Chapter 7 : Nutrition in Plants 7.2 Leaf Structure and function External Structure of Leaf • Network of Veins – Veins carry water, mineral salts and manufacture food. • Lamina – Large flat surface so as to obtain maximum light for photosynthesis. Lamina allow carbon dioxide to rapidly reach inner cells. • Petiole – Holds lamina away from stem. So as to obtain sufficient sunlight and air. Internal Structure of Leaf 1. Upper Epidermis Waxy Cuticle: Prevent loss of water, transparent to allow sunlight to pass through Single layer of closely packed cells. Internal Structure of Leaf 2. Palisade Mesophyll Closely packed, long and cylindrical cells Contains numerous chloroplast : Absorb maximum sunlight for photosynthesis. Internal Structure of Leaf 3. Spongy Mesophyll Irregularly shaped cells Contains numerous large intercellular spaces : for diffusion of gases Fewer chloroplast as compared to palisade mesophyll. Internal Structure of Leaf 3. Spongy Mesophyll Cells are layered with a thin film of moisture : Allow Carbon Dioxide to dissolve in Presence of vascular bundle (xylem + phloem) Internal Structure of Leaf 4. Lower Epidermis Single layer of closely packed cells Cuticle Many minute opening known as stoma : Important of gaseous exchange Guard Cells • Bean-shaped • Contains chloroplast. Difference between Guard Cells and Epidermis Cells Shape Chloroplast Cell Wall Function Guard Cells Bean-shaped Epidermis Cell Irregular shaped √ Cell wall near the stoma is thicker than elsewhere. X Uniform cell thickness Controls diffusion rate Does not control diffusion Internal Structure of Lamina: Guard Cells • During Day, 1. Water potential of guard cell decreases. 2. Water from neighbouring cells would start to enter guard cells by osmosis 3. Guard cells become turgid. Internal Structure of Lamina: Guard Cells • During Night, 1. Water potential of guard cell increases. 2. Water from neighbouring cells would start to leave guard cells by osmosis 3. Guard cells become flaccid. How does Carbon Dioxide and Water enter the leaf? • Carbon dioxide enter the leaf through the stomata. • Water is transported in the leaf through xylem (vascular bundle) How does Carbon Dioxide enter the leaf? • Through stomata • Carbon dioxide is used for photosynthesis • During the day, carbon dioxide is rapidly used up during photosynthesis • As a result, there is a lower concentration of carbon dioxide in leaf. • Carbon dioxide from air would then diffuse into leaf through stomata and goes into mesophyll cells. How does Water enter the leaf? • Vascular bundle – xylem and phloem • Xylem transports water and dissolved minerals from roots to leaf • Water and mineral salts move from xylem to mesophyll cells. • Water is used for photosynthesis.