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  • Translocation
    • Unlike water in xylem, the organic compounds that are transported through phloem can move either up or down the plant stem, often simultaneously
    • 'Mass flow theory' is one theory of how transportation through the phloem takes place
      • Photosynthesising cells in the leaves of plants (Source cells) produce glucose
        • The glucose is converted to sucrose for transportation in the phloem sieve tube to all parts of the plant
          • At the roots or shoots (growing plants known as sink cells), the sucrose is converted back into glucose so it can be used in aerobic respiration for growth (mitosis cell division)
            • If it is not used in respiration in the sink cells, the sucrose is converted to starch and stored
      • Problems with the mass flow theory
        • Suggests that the flow of sucrose from source to sink cells is a passive process
          • However, we now know that the transportation of organic molecules in phloem occurs too quickly for it to be diffusion alone and it is believed that it is an active process and that the cells making up the phloem are living cells
            • Mass flow is also unable to explain how movement can occur in opposite directions at the same time and different rates of sucrose flow
              • Other hypotheses that have been put forward include: Diffusion and cytoplasmic streaming, but neither can account for all the observations made
    • Translocation Experiments
      • Ringing Experiments
        • Cylinders of bark can be taken from the plant stem, removing the phloem but leaving the xylem intact
          • The composition of phloem shows that sucrose and other organic solutes are present above the ring but absent from the region below the ring
            • This indicates that translocation occurs in phloem rather than in xylem
      • Aphids and autoradiography
        • Aphids, such as greenfly, have specialised mouthparts called stylets, which they use to penetrate phloem tubes to sup off the sugary sap inside
          • If the aphids are anaesthetised with carbon dioxide and the stylet cut off, the stylet remains in the phloem, so pure phloem sap can be collected through the stylet for analysis
            • The plant is supplied with radioactive carbon dioxide (14CO2) and allowed to photosynthesise. The carbohydrates it makes, including sucrose will be radioactively labelled
              • The movement of these radioactive molecules in the shoot can be traced either by using a photographic film (autoradiograph) or with a geiger counter
                • An autoradiograph is an image produced on photographic film, the presence of radioactive isotopes will cause the film to 'fog' revealing the location of the labelled carbohydrates. This corresponds to the location of the phloem tissue in the stem
        • The use of aphids and radioactive tracing has demonstrated than;
          • Sucrose is transported in phloem
          • Sucrose is transported bi-directionally to sinks
          • Translocation is a rapid process


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