Unit 1 Section 6.1 Xylem and Phloem

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  • Xylem and Phloem
    • why do plants need transport systems?
      • like animals, plants are a multicellular organisms
        • meaning it has a small surface area to volume ratio
      • plants could exchange substances by direct diffusion but that would be too slow
      • plants need a transport system to move substances to and from individual cells quickly
    • location of xylem and phloem tissues
      • Phloem Tissues
        • Phloem transports dissolved substances like sugars up and down a plant
      • Xylem Vessels
        • Xylem transports water and mineral ions up a plants stem to the leaves
        • Xylem acts as a support for the plant
      • xylem and phloem are found throughout a plant
    • adaptations of xylem vessels
      • xylem vessels are very long
        • they are tube like structures formed from cells joined end to end
          • there are no end walls, they form a continuous tube for water to flow through the middle easily
            • the cells are dead, so they contain no cytoplasm
      • the cell walls are thickened with a woody substance called lignin
        • which helps support the walls and stops them from collapsing inwards
          • the lignin is present in spiral patterns, which allows flexibility and prevents the stem from breaking
            • the amount of lignin increases as the cell gets older
      • water and mineral ions move into and out of the vessels through small pits in the walls where there's no lignin
        • this is how other types of cells are supplied with water
    • adaptations of phloem tissue
      • phloem tissue transports sugars like sucrose
        • like xylem phloem is formed from cells arranged in tuibes
          • but unlike xylem, its purely a transport tissue
            • phloem tissue contains phloem fibres, phloem parenchyma, sieve tube elements and companion cells
              • sieve tube  elements and companion cells are the most important cell types in phloem for transport
                • sieve tube elements
                  • these are living cells that form the tube for transporting sugars through the plant
                    • the sieve parts are the end walls, which have lots of holes in them to allow sugars to pass through
                      • unusually for living cells sieve tube elements have no nucleus, a very thin layer of cytoplasm and few organelles
                • Companion cells
                  • the lack of a nucleus and other organelles in sieve tubes means that they cant survive on their own
                    • so there's a companion cell for every sieve tube element
                      • companion cells carry out the living functions for both themselves and their sieve cells


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