Students should be able to describe the process of transpiration and…

The idea

Transpiration is the loss of water vapour from the leaves and stems of a plant, mainly through stomata. Water evaporates from the surfaces of cells inside the leaf, diffuses through the air spaces and exits through stomata. This loss creates a pull that draws water up the xylem from the roots. Stomata are small pores, mostly on the lower surface of a leaf, each surrounded by two guard cells. When guard cells take in water they become turgid and the pore opens; when they lose water they become flaccid and the pore closes. This controls gas exchange and water loss. Translocation is the movement of dissolved sugars and other substances through the phloem from sources, such as leaves, to sinks, such as growing regions and storage organs.

What earns the marks

• Define transpiration as the loss of water vapour from plant surfaces, mainly through stomata.
• Describe the pathway of water loss: evaporation from cell surfaces inside the leaf, diffusion through air spaces, and exit through stomata.
• Explain that transpiration creates a transpiration pull that moves water up the xylem from the roots.
• Describe stomata as pores surrounded by guard cells that open and close to control gas exchange and water loss.
• Explain that guard cells become turgid when water enters, opening the pore, and flaccid when water leaves, closing the pore.
• Define translocation as the transport of dissolved sugars through the phloem from sources to sinks.
• Compare transpiration and translocation in terms of the substance moved, the vessel used and the direction of movement.

Common mistakes and how to put them right

• Stating that transpiration is the movement of water up the xylem.

  Put it right: State that transpiration is the loss of water vapour, while the transpiration pull moves water up the xylem.

• Confusing stomata with guard cells.

  Put it right: State that stomata are the pores and guard cells are the cells that surround and control them.

• Saying that translocation occurs in the xylem.

  Put it right: State that translocation occurs in the phloem.

• Describing stomata as always open.

  Put it right: Explain that guard cells open and close the pore in response to water availability.

Exam tips

• Use the terms 'water vapour', 'diffusion' and 'stomata' when describing transpiration.
• When describing stomata, name both the pore and the guard cells and state what each does.
• For translocation, state the substance, the vessel and the direction from source to sink in one sentence.