Transport of Water in the Xylem (AQA A-Level Biology): Revision Notes

Transport of Water in the Xylem

Plants absorb water through their root hair cells, which are extensions of root cells that increase surface area for absorption. This water must travel from the roots all the way up to the leaves, sometimes over 100 metres in tall trees. The transport system that carries water upwards consists of xylem vessels - hollow, thick-walled tubes that form continuous pathways from roots to leaves.

The driving force for water movement through plants is transpiration - the evaporation of water from leaf surfaces. This process is powered by solar energy and operates passively, meaning the plant doesn't need to spend metabolic energy to move water upwards.

Water movement out through stomata

Water exits the plant through small pores called stomata, located mainly on the undersides of leaves. The process works because:

• Atmospheric humidity is typically lower than the humidity in the air spaces inside leaves
• This creates a water potential gradient from inside the leaf to the external environment
• When stomata are open, water vapour molecules diffuse from the humid air spaces to the drier atmosphere
• As water is lost through diffusion, it's replaced by water evaporating from the cell walls of surrounding mesophyll cells

Plants can control their rate of transpiration by adjusting the size of their stomatal openings, allowing them to balance water loss with gas exchange needs.

Water movement across leaf cells

When mesophyll cells lose water through evaporation into air spaces, a chain reaction begins that draws water from the xylem vessels. This happens through the following sequence:

Mesophyll cells lose water to air spaces by evaporation due to heat from the sun. As these cells lose water, their water potential decreases (becomes more negative). Water then moves by osmosis from neighbouring cells with higher water potential into the mesophyll cells.

This water loss from neighbouring cells lowers their water potential in turn, causing them to draw water from their neighbours by osmosis. This process continues cell by cell, creating a water potential gradient that extends all the way back to the xylem vessels.

This water potential gradient effectively pulls water from the xylem, across the leaf mesophyll, and finally into the atmosphere.

Movement of water up the xylem stem

The upward movement of water through xylem vessels is explained by the cohesion-tension theory. This theory describes how water moves from roots to leaves through the following mechanism:

Water evaporates from mesophyll cells, creating the transpiration pull described above. Water molecules form hydrogen bonds with each other, a property called cohesion. Due to cohesion, water molecules stick together and form a continuous, unbroken column throughout the xylem vessels.

When transpiration creates a pull at the top of this water column (in the leaves), the entire column moves upward due to cohesive forces between molecules. This creates tension within the xylem vessels - essentially a negative pressure that pulls the water column upward.

The transpiration pull puts the entire xylem under tension, which is why this mechanism is called the cohesion-tension theory.

Evidence supporting cohesion-tension theory

Several pieces of evidence support the cohesion-tension theory:

Vessel continuity: Xylem vessels have no end walls, forming continuous tubes from roots to leaves. This structure is essential for maintaining unbroken water columns required by the cohesion-tension theory.

Complete water transport pathway

This entire process is passive, requiring no metabolic energy from the plant. The energy driving the system comes from solar radiation, which provides the heat for evaporation. Xylem vessels themselves are dead cells, consisting only of cell walls with no cytoplasm, which creates the hollow tubes necessary for efficient water transport.