2.3.2 Plant Organ Systems

What is translocation?

Translocation is the movement of food substances made in the leaves up or down the phloem, for use immediately or storage.

What is transpiration?

Transpiration is the loss of water vapour from the leaves and stems of the plant, as a consequence of gaseous exchange.

How does water move in transpiration?

Water evaporates at the open stomata, and as molecules leave the plant they are pulled up through the xylem, creating a continuous transpiration stream.

What is the transpiration stream?

The continuous movement of water being taken up from the soil through the roots, pulled through the xylem, and evaporating at the leaves.

How does increased temperature affect transpiration?

Higher temperature makes molecules move faster, so evaporation happens quicker. Photosynthesis also increases, opening more stomata, so more water evaporates.

How does increased relative humidity affect transpiration?

High relative humidity reduces the concentration gradient between inside and outside the leaf, so diffusion slows and transpiration decreases.

How does increased air movement affect transpiration?

Wind removes water vapour around the leaf, making the concentration gradient steeper. Diffusion speeds up, increasing the rate of transpiration.

How does increased light intensity affect transpiration?

More light increases photosynthesis, so stomata open more to let in carbon dioxide. This allows more water vapour to escape, increasing transpiration.

What are the adaptations of guard cells?

Guard cells are kidney shaped, with thin outer walls and thick inner walls. When water is abundant, they swell and open the stomata, allowing gas exchange and water loss. They are also light sensitive.

Why are most stomata found on the underside of leaves?

To minimise water loss by evaporation, as the lower surface is shaded and cooler, while still allowing gas exchange.

How can the rate of transpiration be measured?

By using a potometer, where a plant is placed in a tube of water and the distance travelled by a bubble is measured.