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Unit 3 - Xylem and Phloem

All multicellular organisms require systems to transport fluids around their bodies. In plants, xylem and phloem tissue carry out this job and in this GCSE Biology quiz we take a closer look at these two similar yet different tissues.

Xylem and phloem both transport fluids in plants so what are the differences between them? Well, xylem is dead, woody tissue consisting of tubes and vessels which transport fluid (water containing minerals) from the roots of a plant, up through the stem and into its leaves. Phloem moves dissolved sugars from the leaves to the rest of the plant, including the growing regions and the storage tissues. The cells of the phloem are still living.

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Water is absorbed from the soil by the root hair cells (which also obtain essential mineral ions) by active transport. The solution passes into the xylem of the plant and is carried to the leaves where the plant cells use the nutrients to make sugars and amino acids. The tubes in the xylem are made from elongated cells that are arranged end to end. The cell walls contain the substance lignin which gives strength to the xylem tubes, supporting the plant. The xylem cells are alive at first but as the plant grows, they die. The cytoplasm and cell walls between adjoining cells break down and the result is a dead, empty continuous tube, one cell in diameter that is impermeable to water. Movement of the water and minerals through the xylem tissue is caused by transpiration from the leaves of the plant.

The water and minerals that reach living plant cells are used for photosynthesis, producing food for the plant in the form of sugars. Plants also produce amino acids from nitrate and other ions. Both the sugars and the amino acids are required for cell growth and repair. These substances are carried to growing tissues and storage tissues by the phloem. This movement of fluid through plants is called translocation. The elongated cells that form the tubes of the phloem are alive. The cells are joined end to end and the cell walls between them develop holes that allow liquids to pass through.

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  1. Name the process by which mineral ions move from the soil into the root.
    The concentration of minerals in the water of the soil is very low and is higher in the root hair cells. Uptake of mineral ions therefore takes place against a concentration gradient, so it must be active transport
  2. This plant tissue is woody and dead.
    The xylem was made from living cells which laid down strong woody substances in the cell walls and became the tubes and vessels
  3. If we use a poison which inhibits respiration, which of the following processes is not blocked?
    Xylem is dead tissue, so respiration is not taking place in the xylem. It can continue to function
  4. Which specialised plant tissue transports dissolved sugars?
    Leaves and roots are plant organs - not tissues
  5. Name the special tissue which transports water and mineral ions.
    Cells in the xylem tissue are dead
  6. Water leaves the plant leaves through...
    Stomata are the tiny holes found on the underside of leaves and they let the water vapour out of the plant
  7. Transpiration involves the movement of...
    The water contains dissolved ions from the soil
  8. Can Xylem vessels be blocked by air bubbles?
    They are extremely tiny and so the forces of surface tension are high enough to 'glue' the air bubble to the walls of the xylem vessels, blocking the transpiration stream
  9. Xylem is found in the...
    Water and minerals from the soil are transported from the roots to the leves where they are used in photosynthesis and the synthesis of amino acids
  10. Do plants need living phloem in order to transport sugars?
    Phloem cells are essential to the process of translocation - the movement of food and amino acids through the plant

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