Content Water and Ion Uptake Transpiration and Translocation Learning Outcomes: Candidates should be able to: (a) identify the positions of xylem vessels and phloem in sections of a typical dicotyledonous stem and leaf, under the light microscope, and state their functions. (b) relate the structure and functions of root hairs to their surface area, and to water and ion uptake. (c) state that transpiration is the loss of water vapour from the stomata. (d) briefly explain the movement of water through the stem in terms of transpiration pull. (e) describe the effects of variation of air movement, temperature, humidity and light intensity on transpiration rate how wilting occurs (f) define the term translocation as the transport of food in the phloem tissue.

Transport in Plants Xylem: conduct water and dissolved mineral salts from the roots to other parts of the plant. provide mechanical support within the plant Adaptations: –Dead vessel with no protoplasm. Hence there is a continuous hollow lumen in the vessel to allow the easy passage of water and dissolved mineral salts. –Lignified walls to provide mechanical support Phloem Conducts food substances from the green parts of the plants especially the leaves to the rest of the plant.

Transport in Plants

Water is taken in by osmosis (active transport in desert/ during draught) Mineral salts are taken in by diffusion and active transport Adaptations of root hair cells Long and narrow to increase the surface area to volume ratio which increases the rate of absorption of water and dissolved mineral salts Cell sap contains sugars, amino acids and salts, hence lower potential, creating water potential gradient for water to be taken in by osmosis Large vacoule to contain as much water

Transport in Plants Movement of food substances Translocation: Transport of food substances like sugars and amino acids in phloem from leaves to other parts of the plant Movement of water Conducted upwards from roots to leaves through the xylem This is aided by transpiration pull

Transpiration Loss of water vapour from stomata of leaves Importance of transpiration: Creates suction force to conduct water and dissolved mineral salts up the plant from the roots to the leaves by transpiration pull removes heat from the plant

Transpiration Factors affected by Light In sunlight, stomata open, increasing rate of evaporation. Hence, rate of transpiration increases. In darkness, stomata close and decrease the rate of transpiration. Humidity of air At higher humidity, amount of water vapour is higher. This decreases the water vapour concentration gradient between the leaf and the atmosphere. Hence, rate of transpiration decreases. Wind or air movement Wind carries away water vapour from the surface of the leaf, increasing the water vapour concentration gradient. Hence, rate of transpiration increases. Temperature of air A rise in the temperature of the surroundings increases the rate of evaporation. Hence, rate of transpiration increases.

Wilting Caused by excessive transpiration: rate of water loss is much greater than rate of water uptake Excessive water loss causes plant cells to lose its turgor and becomes flaccid Turgid plantWilted plant Upright stemLimp stem Leaves are held out firmlyLeaves droop Plant cells are firmPlant cells are flaccid

Wilting Advantages: When plants wilt, the leaves folds up reducing the exposed surface area to sunlight. Guard cells becomes flaccid and stomata closed and reduces the rate of transpiration. This prevents further loss of water. Disadvantages: When stomata is closed, it reduces the entry of carbon dioxide and this will reduces the rate of photosynthesis for the plant.