(a) What is transpiration? (b)(i) Name two types of transpiration (ii) List two factors which affect the rate of transpiration. (c) Describe the mechanisms ...

(a) Transpiration

Transpiration is the loss of water in the form of water vapour from the aerial parts of a plant, mainly through the stomata of the leaves, but also through the cuticle and lenticels. The evaporation of water from the moist surfaces of the mesophyll cells into the air spaces, followed by diffusion of the vapour out through the stomata, creates a suction pull that draws water up the plant.

(b)(i) Two types of transpiration

• Stomatal transpiration - loss of water vapour through the stomata (the most important, accounting for the greatest proportion).
• Cuticular transpiration - loss of water vapour directly through the cuticle of the leaf and stem surface.

(Lenticular transpiration, through the lenticels of woody stems, is a third minor type.)

(b)(ii) Two factors affecting the rate of transpiration

• Temperature - a rise in temperature increases evaporation and so raises the rate.
• Humidity - high atmospheric humidity lowers the rate because the vapour-pressure gradient between the leaf and the air is reduced.

(Others include light intensity, wind or air movement, and available soil water.)

(c) Mechanism of opening and closing of the stomata

Each stoma is bordered by two guard cells whose inner walls (next to the pore) are thick and inelastic while the outer walls are thin and elastic. Opening and closing depend on changes in the turgor of these guard cells, controlled by movement of ions and sugars.

Opening (usually in daylight): In light, guard cells photosynthesise and potassium ions (K⁺) are actively pumped into the guard cells. This lowers the osmotic potential of the guard cells, so water enters them by osmosis from the neighbouring epidermal cells. The guard cells become turgid; because the thin outer wall stretches more than the thick inner wall, the guard cells curve outward and the pore between them opens.

Closing (usually at night or when water is scarce): K⁺ ions move out of the guard cells, raising their osmotic potential, so water leaves them by osmosis. The guard cells become flaccid, lose their curvature, straighten and come together, closing the pore.

(d)(i) Two similarities between transpiration and sweating

• Both involve the loss of water from the body of the organism to the environment.
• Both bring about cooling of the organism by the evaporation of water.

(d)(ii) Two differences between transpiration and sweating