Biology XI: 11 Transport in plants(6 Transpiration and its Relation

Transpiration and its Relation with Photosynthesis Transpiration Occurs manly through openings called stomata. Stomata: Open in the day and close during the night Also contribute in the exchange of O2 and CO2 Opening and closing of stomata is influenced by the turgidity of the guard cells Inner walls of the guard cells (towards stomatal opening): Thick and elastic When turgidity increases within two guard cells flanking each stomatal pore, the thin outer walls bulge, and the inner walls assume a crescent shape. Radial orientation of microfibrils in the cell wall of the guard cells makes it easier for the stomata to open. When turgidity decreases within the guard cells, the inner walls regain their original shape and stomata closes. Based on the distribution of stomata, 2 types of leaves: Dorsiventral: More number of stomata on the lower surface of leaves; found in dicots Isobilateral: Equal number of stomata on both sides of leaves; found in monocots Transpiration provides the transpirational pull which is responsible for the upward movement of water in tall plants. Relationship of Transpiration with Photosynthesis Process of photosynthesis requires water. Xylem provides water to the site of photosynthesis (leaves). As the water evaporates, in order to maintain a thin film of water over the cells, water is pulled from the xylem to the leaves. Also, due to the lower concentration of water in the atmosphere, water diffuses into the atmosphere to create a pull. Actively photosynthesising plants need more water, and availability of water can be a limiting factor for photosynthesis. C4 photosynthetic pathway makes sure that the minimum loss of water for the maximum CO2 is fixed. For same amount of CO2 fixed, a C4 plant loses only half the amount of water lost by a C3 plant. Importance of Transpiration Creates transpirational pull for transport Supplies water for photosynthesis Transports minerals from soil to all parts of a plant Cools the surface of the leaves (due to the evaporation of water) Keeps the cells turgid; hence, maintains their shape