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Transport of Water Across the Root osmosisWater is absorbed from the soil by osmosis down the gradientWater moves down the gradient Water only enters the.

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Presentation on theme: "Transport of Water Across the Root osmosisWater is absorbed from the soil by osmosis down the gradientWater moves down the gradient Water only enters the."— Presentation transcript:

1 Transport of Water Across the Root osmosisWater is absorbed from the soil by osmosis down the gradientWater moves down the gradient Water only enters the root near the root tip root hairsincrease the surface areaHere there are root hairs which increase the surface area for osmosis Water potential is higher in the epidermal cells than in the central cells down the gradientWater moves across the cortex down the gradient to xylem vessels, symplastapoplastWater can move via the symplast or apoplast routes

2 Transverse section of a root endodermis Casparian strip in the walls of the endodermal cells xylem vessels stele Subject to copyright clearance a suitable image showing a transverse section of a root could be inserted here. e.g. one similar to that found at:

3 Diagram of transverse root section epidermis with root hairs cortex endodermis xylem phloem pericycle

4 Water is transported across the root by two routes Apoplast route Symplast route between the cells via the cell walls cell cytoplasm to cell cytoplasm

5 The Symplast Route cytoplasmThrough the cytoplasm Water enters the root hair cells across the partially permeable membrane by osmosis Water moves from higher in the soil to the lower in the cell Water moves across the root from cytoplasm to cytoplasm down the gradient plasmadesmataIt passes from one cell to the other via plasmadesmata Water moves into the xylem by osmosis The only way across the endodermisThe only way across the endodermis Normally the most important pathway

6 The Apoplast Route cellulose cell wallintercellular spacesWater moves through the cellulose cell wall and intercellular spaces The permeable fibres of cellulose do no resist water flow Water cannot pass the endodermis by this route Because the Casparian strip in the endodermis cell wall is impermeable to water Due to the waterproof band of suberin So all water must pass the endodermis via the cytoplasmSo all water must pass the endodermis via the cytoplasm Therefore it is under cellular control Apoplast route is important when transpiration rates are high as it is faster and requires no energy

7 The Casparian strip acts as an apoplast block The Casparian strip is made of suberin, which is impermeable to water Water is unable to pass through the endodermis by the apoplast route The endodermis actively transports salts into the root xylem Lowering the in the xylem, so water moves in down the gradient by osmosis Water moves up the stem in the xylem vessels Subject to copyright clearance a suitable image showing the casparian strip could be inserted here. e.g. one similar to that found at:

8 Transverse Section of a Stem (Dicot) Vascular bundles Subject to copyright clearance a suitable image showing a transverse section of a stem could be inserted here. e.g. one similar to that found at:

9 Vascular bundle from a stem Xylem vessels Phloem Epidermis Subject to copyright clearance a suitable image showing a vascular bundle could be inserted here. e.g. one similar to that found at:

10 Xylem vessels with different types of lignin strengthening the cell walls Xylem vessels form continuous tubes lignin fibres strengthen the cell walls so do not collapse when pressure inside falls no cell contents (dead) Subject to copyright clearance a suitable image showing xylem vessels with different strengthening could be inserted here. e.g. one similar to that found at:

11 Mechanisms for the Transport of Water up the Xylem 1.Capillarity 2.Root Pressure 3.Cohesion-Tension

12 Capillarity Water rises up narrow tubes due to the adhesive forces between the water molecules and the wall of the tube Xylem vessels are very narrow Water rises higher in narrower tubes 1.Water will only rise 50mm2.The flow rate is slower than the rate observed in xylem Limitations

13 Root Pressure Root pressure causes the mercury to rise in the manometer Cut stump of a well watered plant Water Mercury Manometer

14 Root Pressure pushedWater is pushed up the xylem by hydrostatic pressure pumpedMineral salts are pumped into the xylem vessels in the root by the endodermal cells Lowering theLowering the in the xylem osmosisWater moves in from the surrounding cells by osmosis Raising the hydrostatic pressureRaising the hydrostatic pressure so pushing water up the xylem What would happen if the roots were deprived of O 2 ? The pumping of the ions would stop as it requires ATP produced in aerobic respiration. O 2 required for aerobic respiration

15 Root Pressure: Evidence 1.Cut stumps of plants exude water from their cut ends 2.In certain conditions some leaves exude water from their leaves = guttation 3.Pressures recorded by mercury manometers attached to the cut stumps could push water in the xylem up to 30m

16 Guttation Water droplets exude from the leaves Subject to copyright clearance a suitable image showing guttation could be inserted here. e.g. one similar to that found at:

17 Limitations of the Root Pressure Hypothesis The pressure measured is not enough to get water to the top of trees Only find root pressure in spring Relies on the use of the plants energy (ATP) for active transport

18 Cohesion - Tension pulled transpirationWater is pulled up the xylem by the water lost in transpiration The sun provides the energy to pull the water up by providing the energy for evaporation Water moves up the xylem by mass flow from the higher pressure in roots to the lower pressure in the leaves cohesive forcesThe column of water does not break because of the cohesive forces between the water molecules Hydrogen bondsHydrogen bonds between individual water molecules is the force of attraction

19 Evidence for the Cohesion Tension Hypothesis 1.Cut stems attached to a tube containing water over mercury can pull the mercury up almost 1m 2.Dendrographs record that tree trunks have a narrower diameter during the day when transpiration rate is high i.e. when most tension is created. 3.Puncturing the xylem of the stem of a transpiring shoot under water containing a dye causes the dye to move into the xylem both ways. The dye must be pulled in so the xylem is under tension.

20 Variation in trunk diameter and transpiration rate over 24 hours The diameter of the trunk decreases as transpiration rate increases Evaporation from the leaves pulled Evaporation from the leaves draws water from the xylem by osmosis, water is pulled up the xylem creating a tension. The tension pulls the xylem vessel walls in, so the trunk diameter gets smaller The trunk has a larger diameter when there is less transpiration 2400 Transpiration rate Branch diameter This supports the cohesion tension hypothesis but not root pressure.

21 Water movement across the leaf upper epidermis palisade mesophyll spongy mesophyll lower epidermis stoma cuticle water vapour diffuses into the air down gradient xylem lowest in the air water evaporates from the spongy mesophyll cell surface lowering cell water evaporates from the spongy mesophyll cell surface lowering cell water moves into cells down gradient by osmosis water is pulled along the xylem

22 The Cohesion Tension Hypothesis for Movement of Water up the Xylem Vessels Water evaporates from the spongy mesophyll cells and diffuses into the atmosphere Transpiration Lower in the leaf cells Water moves from down the gradient Water is pulled up xylem vessels Lower pressure/tension at top of xylem Cohesive forces between water molecules prevent water column breaking Water moves across root from soil down gradient Via the apoplast and symplast paths

23 Questions 1.Explain, in terms of water potential how water moves from the soil to the endodermis in a root(5marks)5marks 2. Explain why, in summer, the diameter of a branch is smaller at noon than at midnight.(4 marks)4 marks 3.Explain the root pressure hypothesis for water movement in the xylem.(3 marks)3 marks 4.Give two limitations of this hypothesis,(2marks)2marks Click on the marks above to check your answers click here to end

24 Answer Q1 Water is absorbed from the soil by the root hairs By osmosis down the water potential gradient The water potential is higher in the epidermal cells than in the xylem in the root centre Water moves from cell to cell through the cytoplasm down the water potential gradient Water also moves through the fibres of the cell wall and intercellular spaces But must go through the endodermal cells due to the Casparian strip Any 5 points Back to question

25 Answer Q2 Temperature higher at noon so transpiration rate higher More water evaporates from the surface of the mesophyll cells Reducing the the water potential Water moves from the xylem in the leaves into the cells Creating a tension pulling the water up the xylem This pulls the xylem vessels in so reducing the diameter of the trunk Any four points Back to question

26 Answer Q3 Root pressure is a hydrostatic pressure pushing water up the xylem Mineral ions are actively transported out of the endodermal cells into the xylem vessels Lowering the water potential in the xylem So water moves in from the surrounding cells by osmosis / down the water potential gradient Raising the hydrostatic pressure Any three points Back to question

27 Answer Q4 Back to question The pressure measured is not enough to get water to the top of trees Only find root pressure in spring Relies on the use of the plants energy (ATP) for active transport Any two

28 Now think of some synoptic links and make a list. Try out this web site to review transport of water in plants


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