end walls and cytoplasm, leaving a continuous, unobstructed pathway for water to move through. The walls of the xylem vessels are thickened with lignin, a substance that makes them strong and prevents them from collapsing under the tension created by transpiration.

The movement of water through the xylem vessels is also facilitated by the properties of water itself. Water molecules are polar, meaning they have a positive and a negative end. This allows them to form hydrogen bonds with each other, creating a continuous chain of water molecules from the roots to the leaves. This is known as cohesion. Additionally, water molecules are attracted to the walls of the xylem vessels, a phenomenon known as adhesion. Together, cohesion and adhesion help to maintain the continuous flow of water up the plant against the force of gravity.

In summary, the movement of water through xylem vessels is a complex process involving transpiration, the physical structure of the xylem vessels, and the properties of water itself. It is a vital process for the survival of the plant, enabling it to transport water from the roots, where it is absorbed from the soil, to the leaves, where it is used in photosynthesis and transpired into the atmosphere.