3 Fig. 3.1 shows a cross-section through a leaf of a crop plant. Fig. 3.1 (a) Name the structures labelled P, Q and R on Fig. 3.1. [3] (b) Describe the role...

Labelled answer diagram:

(a) Structures labelled on Fig. 3.1: [3]

• P = Upper epidermis (or cuticle, the waxy waterproof layer on the leaf surface). [1]
• Q = Xylem vessel (within the vascular bundle/midrib of the leaf). [1]
• R = Stoma (stomatal pore, the opening in the lower epidermis). [1]

(b) Role of structure Q (xylem) in the movement of water through the plant: [2]

1. The xylem carries water (and dissolved mineral ions) from the roots, through the stem and into the leaf. [1]
2. It provides a continuous water column that supplies the mesophyll cells, where the water is used in photosynthesis or evaporates during transpiration. [1]

(c) How water vapour is lost from the leaf during transpiration: [3]

1. Water evaporates from the wet surfaces of the mesophyll cells into the air spaces (intercellular spaces) inside the leaf. [1]
2. The water vapour then diffuses out through the open stomata into the surrounding atmosphere. [1]
3. This diffusion is driven by the concentration gradient (water potential gradient) between the moist interior of the leaf and the drier air outside. The greater the difference, the faster the rate of transpiration. [1]

(d) Two environmental factors that increase the rate of transpiration: [2]

1. Higher temperature - increases the kinetic energy of water molecules, causing faster evaporation from mesophyll surfaces. [1]
2. Increased wind speed - blows away the layer of humid air around the leaf surface, maintaining a steep diffusion gradient for water vapour. [1]

Other acceptable factors: lower humidity; increased light intensity (which causes stomata to open wider).

(e) Why wilting occurs when transpiration exceeds water absorption: [2]

1. The cells lose water faster than it can be replaced by absorption from the roots, so the water content of the cells decreases. [1]
2. The cells lose turgor pressure and become flaccid (limp). Without the internal pressure that normally keeps them rigid, the stems and leaves droop - this visible drooping is wilting. [1]