Transport in plants

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  • Transport in plants
    • Why do plants need a transport system?
      • All living things need to take substances from their environment and return wastes to their environment.
        • Every cell of a multicellular plant needs a regular supply of water and nutrients.
      • In a  large multicellular plant the epithelial (surface) cells, which are close to the supply, could gain all they need by diffusion.
        • But there are many cells inside the plant that are further from the supply. These cells would not receive enough water or nutrients to survive
      • The particular  problem in plants is that the roots can obtain water easily, but they cannot absorb sugars from the soil. The leaves can produce sugars, but cannot obtain water form the air.
      • Substances that need to be moved
        • The transport system in plants move water in special tissue called vascular tissue
          • Water and soluble minerals travel upwards in xylem tissue
          • Sugars travel up or down in phloem tissue
        • Both tissues are highly specialised
    • The vascular tissue
      • The vascular tissue is distributed throughout the plant
      • The xylem and phloem are found together in vascular bundles.
        • These bundles also often contain other types of tissue that give the bundle some strength and help to support the plants
    • Xylem and phloem in the young root
      • The vascular bundle is found at the centre of a young root
      • There is a large central core of xylem, often in the shape of an X.
      • The phloem is found in between the arms of the X shaped xylem.
      • The arrangement provides strength to withstand the pulling forces to which roots are exposed
      • Around the vascular bundle in a special sheath of cells called the endodermis.
        • The endodermis has a key role in getting water into the xylem vessels. Just inside the endoderimis is a layer of meristem cells called the pericycle (cells that are able to divide)
    • Xylem and phloem in the stem
      • The vascular bundles are found near the the outer edge of the stem.
        • In non-woody plants the bundles are separate and discrete.
        • In woody plants the bundles are separate in young stems, but become continuous in older stems
          • This means there is a complete ring of vascular tissue just under the bark of a tree.
      • This arrangement provides strength and flexibility to withstand the bending forces to which stems and branches are exposed
      • The xylem is found towards the inside of each vascular bundle.
      • The phloem is found towards the outside of the bundle.
      • In between the xylem and phloem is a layer of cambium. The cambium is a layer of meristem cells that divide to produce new xylem and phloem
    • Xylem and phloem in the leaf
      • The vascular bundles form in the midrib and veins of a leaf.
      • A dicotyledon leaf has a branching network of veins that get smaller as they spread away from the midrib. Within each vein, the xylem can be seen on top of the phloem

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