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Lecture 14

Lecture 14 - Water and Fluid Transport - Moving Fluids in Plants

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Western University
Biology 2601A/B
Graeme Taylor

LECTURE 14 – WATER AND FLUID TRANSPORT – MOVING FLUID IN PLANTS Plants need a Circulatory System  To transport sugars from the leaves to other tissues  To send hormones and signals throughout the organism  To move water and nutrients from the soil to the leaves Xylem and Phloem  Xylem – translocates water and inorganic nutrients (such as nitrogen, phosphorus and potassium) from the roots to the leaves  Phloem – translocates sugars, proteins and signaling molecules o From source tissues (e.g. leaves) to sink tissues (e.g. roots) Xylem  Made of tracheids (long, thin cells) and vessel elements (short, wide cells)  These are dead cells – start off as live cells that undergo cell death to act as pipes for water  Water moves between the tracheids only through pits while between vessel elements water moves mostly through perforation plates (compound or simple) and subsidiary flow through pits  Interconnect o Laterally (both) o Edge-to-edge (tracheids) o End-to-end (vessel elements)  Supported by fibers and other lignified cells (in trees) to ensure stability Vessel Elements  Primary vessel type in angiosperms  End-to-end stacking plus perforation plates = continuous tubes  Pits connect vessel elements laterally  Some lateral movement – but slow, compared to vertical Tracheids  Secondary in angiosperms  Primary vessel type in gymnosperms o Have permeable pit membranes o Allows lateral movement to provide long connected vessels o Water movement in gymnosperms is very slow How do I know it’s a Circulatory System?  Pump – to push fluid through vessels  Fluid – to trap oxygen and other nutrients  Vessels or spaces – to transport fluids to tissues  In trees, however, there is no pump o Sometimes, root pressure can push water into the xylem o Usually, water is being pulled, rather than pushed – main mechanism in plants The Physics of Pulling Fluid Through Tubes  Evaporation at the leaves – creates low pressure system at extremities  This results in pulling of water up tubes from the roots into the system Cohesion-Tension Theory  Evaporation at the leaves causes negative pressure – in the pores of the cell wall on the cell surface  Negative pressure = tension  The cohesive properties of the water molecules transfer this tension through the length of the water column  Hydrogen bonding is what allows evaporation at the surface of the leaves, and draws up molecules behind it, causing water to be pulled up through xylem system  Requires a continuous water column – if this is broken (such as an air bubble), there is no cohesion which allows it to be continuously pulled  This is a lot of tension o Dense wood to support xylem – prevents xylem from imploding o Danger of cavitation Capillary Action (Capillarity)  Capillary action – ability of a liquid to flow in narrow spaces without the assistance, and in opposition to, external forces such as gravity  Requires ‘wettable’ walls – strong adhesion between liquid and walls of vessel  Requires surface tension – interactions between water molecules, allows for further adhesion  Related to diameter of tube – xylem have small diameters, and are thus, efficient  Could account for a few meters  Cellulose fibers create a network (crisscrossing pattern) – when expanded to see a single pore, as a water film is formed, depending on the radius, an air water interface is formed  Smaller radius = stronger meniscus  Radius of the curvature dictates the pressure that is being created at the surface  Small radius, small pores = large negative pressures can be generated o This is the basis of driving water molecules which flow down a pressure gradient Water Moves Down a Pressure Gradient  Described as a water potential (Ψ) measured in mPA (pressure units)  Water potential of soil would be about -0.1 MPa and the water potential of the leaves would be about
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