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Lesson 3 Learning Objectives 1.Relate the structure and functions of root hairs to their surface area and to water and ion uptake. 2.Explain the movement of water from the soil to the root hairs. 3.Explain the mechanism of root pressure. 4.Explain the mechanism of capillary action. 5.State that root pressure and capillary action are ways to draw water up the stem 6.Define transpiration 7.Explain the mechanism of transpiration producing a pull from above. Outline Transport of water – Soil to root hairs (osmosis) – Up the stem (root pressure, capillary action, transpiration pull) Transport of water • From soil into root hairs – Osmosis • Up the stem – Root pressure – Capillary action – Transpiration pull Transport of water into the root hairs cytoplasm soil particles vacuole 2 1 2 The thin film of liquid surrounding each soil particle is a dilute solution of mineral salts. cell surface membrane of root hair cell nucleus cell wall film of liquid (dilute solution of mineral salts) 1 Each root hair is a fine tubular outgrowth of an epidermal cell. It grows between the soil particles, coming into close contact with the water surrounding them. 3 4 The sap in the root hair cell is a relatively concentrated solution of sugars and various salts. Thus, the sap has a lower water potential than the soil solution. These two solutions are separated by the partially permeable cell surface membrane of the root hair cell. Water enters the root hair by osmosis. a section of root showing the path of water through it 5 C water entering the root hair 4 B A 3 root hair xylem piliferous layer phloem The entry of water dilutes the sap. The sap of the root hair cell now has a higher water potential than that of the next cell (cell B). Hence, water passes by osmosis from the root hair cell into the inner cell. cortex 5 Similarly, water passes from cell B into the next cell (cell C) of the cortex. This process continues until the water enters the xylem vessels and moves up the plant. As water enters the cells, it can take one of two routes depending upon whether or not it has entered the cytoplasm of the cell. These routes are called the – Apoplastic • Water is transported between cells up to the endodermis. – Symplastic • Water is transported through the cell membrane and cytoplasm. • It takes longer to transport and requires ATP. • Water can pass through the endodermis. Transport of mineral salts into root hairs • Diffusion – When the concentration of mineral salts in the soil solution is higher than that in the root hair cell. • Active transport – When the concentration of ions in the soil solution is lower than that in the root hair cell. – The energy comes from respiration in the root hair cells. Transport of water up the stem • Root pressure • Capillary action • Transpiration pull Root Pressure • By using active transport, ions in the living cells around the xylem vessels in the root are pumped into the vessels. • Water potential in the xylem vessels is lowered. • Water passes from the living cells into the xylem vessels by osmosis. • Accumulation of water in the xylem vessels create a pressure called root pressure. • Root pressure pushes the water up the stem. Capillary Action Water also moves up inside fine capillary tube by capillary action • Water molecules attract other water molecules by the force of cohesion. • Water sticks to the inner surface of the xylem vessels by adhesion. • Both cause water to move up xylem stem without breaking. • This mechanism does not account for the greater distance that water can travel in trees. Transpiration Pull Transpiration is the loss of water vapour from stomata of the leaves of the plant. • When water vapour is lost from the leaves, there is a decrease in water concentration in the leaves. • This creates a suction force that pull water up from the stems. • This is called transpirational pull. • Transpiration stream is the stream of water going up the stem. Learning Objectives 1.Relate the structure and functions of root hairs to their surface area and to water and ion uptake. 2.Explain the movement of water from the soil to the root hairs. 3.Explain the mechanism of root pressure. 4.Explain the mechanism of capillary action. 5.State that root pressure and capillary action are ways to draw water up the stem. 6.Define transpiration 7.Explain the mechanism of transpiration producing a pull from above.