Biology 13.7 Movement of Water Through Roots

Vast majority of plants = terrestrial organisms, so they need to conserve water and are covered by a waterproof layer.

Means they can't absorb water over body surface - have a special exchange surface = root hairs.

Uptake of water by roots

Root hairs = exchange surfaces in plants - responsible for absorption of water and mineral ions.

Plants lose water by transpiration so root hairs must absorb water to replace this.

Each root hair = long thin extention of root epidermal cell.

They are efficient exchange surfaces for water and mineral ions because:

• Provide large surface area (they are long extensions) and occur in thousands on each root.
• They have thin surface layer (cell membrane and cellulose cell wall) - materials can cross easily.

Root hairs arise from epidermal cells just behind the tips of young roots.

Hairs grow around soil particles so in damp conditions they are surrounded by a soil solution that is mostly water.

Soil solution = high water potential, whereas root hairs contain sugars, amino acids and mineral ions = low water potential.

Water therefore moves by osmosis into the root hairs down w.p. gradient.

Water continues through roots in two ways:

• Apoplastic pathway (the apoplast)
• Symplastic pathway (the symplast)

The apoplastic pathway

Water = drawn into endodermal cells and pulls more water behind it (water molecules have cohesive properties).

Creates a tension that draws water along cell walls of the cells of the root cortex.

Many water-filled spaces exist in meshlike structures of cellulose cell walls of these cells = little or no resistance to the pull…