IB Biology Topic B3.2.7: Transport of Water from Roots to Leaves During Transpiration Notes

Transpiration Pulls Water from Roots to Leaves

1. Water is lost as vapour from the leaves, and this loss pulls a continuous column of water up from the roots.
2. This whole movement of water up the plant is called the transpiration stream.
3. The water travels upward through the xylem vessels, the dead tubes that run from root to leaf.

Evaporation in the Leaf Starts the Pull

1. Water absorbed from the soil travels up through the roots, into the xylem, and into the leaf cells.
2. Inside the leaf, water evaporates from the cell walls of the spongy mesophyll into the air spaces.
   1. The water vapour then diffuses out through the stomata, tiny pores on the leaf surface, and into the atmosphere.
   2. This evaporation and diffusion of water from the leaf is transpiration.

Tension in the Xylem Draws Water Upward

1. As water evaporates from the leaf cell walls, it creates a negative pressure (tension) inside the xylem.
2. This tension pulls more water up the xylem vessels toward the leaves.
3. The water column holds together because of cohesion, the attraction between water molecules caused by hydrogen bonding.
4. Water also sticks to the walls of the xylem by adhesion, which helps support the column as it rises.

Keeping the Water Column Continuous

1. Water can only be pulled up if the column in the xylem stays unbroken from root to leaf.
2. The cohesive forces between water molecules hold the column together so it does not break under tension.
3. If air enters the xylem, the column can break and disrupt the flow of water.

Water Uptake at the Roots Replaces What Is Lost

1. Water lost from the leaves is constantly replaced by water taken up at the roots.
2. Water enters the root cells from the soil by osmosis and moves across to the xylem in the centre of the root.

Abiotic Factors Change the Rate of Transpiration

1. Light
   1. Brighter light opens the stomata wider, so more water vapour escapes and transpiration speeds up.
2. Temperature
   1. Higher temperature makes water evaporate faster, increasing the rate of transpiration.
3. Humidity
   1. Moist air outside the leaf reduces the gradient for water vapour to diffuse out, so transpiration slows.
4. Wind
   1. Wind blows away the moist air next to the leaf, keeping the gradient steep and speeding transpiration up.

Why the Transpiration Stream Matters

1. The stream carries dissolved mineral ions from the soil up to the leaves.
2. Evaporation from the leaf surface cools the plant, much like sweating cools the body.
3. Water reaching the leaves keeps cells turgid, supplying the turgor pressure that supports soft plant tissue.

Recap the Stream in One Line

Evaporation at the leaves creates tension, cohesion holds the column together, and water is pulled up the xylem while more is taken in at the roots.