Plant organ system: Students should be able to describe the process of transpiration and translocation, including the structure and function of the stomata.

The idea

Transpiration is the loss of water vapour from plant leaves, mainly through stomata. Water evaporates from the surfaces of mesophyll cells, diffuses through air spaces and exits through stomatal pores. This creates a pull that draws water up the xylem from the roots in a continuous column. Stomata are tiny pores, each surrounded by two guard cells; they open to allow carbon dioxide in for photosynthesis and close to reduce water loss. Translocation is the movement of dissolved sugars and other organic substances through the phloem from sources such as leaves to sinks such as roots, fruits and growing tips. It requires energy and can occur in either direction.

What earns the marks

• Defines transpiration as the loss of water vapour from leaves, mostly through stomata.
• Explains that evaporation and diffusion of water vapour out of stomata create a pulling force that moves water up the xylem.
• Describes stomata as pores, usually on the lower leaf surface, each bounded by two guard cells.
• States that stomata allow carbon dioxide to diffuse in and oxygen and water vapour to diffuse out.
• Explains that guard cells change shape to open or close the stomatal pore, balancing gas exchange with water loss.
• Defines translocation as the transport of dissolved sugars through the phloem from source to sink, requiring energy.
• Recognises that translocation can move substances in different directions, unlike the one-way flow of water in the xylem.

Common mistakes and how to put them right

• Confusing transpiration with translocation.

  Put it right: Transpiration is water loss and xylem transport, while translocation is sugar transport in the phloem.

• Stating that stomata are always open.

  Put it right: Guard cells open and close stomata to control gas exchange and water loss.

• Thinking that translocation only moves sugars downwards.

  Put it right: Sugars move from source to sink, which may be up or down depending on where they are needed.

Exam tips

• Use the terms xylem and phloem correctly: xylem for water and mineral ions, phloem for sugars.
• When describing stomata, mention guard cells and the balance between gas exchange and water loss.
• For translocation, always state the source and the sink and note that energy is required.