Plant Transport: Transpiration and Translocation — AQA GCSE Biology

Plants move water and dissolved substances through two transport tissues: xylem and phloem.

Transpiration (xylem)

Transpiration is the loss of water vapour from the leaves, mainly through the stomata. This loss pulls water up from the roots through the xylem in a continuous transpiration stream.

Water is absorbed by root hair cells (large surface area), travels up the xylem, and evaporates from the leaves.

Factors that increase the transpiration rate

• Higher temperature (faster evaporation),
• more air movement / wind (removes water vapour),
• lower humidity (steeper concentration gradient),
• brighter light (stomata open wider).

Stomata and guard cells

Stomata are pores that allow gas exchange but also let water escape. Guard cells open and close the stomata: they open in the light (for photosynthesis) and close in darkness or when the plant is short of water, to reduce water loss.

Translocation (phloem)

Translocation is the transport of dissolved sugars (made in photosynthesis) through the phloem from the leaves to other parts of the plant for respiration or storage.

Comparing xylem and phloem

Exam tips

• Define transpiration and translocation precisely.
• Learn the four factors that increase transpiration and explain each.
• Guard cells control stomata to balance gas exchange and water loss.
• Distinguish xylem (water, up) from phloem (sugars, both ways).