Xylem and phloem

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  • Xylem and phloem
    • Structure of xylem
      • Xylem is used to transport water and minerals from the roots up to the leaves and other parts of the plant.
      • Xylem tissues consists of tubes to carry the water and dissolved minerals, fibres to help support the plant and living parenchyma cells
      • Xylem vessels
        • In dicotyledonous plants, the obvious  features of xylem and the xylem vessel elements.
          • These are long cells with thick walls that have been impregnated by lignin.
        • As the xylem develops the lignin waterproofs the walls of the cells.
          • As a result the cells die, and their end walls and contents decay
            • This leaves a long column of dead cells with no contents - a tube with no end walls (xylem vessels)
        • The lignin strengthens the vessel walls and prevents the vessel from collapsing. This keeps the vessels open even at times when water may be in short supply
        • The lignin thickening forms patterns in the cell wall. These may be spiral, annular (rings) or reticulate (a network of broken rings)
          • This prevents the vessel from being too rigid and allows flexibility of the stem or branch
        • In some places this lignification is not complete. It leaves pores in the wall of the vessel, which are called pits or bordered pits.
          • These allow water to leave one vessel and pass into another adjacent vessel, or pass into the living parts of the plants
    • Adaptations of xylem to its function
      • Xylem tissue can carry water and minerals from roots to the very top of the plant because:
        • it is made from dead cells aligned end-to-end to form a continuous column
        • the tubes are narrow so the water column doesn't break easily and capillary action can be effective.
        • pits in the lignified walls allow water to move sideways from one vessel to another
        • lignin deposited in the walls in spiral, annular or reticulate patterns allows xylem to stretch as the pant grows and enables the stem/branch to bend
      • The flow of water in impeded because:
        • there are no end walls
        • there are no cell contents
        • there is no nucleus/cytoplasm
        • lignin thickening prevents the walls from collapsing
    • Structure of phloem
      • The function of the phloem is to transport sugars from one part of the plant to another. Could be up or down the stem
      • Phloem tissue consists of two types of cell;the sieve tube elements and the companion cells.
      • Sieve tubes
        • Not true cells, contain very little cytoplasm and no nucleus
        • Lined up end-to-end to form a tube, in which the plant transports sucrose
        • Unlike xylem vessels, this tube contains cross-walls at intervals.
          • These cross-walls are perforated by many pores to allow the sap to flow.
        • Hence the cross-walls are called sieve plates and the tubes are called sieve tubes. The sieve tubes have very thin walls and are usually 5/6 sided
      • Companion cells
        • In between the sieve tubes are small cells, each with a large nucleus and dense cytoplasm
        • Have numerous mitochondria to produce the ATP needed for active processes.
          • The companion cells carry out the metabolic processes needed by the sieve tube elements. Includes using ATP as a source of energy to load sucrose into the sieve tubes.
        • The cytoplasm of the companion cells and the sieve tube elements are linked through many plasmodesmata.
          • These are gaps in the cell walls allowing communication and flow of substances  between the cells


Daniel Meads


Where is the information on the Phloem?

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