Transpiration

Water Movement Mass flow of water in xylem vessels – pressure lowered as water leaves vessels. Water moves up from the roots where the pressure is higher. Diffusion of water through cells from xylem. Water vapour diffuses through leaf air spaces. If the concentration of water vapour in the leaf is higher than outside, water vapour will diffuse out of the leaf through the stomata. Capillary action (water molecules attracted to polar molecules in xylem walls (adhesion)).

Cross Section of A Leaf

Opening/ Closing Stomata Stomata can be closed to prevent water loss. Guard cells curve apart when turgid. When flaccid, the edges of the cells lie close together. However, this drastically slows transpiration and means that no carbon dioxide can enter the leaf (= no photosynthesis).

Stomata Diagrams Animation link

Factors Affecting Water Loss Diffusion rate is affected by: Surface area (the higher the surface area, the greater the rate of diffusion). Difference in concentration (the greater the concentration gradient, the higher the rate of diffusion). Length of the diffusion path (the greater the length, the lower the rate of diffusion).

Preventing Excess Water Loss Cuticle (waxy layer) on leaf which is impermeable to water. Most stomata found on underside of leaf as it is cooler in dicotyledons. Thick leaves = reduced water loss. Spines/ hairs increasing boundary layer (undisturbed layer of air). Stomata closed at certain times of the day. Stomata may be sunken and found in pits.