Plant organ system: Students should be able to explain the effect of changing temperature, humidity, air movement and light intensity on the rate of transpiration.

The idea

Transpiration is the loss of water vapour from leaf surfaces, mainly through stomata. Water evaporates from moist cell walls into air spaces and diffuses out when the stomata are open. The rate depends on the diffusion gradient between the humid air inside the leaf and the drier air outside. Raising temperature increases kinetic energy and evaporation, so the gradient steepens and the rate rises. High humidity reduces the gradient, slowing diffusion. Air movement sweeps away humid air, maintaining a steep gradient and increasing the rate. Light intensity controls stomatal opening: in most plants stomata open in bright light, increasing the rate, while darkness closes them and reduces it. Explaining means linking each change to the gradient or to stomatal aperture, not merely naming a direction.

What earns the marks

• Transpiration is the diffusion of water vapour out of a leaf, mainly through stomata, down a concentration gradient.
• Higher temperature increases the kinetic energy of water molecules, so evaporation into the leaf air spaces and diffusion out increase the rate.
• Higher humidity reduces the water vapour concentration gradient between the leaf interior and the outside air, so the rate falls.
• Air movement removes humid air from around the leaf, keeping the gradient steep and increasing the rate.
• Greater light intensity usually opens stomata, providing more routes for diffusion and increasing the rate; darkness closes them and reduces it.
• A full explanation links each environmental change to either the diffusion gradient or the size of the stomatal opening.

Common mistakes and how to put them right

• Saying transpiration is the movement of water up the stem.

  Put it right by stating it is loss of water vapour from leaves, mainly through stomata.

• Claiming high humidity increases transpiration because water is present.

  Put it right by explaining that a smaller concentration gradient slows diffusion.

• Treating light as directly speeding up diffusion.

  Put it right by explaining that light mainly affects stomatal aperture, which controls how easily vapour escapes.

Exam tips

• For each factor, state the direction of change and then give the reason using the words gradient or stomata.
• Use comparative language such as increases, decreases, steeper or shallower to make the explanation precise.
• If asked to explain a graph, read the trend first, then link each section to temperature, humidity, air movement or light intensity.