Transport in plants- phloem

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  • Transport in plants- Phloem
    • Adaptations
      • Transport solutes around plants
      • Sieve tube elements: Living cells that form tube for transporting solutes
      • Companion cell: Carries out living functions e.g providing energy for AT
    • Translocation
      • Movement of solutes/ assimilates to where they're needed in plant
      • Requires energy, happens in phloem
      • Moves solutes from 'source' (where it's made, high conc) to 'sink' (where it's used, low conc)
      • Enzymes maintain conc gradient by changing solutes at sink. Always lower conc at sink than source
    • Mass Flow
      • 1: AT used to load solutes from companion cells to sieve tubes at source e.g leaves. Lowers water pot in sieve tubes, water enters by osmosis from xylem/ companion cells. High pressure inside sieve tubes at source
      • 2: Solutes removed from phloem at sink end. Increases water pot inside sieve tubes, water leaves by osmosis. Lowers pressure inside sieve tubes
      • 3: Resulting pressure gradient from source to sink. Gradient pushes solutes towards sink. At sink, solutes either used/stored
    • Evidence for/against mass flow
      • Not all solutes move at same speed- should do if movement is by mass flow
      • Function of sieve plates unclear- should hinder mass flow
      • Sucrose delivered at same rate to all regions rather than quicker to ones with lower conc
      • Companion cells possess many mitochondria, readily produce ATP
      • Increases in sucrose in leaf followed  by similar increases in phloem later
      • Metabolic poisons/ lack of oxygen inhibit translocation of sucrose in phloem
      • Downward flow in phloem increases in daylight, ceases at night/when leaves are shed
      • Pressure within sieve tubes, sap released when they are cut
      • Concentration of sucrose higher in leaves, lower in roots


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