79 Transpiration
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114
Key Idea: Water moves through the xylem primarily as a result of evaporation from the leaves and the
PR E V ON IEW LY
cohesive and adhesive properties of water molecules.
ffIn a vascular plant, water moves from the roots to the leaves via the xylem. Approximately 99% of the water
absorbed from the soil by a plant's roots is lost by evaporation from the plant's leaves and stems. This loss of water is called transpiration and occurs mostly through pores in the leaf called stomata (sing. stoma).
ffPlants rely on an increase in solute concentration from the roots to the leaves to transport water up the plant. Water flows passively from a low solute concentration (high water concentration) to a high solute concentration (low water concentration). This gradient, called transpiration pull, is primarily responsible for water moving up the plant, although cohesion-tension and root pressure also play a part (see below).
ffThe transpiration stream provides a constant supply of water for essential life processes and helps the plant to maintain an adequate uptake of minerals from the soil. Evaporative water loss also cools the plant.
Air
Water
Evaporative loss of water from the leaves as water vapour
The role of stomata
Water loss occurs mainly through stomata (pores in the leaf). The rate of water loss can be regulated by specialised guard cells each side of the stoma, which open or close the pore.
ffStomata open: gas exchange and
Leaves
transpiration rate increase.
Highest solute concentration Lowest water concentration
ffStomata closed: gas exchange and transpiration rates decrease.
Water flows passively from a low solute concentration (high water concentration) to a high solute concentration (lower water concentration). This gradient is the driving force in the transport of water up a plant.
G
S
G
EII
Water
The continuous flow of water is called the transpiration stream. It is primarily responsible for water moving up the plant.
Solute particle
Soil
Highest water concentration Lowest solute concentration
Water
1. What is transpiration?
2. How does the plant regulate the amount of water lost from the leaves?
WEB
KNOW
79
LINK
80
No Cla t fo ssr r o Us om e
Xylem
LINK
81
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