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Why plant distributes sugar in the form of sucrose?

  1. Apr 1, 2017 #1
    Few months back, in the final rounds of biology olympiad for Highschool students, i was asked, "Plants distributes sugar in the phloem as sucrose molecules, why did plant do this, and is there any advantages? Why not other form of sugar like glucose?"

    Can anyone help me? :3
    Also i was wondering why animals store energy in form of fat or glycogen while plants store them in form of amylum
  2. jcsd
  3. Apr 2, 2017 #2

    jim mcnamara

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    Staff: Mentor

    There are two type of sucrose translocation in the phloem.

    In the first mechanism, sucrose enters the phloem by attaching to sucrose transporter proteins embedded in the plasma membranes of the sieve elements and companions cells. In the second mechanism, sucrose enters the companion cells of the phloem through small plasmodesmata (openings in the cell wall), and is converted to larger sugars. Ex., raffinose, stachyose. So, glucose cannot move from the leaf down through the phloem because there is no transport mechanism.

    So an answer is: the mechanisms of transport only "work" for sucrose.

    Starch is a synonym for amylum, and more often used. Starch is stored as small inclusions - granules or grains - because it is not soluble. That means cells can contain lots of starch without altering osmotic pressure, like glucose or sucrose would do. Concentrations of simple sugars would simply increase in storage tissue like roots, to the point where the cytoplasm would have a very high concentration. Net movement of additional simple carbs into the cell would become difficult - or close to impossible. If simple carbs come into the storage cell and then are immediately converted to starch, they no longer negatively affect the movement. So, there is no problem moving new simple carbs into that same cell.

    Starch is a collective term - there are multiple type of starch molecules. Some types absorb water, some like amylopectin do not absorb water.

    Glycogen works similarly to starch - it keeps glucose concentrations low in animal tissues where it is stored near the liver. And, like starch, converting glycogen back to simple sugars is easy to do and efficient (in biochemical terms this is called hydrolysis).
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