Translocation

    Cards (18)

    • What are solutes
      dissolved substances - solutes are sometimes called assimilates (these are substances that get incorporated into the plant tissue)
    • What is the function of the phloem
      transports organic substances/solutes (mainly sucrose) in plants
    • What does organic mean
      contains carbon
    • Phloem tissue consists of 2 types of living cells
      sieve tube elements form the tube for transporting solutes
      companion cells contain many mitochondria to make ATP through aerobic respiration for the active transport of solutes

      when says: to allow continuous flow of organic substances also right to reduce resistance to flow of sucrose
    • How is the phloem adapted for its function
      • sieve tubes have no nucleus, little cytoplasm + few organelles = to allow continuous flow of organic substances (sucrose)/ to reduce resistance to flow of sucrose
      • end walls of sieve tubes are perforated with holes so it allows continuous flow of organic substances through sieve tube/ to reduce resistance to flow of sucrose
      • companion cells contain many organelles e. mitochondria for aerobic respiration to produce ATP for active transport of sucrose into sieve tube elements + many carrier proteins for active transport of sucrose
    • What is translocation
      the movement of organic substances/solutes (e.g. sucrose and amino acids) in the plant from source to sink - its an energy requiring process that happens in the phloem
      source = produce carbohydrates/assimilates (so they are in high concentration at the sink) like sucrose during photosynthesis e.g. leaves
      sink = areas which need energy such as growing regions (roots) or storage organs (potatoes) - where assimilates are used or converted to a storage molecule (so low concentrations here)so a conc gradient from source to sink
    • More on translocation
      occurs in both directions (e.g in winter moves down to roots to store sucrose as starch) there4 sucrose can move in both directions up and down the phloem
    • How are the products of photosynthesis transported around the plant
      by the mass flow hypothesis
    • Why is sucrose transported through the phloem rather than glucose
      after being synthesised during photosynthesis, glucose is converted to sucrose for transport as its more stable and less reactive than glucose
    • Mass flow hypothesis pt1
    • Mass flow hypothesis pt2
    • Mass flow hypothesis pt3
      the higher the conc of sucrose at the source, the higher the rate of translocation
    • Exam Q
    • Evidence for mass flow hypothesis pt1
    • Evidence for mass hypothesis pt2
      pressure in the phloem can be investigated using aphids. their mouth parts can pierce the phloem allowing it to feed. scientists remove the aphids body leaving the mouth parts behind in the plant which allows sap to flow out. the phloem sap flows out quicker nearer the leaves than further down the stem
      • due to pressure gradient - higher pressure at leaves (source) and lower pressure at sink, due to movement of water into and out of phloem by osmosis
    • Evidence foe mass flow hypothesis pt3
      • 14C can be used as radioactive tracer to track movement of organic substances in a plant
      • radioactive CO2 can be supplied to plant which is used for photosynthesis so its incorporated into sucrose at the source when cells photosynthesise + will then be moved around plant in translocation
      • radioactivity can be tracked using autoradiography (= plant is placed onto photographic film)
      • radioactivity observed more at leaves first then further away overtime as radioactive sucrose is actively transported into phloem + translocated towards sink via mass flow
    • More on autoradiography
    • Evidence against mass flow hypothesis
      sugars move to all areas of the plant not just form source to sink with the highest water potential
      the pressure gradient wouldn't be high enough for a sufficient rate of flow through sieve plates so sieve plates would create a barrier to mass flow
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